ASICs...THE COURSE (1 WEEK)
1
VERILOG HDL
Key terms and concepts: syntax and semantics ? operators ? hierarchy ? procedures and assign-
ments ? timing controls and delay ? tasks and functions ? control statements ? logic-gate modeling
? modeling delay ? altering parameters ? other Verilog features: PLI
History: Gateway Design Automation developed Verilog as a simulation language ? Cadence
purchased Gateway in 1989 ? Open Verilog International (OVI) was created to develop the
Verilog language as an IEEE standard ? Verilog LRM, IEEE Std 1364-1995 ? problems with a
normative LRM
11.1 A Counter
Key terms and concepts: Verilog keywords ? simulation language ? compilation ? interpreted,
compiled, and native code simulators
`timescale 1ns/1ns // Set the units of time to be nanoseconds. //1
module counter; //2
reg clock; // Declare a reg data type for the clock. //3
integer count; // Declare an integer data type for the count. //4
initial // Initialize things; this executes once at t=0. //5
begin //6
clock = 0; count = 0; // Initialize signals. //7
#340 $finish; // Finish after 340 time ticks. //8
end //9
/* An always statement to generate the clock; only one statement
follows the always so we don't need a begin and an end. */ //10
always //11
#10 clock = ~ clock; // Delay (10ns) is set to half the clock
cycle. //12
/* An always statement to do the counting; this executes at the same
time (concurrently) as the preceding always statement. */ //13
always //14
begin //15
11
2 SECTION 11 VERILOG HDL ASICS... THE COURSE
// Wait here until the clock goes from 1 to 0. //16
@ (negedge clock); //17
// Now handle the counting. //18
if (count == 7) //19
count = 0; //20
else //21
count = count + 1; //22
$display("time = ",$time," count = ", count); //23
end //24
endmodule //25
11.2 Basics of the Verilog Language
Key terms and concepts: identifier ? Verilog is case-sensitive ? system tasks and functions
begin with a dollar sign '$'
identifier ::= simple_identifier | escaped_identifier
simple_identifier ::= [a-zA-Z][a-zA-Z_$]
escaped_identifier ::=
\ {Any_ASCII_character_except_white_space} white_space
white_space ::= space | tab | newline
module identifiers; //1
/* Multiline comments in Verilog //2
look like C comments and // is OK in here. */ //3
// Single-line comment in Verilog. //4
reg legal_identifier,two__underscores; //5
reg _OK,OK_,OK_$,OK_123,CASE_SENSITIVE, case_sensitive; //6
reg \/clock ,\a*b ; // Add white_space after escaped identifier. //7
//reg $_BAD,123_BAD; // Bad names even if we declare them! //8
initial begin //9
legal_identifier = 0; // Embedded underscores are OK, //10
two__underscores = 0; // even two underscores in a row. //11
_OK = 0; // Identifiers can start with underscore //12
OK_ = 0; // and end with underscore. //13
OK$ = 0; // $ sign is OK, but beware foreign keyboards. //14
OK_123 =0; // Embedded digits are OK. //15
CASE_SENSITIVE = 0; // Verilog is case-sensitive (unlike VHDL). //16
ASICs... THE COURSE 11.2 Basics of the Verilog Language 3
case_sensitive = 1; //17
\/clock = 0; // An escaped identifier with \ breaks rules, //18
\a*b = 0; // but be careful to watch the spaces! //19
$display("Variable CASE_SENSITIVE= %d",CASE_SENSITIVE); //20
$display("Variable case_sensitive= %d",case_sensitive); //21
$display("Variable \/clock = %d",\/clock ); //22
$display("Variable \\a*b = %d",\a*b ); //23
end //24
endmodule //25
11.2.1 Verilog Logic Values
Key terms and concepts: predefined logic-value system or value set ? four logic values: '0',
'1', 'x', and 'z' (lowercase) ? uninitialized or an unknown logic value (either '1', '0', 'z',
or in a state of change) ? high-impedance value (usually treated as an 'x' value) ? internal logic-
value system resolves conflicts between drivers on the same node
11.2.2 Verilog Data Types
Key terms and concepts: data types ? nets ? wire and tri (identical) ? supply1 and supply0
(positive and negative power) ? default initial value for a wire is 'z' ? integer, time, event, and
real data types ? register data type (keyword reg) ? default initial value for a regis 'x' ? a reg
is not always equivalent to a register, flip-flop, or latch ? scalar ? vector ? range ? access (or
expand) bits in a vector using a bit-select, or as a contiguous subgroup of bits using a part-
select ? no multidimensional arrays ? memory data type is an array of registers ? integer arrays
? time arrays ? no real arrays
module declarations_1; //1
wire pwr_good, pwr_on, pwr_stable; // Explicitly declare wires. //2
integer i; // 32-bit, signed (2's complement). //3
time t; // 64-bit, unsigned, behaves like a 64-bit reg. //4
event e; // Declare an event data type. //5
real r; // Real data type of implementation defined size. //6
// An assign statement continuously drives a wire: //7
assign pwr_stable = 1'b1; assign pwr_on = 1; // 1 or 1'b1 //8
assign pwr_good = pwr_on & pwr_stable; //9
initial begin //10
i = 123.456; // There must be a digit on either side //11
r = 123456e-3; // of the decimal point if it is present. //12
t = 123456e-3; // Time is rounded to 1 second by default. //13
4 SECTION 11 VERILOG HDL ASICS... THE COURSE
$display("i=%0g",i," t=%6.2f",t," r=%f",r); //14
#2 $display("TIME=%0d",$time," ON=",pwr_on, //15
" STABLE=",pwr_stable," GOOD=",pwr_good); //16
$finish; end //17
endmodule //18
module declarations_2; //1
reg Q, Clk; wire D; //2
// Drive the wire (D): //3
assign D = 1; //4
// At a +ve clock edge assign the value of wire D to the reg Q: //5
always @(posedge Clk) Q = D; //6
initial Clk = 0; always #10 Clk = ~ Clk; //7
initial begin #50; $finish; end //8
always begin //9
$display("T=%2g", $time," D=",D," Clk=",Clk," Q=",Q); #10; end //10
endmodule //11
module declarations_3; //1
reg a,b,c,d,e; //2
initial begin //3
#10; a = 0;b = 0;c = 0;d = 0; #10; a = 0;b = 1;c = 1;d = 0; //4
#10; a = 0;b = 0;c = 1;d = 1; #10; $stop; //5
end //6
always begin //7
@(a or b or c or d) e = (a|b)&(c|d); //8
$display("T=%0g",$time," e=",e); //9
end //10
endmodule //11
module declarations_4; //1
wire Data; // A scalar net of type wire. //2
wire [31:0] ABus, DBus; // Two 32-bit-wide vector wires: //3
// DBus[31] = leftmost = most-significant bit = msb //4
// DBus[0] = rightmost = least-significant bit = lsb //5
// Notice the size declaration precedes the names. //6
// wire [31:0] TheBus, [15:0] BigBus; // This is illegal. //7
reg [3:0] vector; // A 4-bit vector register. //8
reg [4:7] nibble; // msb index < lsb index is OK. //9
integer i; //10
initial begin //11
i = 1; //12
vector = 'b1010; // Vector without an index. //13
nibble = vector; // This is OK too. //14
ASICs... THE COURSE 11.2 Basics of the Verilog Language 5
#1; $display("T=%0g",$time," vector=", vector," nibble=", nibble);//15
#2; $display("T=%0g",$time," Bus=%b",DBus[15:0]); //16
end //17
assign DBus [1] = 1; // This is a bit-select. //18
assign DBus [3:0] = 'b1111; // This is a part-select. //19
// assign DBus [0:3] = 'b1111; // Illegal : wrong direction. //20
endmodule //21
module declarations_5; //1
reg [31:0] VideoRam [7:0]; // An 8-word by 32-bit wide memory. //2
initial begin //3
VideoRam[1] = 'bxz; // We must specify an index for a memory. //4
VideoRam[2] = 1; //5
VideoRam[7] = VideoRam[VideoRam[2]]; // Need 2 clock cycles for this.//6
VideoRam[8] = 1; // Careful! the compiler won't complain about this!//7
// Verify what we entered: //8
$display("VideoRam[0] is %b",VideoRam[0]); //9
$display("VideoRam[1] is %b",VideoRam[1]); //10
$display("VideoRam[2] is %b",VideoRam[2]); //11
$display("VideoRam[7] is %b",VideoRam[7]); //12
end //13
endmodule //14
module declarations_6; //1
integer Number [1:100]; // Notice that size follows name //2
time Time_Log [1:1000]; // - as in an array of reg. //3
// real Illegal [1:10]; // Illegal. There are no real arrays. //4
endmodule //5
11.2.3 Other Wire Types
Key terms and concepts: wand, wor, triand, and trior model wired logic ? ECL or EPROM,
? one area in which the logic values 'z' and 'x' are treated differently ? tri0 and tri1 model
resistive connections to VSS or VDD ? trireg is like a wire but associates some capacitance
with the net and models charge storage ? scalared and vectored are properties of vectors ?
small, medium, and large model the charge strength of trireg
11.2.4 Numbers
Key terms and concepts: constant numbers are integer or real constants ? integer constants
are written as width'radix value ? radix (or base): decimal (d or D), hex (h or H), octal (o
or O), or binary (b or B) ? sized or unsized (implementation dependent) ? 1'bx and 1'bz for 'x'
6 SECTION 11 VERILOG HDL ASICS... THE COURSE
and 'z' ? parameter (local scope) ? real constants 100.0 or 1e2 (IEEE Std 754-1985) ? reals
round to the nearest integer, ties away from zero
module constants; //1
parameter H12_UNSIZED = 'h 12; // Unsized hex 12 = decimal 18. //2
parameter H12_SIZED = 6'h 12; // Sized hex 12 = decimal 18. //3
// Note: a space between base and value is OK. //4
// Note: ‘’ (single apostrophes) are not the same as the ' character.//5
parameter D42 = 8'B0010_1010; // bin 101010 = dec 42 //6
// OK to use underscores to increase readability. //7
parameter D123 = 123; // Unsized decimal (the default). //8
parameter D63 = 8'o 77; // Sized octal, decimal 63. //9
// parameter ILLEGAL = 1'o9; // No 9's in octal numbers! //10
// A = 'hx and B = 'ox assume a 32 bit width. //11
parameter A = 'h x, B = 'o x, C = 8'b x, D = 'h z, E = 16'h ????; //12
// Note the use of ? instead of z, 16'h ???? is the same as 16'h
zzzz. //13
// Also note the automatic extension to a width of 16 bits. //14
reg [3:0] B0011,Bxxx1,Bzzz1; real R1,R2,R3; integer I1,I3,I_3; //15
parameter BXZ = 8'b1x0x1z0z; //16
initial begin //17
B0011 = 4'b11; Bxxx1 = 4'bx1; Bzzz1 = 4'bz1; // Left padded. //18
R1 = 0.1e1; R2 = 2.0; R3 = 30E-01; // Real numbers. //19
I1 = 1.1; I3 = 2.5; I_3 = -2.5; // IEEE rounds away from 0. //20
end //21
initial begin #1; //22
$display //23
("H12_UNSIZED, H12_SIZED (hex) = %h, %h",H12_UNSIZED, H12_SIZED); //24
$display("D42 (bin) = %b",D42," (dec) = %d",D42); //25
$display("D123 (hex) = %h",D123," (dec) = %d",D123); //26
$display("D63 (oct) = %o",D63); //27
$display("A (hex) = %h",A," B (hex) = %h",B); //28
$display("C (hex) = %h",C," D (hex) = %h",D," E (hex) = %h",E); //29
$display("BXZ (bin) = %b",BXZ," (hex) = %h",BXZ); //30
$display("B0011, Bxxx1, Bzzz1 (bin) = %b, %b, %b",B0011,Bxxx1,Bzzz1);//31
$display("R1, R2, R3 (e, f, g) = %e, %f, %g", R1, R2, R3); //32
$display("I1, I3, I_3 (d) = %d, %d, %d", I1, I3, I_3); //33
end //34
endmodule //35
ASICs... THE COURSE 11.2 Basics of the Verilog Language 7
11.2.5 Negative Numbers
Key terms and concepts: Integers are signed (two’s complement) or unsigned ? Verilog only
“keeps track” of the sign of a negative constant if it is (1) assigned to an integer or (2) assigned
to a parameter without using a base (essentially the same thing) ? in other cases a negative
constant is treated as an unsigned number ? once Verilog “loses” a sign, keeping track of signed
numbers is your responsibility
module negative_numbers; //1
parameter PA = -12, PB = -'d12, PC = -32'd12, PD = -4'd12; //2
integer IA , IB , IC , ID ; reg [31:0] RA , RB , RC , RD ; //3
initial begin #1; //4
IA = -12; IB = -'d12; IC = -32'd12; ID = -4'd12; //5
RA = -12; RB = -'d12; RC = -32'd12; RD = -4'd12; #1; //6
$display(" parameter integer reg[31:0]"); //7
$display ("-12 =",PA,IA,,,RA); //8
$displayh(" ",,,,PA,,,,IA,,,,,RA); //9
$display ("-'d12 =",,PB,IB,,,RB); //10
$displayh(" ",,,,PB,,,,IB,,,,,RB); //11
$display ("-32'd12 =",,PC,IC,,,RC); //12
$displayh(" ",,,,PC,,,,IC,,,,,RC); //13
$display ("-4'd12 =",,,,,,,,,,PD,ID,,,RD); //14
$displayh(" ",,,,,,,,,,,PD,,,,ID,,,,,RD); //15
end //16
endmodule //17
parameter integer reg[31:0]
-12 = -12 -12 4294967284
fffffff4 fffffff4 fffffff4
-'d12 = 4294967284 -12 4294967284
fffffff4 fffffff4 fffffff4
-32'd12 = 4294967284 -12 4294967284
fffffff4 fffffff4 fffffff4
-4'd12 = 4 -12 4294967284
4 fffffff4 fffffff4
8 SECTION 11 VERILOG HDL ASICS... THE COURSE
11.2.6 Strings
Key terms and concepts: ISO/ANSI defines characters, but not their appearance ? problem
characters are quotes and accents ? string constants ? define directive is a compiler directive
(global scope)
module characters; /* //1
" is ASCII 34 (hex 22), double quote. //2
' is ASCII 39 (hex 27), tick or apostrophe. //3
/ is ASCII 47 (hex 2F), forward slash. //4
\ is ASCII 92 (hex 5C), back slash. //5
` is ASCII 96 (hex 60), accent grave. //6
| is ASCII 124 (hex 7C), vertical bar. //7
There are no standards for the graphic symbols for codes above 128.//8
′ is 171 (hex AB), accent acute in almost all fonts. //9
“ is 210 (hex D2), open double quote, like 66 (in some fonts). //10
” is 211 (hex D3), close double quote, like 99 (in some fonts). //11
‘ is 212 (hex D4), open single quote, like 6 (in some fonts). //12
’ is 213 (hex D5), close single quote, like 9 (in some fonts). //13
*/ endmodule //14
module text; //1
parameter A_String = "abc"; // string constant, must be on one line//2
parameter Say = "Say \"Hey!\""; //3
// use escape quote \" for an embedded quote //4
parameter Tab = "\t"; // tab character //5
parameter NewLine = "\n"; // newline character //6
parameter BackSlash = "\\"; // back slash //7
parameter Tick = "\047"; // ASCII code for tick in octal //8
// parameter Illegal = "\500"; // illegal - no such ASCII code //9
initial begin //10
$display("A_String(str) = %s ",A_String," (hex) = %h ",A_String); //11
$display("Say = %s ",Say," Say \"Hey!\""); //12
$display("NewLine(str) = %s ",NewLine," (hex) = %h ",NewLine); //13
$display("\\(str) = %s ",BackSlash," (hex) = %h ",BackSlash); //14
$display("Tab(str) = %s ",Tab," (hex) = %h ",Tab,"1 newline..."); //15
$display("\n"); //16
$display("Tick(str) = %s ",Tick," (hex) = %h ",Tick); //17
#1.23; $display("Time is %t", $time); //18
ASICs... THE COURSE 11.3 Operators 9
end //19
endmodule //20
module define; //1
`define G_BUSWIDTH 32 // Bus width parameter (G_ for global). //2
/* Note: there is no semicolon at end of a compiler directive. The
character ` is ASCII 96 (hex 60), accent grave, it slopes down from
left to right. It is not the tick or apostrophe character ' (ASCII 39
or hex 27)*/ //3
wire [`G_BUSWIDTH:0]MyBus; // A 32-bit bus. //4
endmodule //5
11.3 Operators
Key terms and concepts: three types of operators: unary, binary, or a single ternary operator ?
similar to C programming language (but no ++ or --)
module operators; //1
parameter A10xz = {1'b1,1'b0,1'bx,1'bz}; // Concatenation and //2
parameter A01010101 = {4{2'b01}}; // replication, illegal for real.//3
// Arithmetic operators: +, -, *, /, and modulus % //4
parameter A1 = (3+2) %2; // The sign of a % b is the same as sign of
a. //5
// Logical shift operators: << (left), >> (right) //6
parameter A2 = 4 >> 1; parameter A4 = 1 << 2; // Note: zero fill. //7
Verilog unary operators
Opera-
tor Name Examples
! logical negation !123 is 'b0 [0, 1, or x for ambiguous; legal for real]
~ bitwise unary negation ~1'b10xz is 1'b01xx
& unary reduction and & 4'b1111 is 1'b1, & 2'bx1 is 1'bx, & 2'bz1 is 1'bx
~& unary reduction nand ~& 4'b1111 is 1'b0, ~& 2'bx1 is 1'bx
| unary reduction or Note:
~| unary reduction nor Reduction is performed left (first bit) to right
^ unary reduction xor Beware of the non-associative reduction operators
~^ ^~ unary reduction xnor z is treated as x for all unary operators
+ unary plus +2'bxz is +2'bxz [+m is the same as m; legal for real]
- unary minus -2'bxz is x [-m is unary minus m; legal for real]
10 SECTION 11 VERILOG HDL ASICS... THE COURSE
// Relational operators: <, <=, >, >= //8
initial if (1 > 2) $stop; //9
// Logical operators: ! (negation), && (and), || (or) //10
parameter B0 = !12; parameter B1 = 1 && 2; //11
reg [2:0] A00x; initial begin A00x = 'b111; A00x = !2'bx1; end //12
parameter C1 = 1 || (1/0); /* This may or may not cause an //13
error: the short-circuit behavior of && and || is undefined. An //14
evaluation including && or || may stop when an expression is known//15
to be true or false. */ //16
// == (logical equality), != (logical inequality) //17
parameter Ax = (1==1'bx); parameter Bx = (1'bx!=1'bz); //18
parameter D0 = (1==0); parameter D1 = (1==1); //19
// === case equality, !== (case inequality) //20
// The case operators only return true (1) or false (0). //21
parameter E0 = (1===1'bx); parameter E1 = 4'b01xz === 4'b01xz; //22
parameter F1 = (4'bxxxx === 4'bxxxx); //23
// Bitwise logical operators: //24
// ~ (negation), & (and), | (inclusive or), //25
// ^ (exclusive or), ~^ or ^~ (equivalence) //26
parameter A00 = 2'b01 & 2'b10; //27
// Unary logical reduction operators: //28
// & (and), ~& (nand), | (or), ~| (nor), //29
// ^ (xor), ~^ or ^~ (xnor) //30
parameter G1= & 4'b1111; //31
// Conditional expression f = a ? b : c [if (a) then f=b else f=c]//32
Verilog operators (in increasing order of precedence)
?: (conditional) [legal for real; associates right to left (others associate left to right)]
|| (logical or) [A smaller operand is zero-filled from its msb (0-fill); legal for real]
&& (logical and)[0-fill, legal for real]
| (bitwise or) ~| (bitwise nor) [0-fill]
^ (bitwise xor) ^~ ~^ (bitwise xnor, equivalence) [0-fill]
& (bitwise and) ~& (bitwise nand) [0-fill]
== (logical) != (logical) === (case) !== (case) [0-fill, logical versions are legal for real]
< (lt) <= (lt or equal) > (gt) >= (gt or equal) [0-fill, all arelegal for real]
<< (shift left) >> (shift right) [zero fill; no -ve shifts; shift by x or z results in unknown]
+ (addition) - (subtraction) [if any bit is x or z for + - * / % then entire result is unknown]
* (multiply) / (divide) % (modulus) [integer divide truncates fraction; + - * / legal for real]
Unary operators: ! ~ & ~& | ~| ^ ~^ ^~ + -
ASICs... THE COURSE 11.3 Operators 11
// if a=(x or z), then (bitwise) f=0 if b=c=0, f=1 if b=c=1, else f=x//33
reg H0, a, b, c; initial begin a=1; b=0; c=1; H0=a?b:c; end //34
reg[2:0] J01x, Jxxx, J01z, J011; //35
initial begin Jxxx = 3'bxxx; J01z = 3'b01z; J011 = 3'b011; //36
J01x = Jxxx ? J01z : J011; end // A bitwise result. //37
initial begin #1; //38
$display("A10xz=%b",A10xz," A01010101=%b",A01010101); //39
$display("A1=%0d",A1," A2=%0d",A2," A4=%0d",A4); //40
$display("B1=%b",B1," B0=%b",B0," A00x=%b",A00x); //41
$display("C1=%b",C1," Ax=%b",Ax," Bx=%b",Bx); //42
$display("D0=%b",D0," D1=%b",D1); //43
$display("E0=%b",E0," E1=%b",E1," F1=%b",F1); //44
$display("A00=%b",A00," G1=%b",G1," H0=%b",H0); //45
$display("J01x=%b",J01x); end //46
endmodule //47
11.3.1 Arithmetic
Key terms and concepts: arithmetic on n-bit objects is performed modulo 2n ? arithmetic on
vectors (regor wire) are predefined ? once Verilog “loses” a sign, it cannot get it back
module modulo; reg [2:0] Seven; //1
initial begin //2
#1 Seven = 7; #1 $display("Before=", Seven); //3
#1 Seven = Seven + 1; #1 $display("After =", Seven); //4
end //5
endmodule //6
module LRM_arithmetic; //1
integer IA, IB, IC, ID, IE; reg [15:0] RA, RB, RC; //2
initial begin //3
IA = -4'd12; RA = IA / 3; // reg is treated as unsigned. //4
RB = -4'd12; IB = RB / 3; // //5
IC = -4'd12 / 3; RC = -12 / 3; // real is treated as signed //6
ID = -12 / 3; IE = IA / 3; // (two’s complement). //7
end //8
initial begin #1; //9
$display(" hex default"); //10
$display("IA = -4'd12 = %h%d",IA,IA); //11
$display("RA = IA / 3 = %h %d",RA,RA); //12
$display("RB = -4'd12 = %h %d",RB,RB); //13
$display("IB = RB / 3 = %h%d",IB,IB); //14
$display("IC = -4'd12 / 3 = %h%d",IC,IC); //15
12 SECTION 11 VERILOG HDL ASICS... THE COURSE
$display("RC = -12 / 3 = %h %d",RC,RC); //16
$display("ID = -12 / 3 = %h%d",ID,ID); //17
$display("IE = IA / 3 = %h%d",IE,IE); //18
end //19
endmodule //20
hex default
IA = -4'd12 = fffffff4 -12
RA = IA / 3 = fffc 65532
RB = -4'd12 = fff4 65524
IB = RB / 3 = 00005551 21841
IC = -4'd12 / 3 = 55555551 1431655761
RC = -12 / 3 = fffc 65532
ID = -12 / 3 = fffffffc -4
IE = IA / 3 = fffffffc -4
11.4 Hierarchy
Key terms and concepts: module ? the module interface interconnects two Verilog modules
using ports ? ports must be explicitly declared as input, output, or inout ? a reg cannot be
inputor inoutport (to connection of a regto another reg) ? instantiation ? ports are linked
using named association or positional association ? hierarchical name ( m1.weekend) ?
The compiler will first search downward (or inward) then upward (outward)
module holiday_1(sat, sun, weekend); //1
input sat, sun; output weekend; //2
assign weekend = sat | sun; //3
endmodule //4
`timescale 100s/1s // Units are 100 seconds with precision of 1s. //1
module life; wire [3:0] n; integer days; //2
wire wake_7am, wake_8am; // Wake at 7 on weekdays else at 8. //3
assign n = 1 + (days % 7); // n is day of the week (1-7) //4
always@(wake_8am or wake_7am) //5
Verilog ports.
Verilog port input output inout
Characteris-
tics
wire (or other
net)
reg or wire (or other net)
We can read an output port inside a
module
wire (or other
net)
ASICs... THE COURSE 11.5 Procedures and Assignments 13
$display("Day=",n," hours=%0d ",($time/36)%24," 8am = ", //6
wake_8am," 7am = ",wake_7am," m2.weekday = ", m2.weekday); //7
initial days = 0; //8
initial begin #(24*36*10);$finish; end // Run for 10 days. //9
always #(24*36) days = days + 1; // Bump day every 24hrs. //10
rest m1(n, wake_8am); // Module instantiation. //11
// Creates a copy of module rest with instance name m1, //12
// ports are linked using positional notation. //13
work m2(.weekday(wake_7am), .day(n)); //14
// Creates a copy of module work with instance name m2, //15
// Ports are linked using named association. //16
endmodule //17
module rest(day, weekend); // Module definition. //1
// Notice the port names are different from the parent. //2
input [3:0] day; output weekend; reg weekend; //3
always begin #36 weekend = day > 5; end // Need a delay here. //4
endmodule //5
module work(day, weekday); //1
input [3:0] day; output weekday; reg weekday; //2
always begin #36 weekday = day < 6; end // Need a delay here. //3
endmodule //4
11.5 Procedures and Assignments
Key terms and concepts: a procedure is an always or initial statement, a task, or a
function) ? statements within a sequential block (between a begin and an end) that is part
of a procedure execute sequentially, but the procedure executes concurrently with other proce-
dures ? continuous assignments appear outside procedures ? procedural assignments
appear inside procedures
module holiday_1(sat, sun, weekend); //1
input sat, sun; output weekend; //2
assign weekend = sat | sun; // Assignment outside a procedure. //3
endmodule //4
module holiday_2(sat, sun, weekend); //1
input sat, sun; output weekend; reg weekend; //2
14 SECTION 11 VERILOG HDL ASICS... THE COURSE
always #1 weekend = sat | sun; // Assignment inside a procedure. //3
endmodule //4
module assignments //1
//... Continuous assignments go here. //2
always // beginning of a procedure //3
begin // beginning of sequential block //4
//... Procedural assignments go here. //5
end //6
endmodule //7
11.5.1 Continuous Assignment Statement
Key terms and concepts: a continuous assignment statement assigns to a wire like a real
logic gate drives a real wire,
module assignment_1(); //1
wire pwr_good, pwr_on, pwr_stable; reg Ok, Fire; //2
assign pwr_stable = Ok & (!Fire); //3
assign pwr_on = 1; //4
assign pwr_good = pwr_on & pwr_stable; //5
initial begin Ok = 0; Fire = 0; #1 Ok = 1; #5 Fire = 1; end //6
initial begin $monitor("TIME=%0d",$time," ON=",pwr_on, " STABLE=", //7
pwr_stable," OK=",Ok," FIRE=",Fire," GOOD=",pwr_good); //8
#10 $finish; end //9
endmodule //10
module assignment_2; reg Enable; wire [31:0] Data; //1
/* The following single statement is equivalent to a declaration and
continuous assignment. */ //2
wire [31:0] DataBus = Enable ? Data : 32'bz; //3
assign Data = 32'b10101101101011101111000010100001; //4
initial begin //5
$monitor("Enable=%b DataBus=%b ", Enable, DataBus); //6
Enable = 0; #1; Enable = 1; #1; end //7
endmodule //8
11.5.2 Sequential Block
Key terms and concepts: a sequential block is a group of statements between a begin and an
end ? to declare new variables within a sequential block we must name the block ? a sequential
block is a statement, so that we may nest sequential blocks ? a sequential block in an always
ASICs... THE COURSE 11.5 Procedures and Assignments 15
statement executes repeatedly ? an initial statement executes only once, so a sequential block
in an initial statement only executes once at the beginning of a simulation
module always_1; reg Y, Clk; //1
always // Statements in an always statement execute repeatedly: //2
begin: my_block // Start of sequential block. //3
@(posedge Clk) #5 Y = 1; // At +ve edge set Y=1, //4
@(posedge Clk) #5 Y = 0; // at the NEXT +ve edge set Y=0. //5
end // End of sequential block. //6
always #10 Clk = ~ Clk; // We need a clock. //7
initial Y = 0; // These initial statements execute //8
initial Clk = 0; // only once, but first. //9
initial $monitor("T=%2g",$time," Clk=",Clk," Y=",Y); //10
initial #70 $finish; //11
endmodule //12
11.5.3 Procedural Assignments
Key terms and concepts: the value of an expression on the RHS of an assignment within a
procedure (a procedural assignment) updates a reg (or memory element) immediately ? a reg
holds its value until changed by another procedural assignment ? a blocking assignment is one
type of procedural assignment
blocking_assignment ::= reg-lvalue = [delay_or_event_control]
expression
module procedural_assign; reg Y, A; //1
always @(A) //2
Y = A; // Procedural assignment. //3
initial begin A=0; #5; A=1; #5; A=0; #5; $finish; end //4
initial $monitor("T=%2g",$time,,"A=",A,,,"Y=",Y); //5
endmodule //6
T= 0 A=0 Y=0
T= 5 A=1 Y=1
T=10 A=0 Y=0
16 SECTION 11 VERILOG HDL ASICS... THE COURSE
11.6 Timing Controls and Delay
Key terms and concepts: statements in a sequential block are executed, in the absence of any
delay, at the same simulation time—the current time step ? delays are modeled using a timing
control
11.6.1 Timing Control
Key terms and concepts: a timing control is a delay control or an event control ? a delay control
delays an assignment by a specified amount of time ? timescale compiler directive is used to
specify the units of time and precision ? `timescale 1ns/10ps ? (s, ns, ps, or fs and the
multiplier must be 1, 10, or 100) ? intra-assignment delay ? delayed assignment ? an event
control delays an assignment until a specified event occurs ? posedge is a transition from '0'
to '1' or 'x', or a transition from 'x' to '1' (transitions to or from 'z' don’t count) ? events
can be declared (as named events), triggered, and detected
x = #1 y; // intra-assignment delay
#1 x = y; // delayed assignment
begin // Equivalent to intra-assignment delay.
hold = y; // Sample and hold y immediately.
#1; // Delay.
x = hold; // Assignment to x. Overall same as x = #1 y.
end
begin // Equivalent to delayed assignment.
#1; // Delay.
x = y; // Assign y to x. Overall same as #1 x = y.
end
event_control ::= @ event_identifier | @ (event_expression)
ASICs... THE COURSE 11.6 Timing Controls and Delay 17
event_expression ::= expression | event_identifier
| posedge expression | negedge expression
| event_expression or event_expression
module delay_controls; reg X, Y, Clk, Dummy; //1
always #1 Dummy=!Dummy; // Dummy clock, just for graphics. //2
// Examples of delay controls: //3
always begin #25 X=1;#10 X=0;#5; end //4
// An event control: //5
always @(posedge Clk) Y=X; // Wait for +ve clock edge. //6
always #10 Clk = !Clk; // The real clock. //7
initial begin Clk = 0; //8
$display("T Clk X Y"); //9
$monitor("%2g",$time,,,Clk,,,,X,,Y); //10
$dumpvars;#100 $finish; end //11
endmodule //12
module show_event; //1
reg clock; //2
event event_1, event_2; // Declare two named events. //3
always @(posedge clock) -> event_1; // Trigger event_1. //4
always @ event_1 //5
begin $display("Strike 1!!"); -> event_2; end // Trigger event_2. //6
always @ event_2 begin $display("Strike 2!!"); //7
$finish; end // Stop on detection of event_2. //8
always #10 clock = ~ clock; // We need a clock. //9
initial clock = 0; //10
endmodule //11
11.6.2 Data Slip
module data_slip_1 (); reg Clk, D, Q1, Q2; //1
/************* bad sequential logic below ***************/ //2
always @(posedge Clk) Q1 = D; //3
always @(posedge Clk) Q2 = Q1; // Data slips here! //4
/************* bad sequential logic above ***************/ //5
initial begin Clk = 0; D = 1; end always #50 Clk = ~Clk; //6
initial begin $display("t Clk D Q1 Q2"); //7
$monitor("%3g",$time,,Clk,,,,D,,Q1,,,Q2); end //8
initial #400 $finish; // Run for 8 cycles. //9
18 SECTION 11 VERILOG HDL ASICS... THE COURSE
initial $dumpvars; //10
endmodule //11
always @(posedge Clk) Q1 = #1 D; // The delays in the assignments //1
always @(posedge Clk) Q2 = #1 Q1; // fix the data slip. //2
11.6.3 Wait Statement
Key terms and concepts: wait statement suspends a procedure until a condition is true ? beware
“infinite hold” ? level-sensitive
wait (Done) $stop; // Wait until Done = 1 then stop.
module test_dff_wait; //1
reg D, Clock, Reset; dff_wait u1(D, Q, Clock, Reset); //2
initial begin D=1; Clock=0;Reset=1'b1; #15 Reset=1'b0; #20 D=0; end //3
always #10 Clock = !Clock; //4
initial begin $display("T Clk D Q Reset"); //5
$monitor("%2g",$time,,Clock,,,,D,,Q,,Reset); #50 $finish; end //6
endmodule //7
module dff_wait(D, Q, Clock, Reset); //1
output Q; input D, Clock, Reset; reg Q; wire D; //2
always @(posedge Clock) if (Reset !== 1) Q = D; //3
always begin wait (Reset == 1) Q = 0; wait (Reset !== 1); end //4
endmodule //5
module dff_wait(D,Q,Clock,Reset); //1
output Q; input D,Clock,Reset; reg Q; wire D; //2
always @(posedge Clock) if (Reset !== 1) Q = D; //3
// We need another wait statement here or we shall spin forever. //4
always begin wait (Reset == 1) Q = 0; end //5
endmodule //6
11.6.4 Blocking and Nonblocking Assignments
Key terms and concepts: a procedural assignment (blocking procedural assignment
statement) with a timing control delays or blocks execution ? nonblocking procedural
assignment statement allows execution to continue ? registers are updated at end of current
time step ? synthesis tools don’t allow blocking and nonblocking procedural assignments to the
same regwithin a sequential block
module delay; //1
reg a,b,c,d,e,f,g,bds,bsd; //2
initial begin //3
ASICs... THE COURSE 11.6 Timing Controls and Delay 19
a = 1; b = 0; // No delay control. //4
#1 b = 1; // Delayed assignment. //5
c = #1 1; // Intra-assignment delay. //6
#1; // Delay control. //7
d = 1; // //8
e <= #1 1; // Intra-assignment delay, nonblocking assignment //9
#1 f <= 1; // Delayed nonblocking assignment. //10
g <= 1; // Nonblocking assignment. //11
end //12
initial begin #1 bds = b; end // Delay then sample (ds). //13
initial begin bsd = #1 b; end // Sample then delay (sd). //14
initial begin $display("t a b c d e f g bds bsd"); //15
$monitor("%g",$time,,a,,b,,c,,d,,e,,f,,g,,bds,,,,bsd); end //16
endmodule //17
t a b c d e f g bds bsd
0 1 0 x x x x x x x
1 1 1 x x x x x 1 0
2 1 1 1 x x x x 1 0
3 1 1 1 1 x x x 1 0
4 1 1 1 1 1 1 1 1 0
20 SECTION 11 VERILOG HDL ASICS... THE COURSE
11.6.5 Procedural Continuous Assignment
Key terms and concepts: procedural continuous assignment statement (or quasicontinuous
assignment statement) is a special form assign within a sequential block
module dff_procedural_assign; //1
reg d,clr_,pre_,clk; wire q; dff_clr_pre dff_1(q,d,clr_,pre_,clk); //2
always #10 clk = ~clk; //3
initial begin clk = 0; clr_ = 1; pre_ = 1; d = 1; //4
#20; d = 0; #20; pre_ = 0; #20; pre_ = 1; #20; clr_ = 0; //5
#20; clr_ = 1; #20; d = 1; #20; $finish; end //6
initial begin //7
$display("T CLK PRE_ CLR_ D Q"); //8
$monitor("%3g",$time,,,clk,,,,pre_,,,,clr_,,,,d,,q); end //9
endmodule //10
module dff_clr_pre(q,d,clear_,preset_,clock); //1
output q; input d,clear_,preset_,clock; reg q; //2
always @(clear_ or preset_) //3
Verilog assignment statements.
Type of
Verilog
assignment
Continuous
assignment
statement
Procedural
assignment
statement
Nonblocking
procedural
assignment
statement
Procedural
continuous
assignment
statement
Where it can
occur
outside an
always or
initial state-
ment, task, or
function
inside an always
or initial state-
ment, task, or
function
inside an always
or initial state-
ment, task, or
function
always or
initial state-
ment, task, or
function
Example wire [31:0] DataBus;
assign DataBus =
Enable ? Data :
32'bz
reg Y;
always
@(posedge
clock) Y = 1;
reg Y;
always
Y <= 1;
always @(Enable)
if(Enable)
assign Q = D;
else deassign Q;
Valid LHS of
assignment
net register or mem-
ory element
register or mem-
ory element
net
Valid RHS of
assignment
<expression>
net, reg or mem-
ory element
<expression>
net, reg or mem-
ory element
<expression>
net, reg or mem-
ory element
<expression>
net, reg or mem-
ory element
Book 11.5.1 11.5.3 11.6.4 11.6.5
Verilog LRM 6.1 9.2 9.2.2 9.3
ASICs... THE COURSE 11.7 Tasks and Functions 21
if (!clear_) assign q = 0; // active-low clear //4
else if(!preset_) assign q = 1; // active-low preset //5
else deassign q; //6
always @(posedge clock) q = d; //7
endmodule //8
module all_assignments //1
//... continuous assignments. //2
always // beginning of procedure //3
begin // beginning of sequential block //4
//... blocking procedural assignments. //5
//... nonblocking procedural assignments. //6
//... procedural continuous assignments. //7
end //8
endmodule //9
11.7 Tasks and Functions
Key terms and concepts: a task is a procedure called from another procedure ? a task may call
other tasks and functions ? a function is a procedure used in an expression ? a function may not
call a task ? tasks may contain timing controls but functions may not
Call_A_Task_And_Wait (Input1, Input2, Output);
Result_Immediate = Call_A_Function (All_Inputs);
module F_subset_decode; reg [2:0]A, B, C, D, E, F; //1
initial begin A = 1; B = 0; D = 2; E = 3; //2
C = subset_decode(A, B); F = subset_decode(D,E); //3
$display("A B C D E F"); $display(A,,B,,C,,D,,E,,F); end //4
function [2:0] subset_decode; input [2:0] a, b; //5
begin if (a <= b) subset_decode = a; else subset_decode = b; end //6
endfunction //7
endmodule //8
11.8 Control Statements
Key terms and concepts: if, case, loop, disable, fork, and join statements control
execution
22 SECTION 11 VERILOG HDL ASICS... THE COURSE
11.8.1 Case and If Statement
Key terms and concepts: an if statement represents a two-way branch ? a case statement
represents a multiway branch ? a controlling expression is matched with case expressions
in each of the case items (or arms) to determine a match ? the casestatement must be inside
a sequential block (inside an alwaysstatement) and needs some delay ? a casex statement
handles both 'z' and 'x'as don’t care ? the casez statement handles only 'z'bits as don’t
care ? bits in case expressions may be set to '?'representing don’t care values
if(switch) Y = 1; else Y = 0;
module test_mux; reg a, b, select; wire out; //1
mux mux_1(a, b, out, select); //2
initial begin #2; select = 0; a = 0; b = 1; //3
#2; select = 1'bx; #2; select = 1'bz; #2; select = 1; end //4
initial $monitor("T=%2g",$time," Select=",select," Out=",out); //5
initial #10 $finish; //6
endmodule //7
module mux(a, b, mux_output, mux_select); input a, b, mux_select; //1
output mux_output; reg mux_output; //2
always begin //3
case(mux_select) //4
0: mux_output = a; //5
1: mux_output = b; //6
default mux_output = 1'bx; // If select = x or z set output to
x. //7
endcase //8
#1; // Need some delay, otherwise we'll spin forever. //9
end //10
endmodule //11
casex (instruction_register[31:29])
3b'??1 : add;
3b'?1? : subtract;
3b'1?? : branch;
endcase
ASICs... THE COURSE 11.8 Control Statements 23
11.8.2 Loop Statement
Key terms and concepts: A loop statement is a for, while, repeat, or forever statement ?
module loop_1; //1
integer i; reg [31:0] DataBus; initial DataBus = 0; //2
initial begin //3
/************** Insert loop code after here. ******************/
/* for(Execute this assignment once before starting loop; exit loop
if this expression is false; execute this assignment at end of loop
before the check for end of loop.) */
for(i = 0; i <= 15; i = i+1) DataBus[i] = 1; //4
/*************** Insert loop code before here. ****************/
end //5
initial begin //6
$display("DataBus = %b",DataBus); //7
#2; $display("DataBus = %b",DataBus); $finish; //8
end //9
endmodule //10
i = 0;
/* while(Execute next statement while this expression is true.) */
while(i <= 15) begin DataBus[i] = 1; i = i+1; end
i = 0;
/* repeat(Execute next statement the number of times corresponding to
the evaluation of this expression at the beginning of the loop.) */
repeat(16) begin DataBus[i] = 1; i = i+1; end
i = 0;
/* A forever statement loops continuously. */
forever begin : my_loop
DataBus[i] = 1;
if (i == 15) #1 disable my_loop; // Need to let time advance to
exit.
i = i+1;
end
24 SECTION 11 VERILOG HDL ASICS... THE COURSE
11.8.3 Disable
Key terms and concepts: The disable statement stops the execution of a labeled sequential
block and skips to the end of the block ? difficult to implement in hardware
forever
begin: microprocessor_block // Labeled sequential block.
@(posedge clock)
if (reset) disable microprocessor_block; // Skip to end of block.
else Execute_code;
end
11.8.4 Fork and Join
Key terms and concepts: The fork statement and join statement allows the execution of two or
more parallel threads in a parallel block ? difficult to implement in hardware
module fork_1 //1
event eat_breakfast, read_paper; //2
initial begin //3
fork //4
@eat_breakfast; @read_paper; //5
join //6
end //7
endmodule //8
11.9 Logic-Gate Modeling
Key terms and concepts: Verilog has a set of built-in logic models and you may also define
your own models.
ASICs... THE COURSE 11.9 Logic-Gate Modeling 25
11.9.1 Built-in Logic Models
Key terms and concepts: primitives: and, nand, nor, or, xor, xnor ? strong drive
strength is the default ? the first port of a primitive gate is always the output port ? remaining
ports are the input ports
module primitive; //1
nand (strong0, strong1) #2.2 //2
Nand_1(n001, n004, n005), //3
Nand_2(n003, n001, n005, n002); //4
nand (n006, n005, n002); //5
endmodule //6
11.9.2 User-Defined Primitives
Key terms and concepts: a user-defined primitive (UDP) uses a truth-table specification ? the
first port of a UDP must be an output port (no vector or inout ports) ? inputs in a UDP truth
table are '0', '1', and 'x' ? any 'z' input is treated as an 'x' ? default output is 'x' ? any
next state goes between an input and an output in UDP table ? shorthand notation for levels ?
(ab) represents a change from a to b ? (01) represents a rising edge ? shorthand notations for
edges
primitive Adder(Sum, InA, InB); //1
output Sum; input Ina, InB; //2
table //3
// inputs : output //4
00 : 0; //5
01 : 1; //6
10 : 1; //7
11 : 0; //8
Definition of the Verilog primitive 'and' gate
'and' 0 1 x z
0 0 0 0 0
1 0 1 x x
x 0 x x x
z 0 x x x
26 SECTION 11 VERILOG HDL ASICS... THE COURSE
endtable //9
endprimitive //10
primitive DLatch(Q, Clock, Data); //1
output Q; reg Q; input Clock, Data; //2
table //3
//inputs : present state : output (next state) //4
1 0 : ? : 0; // ? represents 0,1, or x (input or present state). //5
1 1 : b : 1; // b represents 0 or 1 (input or present state). //6
1 1 : x : 1; // Could have combined this with previous line. //7
0 ? : ? : -; // - represents no change in an output. //8
endtable //9
endprimitive //10
primitive DFlipFlop(Q, Clock, Data); //1
output Q; reg Q; input Clock, Data; //2
table //3
//inputs : present state : output (next state) //4
r 0 : ? : 0 ; // rising edge, next state = output = 0 //5
r 1 : ? : 1 ; // rising edge, next state = output = 1 //6
(0x) 0 : 0 : 0 ; // rising edge, next state = output = 0 //7
(0x) 1 : 1 : 1 ; // rising edge, next state = output = 1 //8
(?0) ? : ? : - ; // falling edge, no change in output //9
? (??) : ? : - ; // no clock edge, no change in output //10
endtable //11
endprimitive //12
11.10 Modeling Delay
Key terms and concepts: built-in delays ? ASIC cell library models include logic delays as a
function of fanout and estimated wiring loads ? after layout, we can back-annotate ? delay calcu-
lator calculates the net delays in Standard Delay Format, SDF ? sign-off quality ASIC cell
libraries
11.10.1 Net and Gate Delay
Key terms and concepts: minimum, typical, and maximum delays ? first triplet specifies the
min/typ/max rising delay ('0' or 'x' or 'z' to '1') and the second triplet specifies the falling
ASICs... THE COURSE 11.10 Modeling Delay 27
delay (to '0') ? for a high-impedance output, we specify a triplet for rising, falling, and the delay
to transition to 'z' (from '0' or '1'), the delay for a three-state driver to turn off or float
#(1.1:1.3:1.7) assign delay_a = a; // min:typ:max
wire #(1.1:1.3:1.7) a_delay; // min:typ:max
wire #(1.1:1.3:1.7) a_delay = a; // min:typ:max
nand #3.0 nd01(c, a, b);
nand #(2.6:3.0:3.4) nd02(d, a, b); // min:typ:max
nand #(2.8:3.2:3.4, 2.6:2.8:2.9) nd03(e, a, b);
// #(rising, falling) delay
wire #(0.5,0.6,0.7) a_z = a; // rise/fall/float delays
11.10.2 Pin-to-Pin Delay
Key terms and concepts: A specify block allows pin-to-pin delays across a module ? x => y
specifies a parallel connection (or parallel path) ? x and y must have the same number of bits
? x *> y specifies a full connection (or full path) ? every bit in x is connected to y ? x and y may
be different sizes ? state-dependent path delay
module DFF_Spec; reg D, clk; //1
DFF_Part DFF1 (Q, clk, D, pre, clr); //2
initial begin D = 0; clk = 0; #1; clk = 1; end //3
initial $monitor("T=%2g", $time," clk=", clk," Q=", Q); //4
endmodule //5
module DFF_Part(Q, clk, D, pre, clr); //1
input clk, D, pre, clr; output Q; //2
DFlipFlop(Q, clk, D); // No preset or clear in this UDP. //3
specify //4
specparam //5
tPLH_clk_Q = 3, tPHL_clk_Q = 2.9, //6
tPLH_set_Q = 1.2, tPHL_set_Q = 1.1; //7
(clk => Q) = (tPLH_clk_Q, tPHL_clk_Q); //8
(pre, clr *> Q) = (tPLH_set_Q, tPHL_set_Q); //9
endspecify //10
endmodule //11
`timescale 1 ns / 100 fs //1
module M_Spec; reg A1, A2, B; M M1 (Z, A1, A2, B); //2
28 SECTION 11 VERILOG HDL ASICS... THE COURSE
initial begin A1=0;A2=1;B=1;#5;B=0;#5;A1=1;A2=0;B=1;#5;B=0; end //3
initial //4
$monitor("T=%4g",$realtime," A1=",A1," A2=",A2," B=",B," Z=",Z); //5
endmodule //6
`timescale 100 ps / 10 fs //1
module M(Z, A1, A2, B); input A1, A2, B; output Z; //2
or (Z1, A1, A2); nand (Z, Z1, B); // OAI21 //3
/*A1 A2 B Z Delay=10*100 ps unless indicated in the table below. //4
0 0 0 1 //5
0 0 1 1 //6
0 1 0 1 B:0->1 Z:1->0 delay=t2 //7
0 1 1 0 B:1->0 Z:0->1 delay=t1 //8
1 0 0 1 B:0->1 Z:1->0 delay=t4 //9
1 0 1 0 B:1->0 Z:0->1 delay=t3 //10
1 1 0 1 //11
1 1 1 0 */ //12
specify specparam t1 = 11, t2 = 12; specparam t3 = 13, t4 = 14; //13
(A1 => Z) = 10; (A2 => Z) = 10; //14
if (~A1) (B => Z) = (t1, t2); if (A1) (B => Z) = (t3, t4); //15
endspecify //16
endmodule //17
11.11 Altering Parameters
Key terms and concepts: parameter override in instantiated module ? parameters have local
scope ? defparam statement and hierarchical name
module Vector_And(Z, A, B); //1
parameter CARDINALITY = 1; //2
input [CARDINALITY-1:0] A, B; //3
output [CARDINALITY-1:0] Z; //4
wire [CARDINALITY-1:0] Z = A & B; //5
endmodule //6
module Four_And_Gates(OutBus, InBusA, InBusB); //1
input [3:0] InBusA, InBusB; output [3:0] OutBus; //2
Vector_And #(4) My_AND(OutBus, InBusA, InBusB); // 4 AND gates //3
endmodule //4
module And_Gates(OutBus, InBusA, InBusB); //1
parameter WIDTH = 1; //2
input [WIDTH-1:0] InBusA, InBusB; output [WIDTH-1:0] OutBus; //3
ASICs... THE COURSE 11.12 A Viterbi Decoder 29
Vector_And #(WIDTH) My_And(OutBus, InBusA, InBusB); //4
endmodule //5
module Super_Size; defparam And_Gates.WIDTH = 4; endmodule //1
11.12 A Viterbi Decoder
11.12.1 Viterbi Encoder
/******************************************************/
/* module viterbi_encode */
/******************************************************/
/* This is the encoder. X2N (msb) and X1N form the 2-bit input
message, XN. Example: if X2N=1, X1N=0, then XN=2. Y2N (msb), Y1N, and
Y0N form the 3-bit encoded signal, YN (for a total constellation of 8
PSK signals that will be transmitted). The encoder uses a state
machine with four states to generate the 3-bit output, YN, from the
2-bit input, XN. Example: the repeated input sequence XN = (X2N, X1N)
= 0, 1, 2, 3 produces the repeated output sequence YN = (Y2N, Y1N,
Y0N) = 1, 0, 5, 4. */
module viterbi_encode(X2N,X1N,Y2N,Y1N,Y0N,clk,res);
input X2N,X1N,clk,res; output Y2N,Y1N,Y0N;
wire X1N_1,X1N_2,Y2N,Y1N,Y0N;
dff dff_1(X1N,X1N_1,clk,res); dff dff_2(X1N_1,X1N_2,clk,res);
assign Y2N=X2N; assign Y1N=X1N ^ X1N_2; assign Y0N=X1N_1;
endmodule
11.12.2 The Received Signal
/******************************************************/
/* module viterbi_distances */
/******************************************************/
/* This module simulates the front end of a receiver. Normally the
received analog signal (with noise) is converted into a series of
distance measures from the known eight possible transmitted PSK
signals: s0,...,s7. We are not simulating the analog part or noise in
this version, so we just take the digitally encoded 3-bit signal, Y,
from the encoder and convert it directly to the distance measures.
d[N] is the distance from signal = N to signal = 0
d[N] = (2*sin(N*PI/8))**2 in 3-bit binary (on the scale 2=100)
Example: d[3] = 1.85**2 = 3.41 = 110
inN is the distance from signal = N to encoder signal.
30 SECTION 11 VERILOG HDL ASICS... THE COURSE
Example: in3 is the distance from signal = 3 to encoder signal.
d[N] is the distance from signal = N to encoder signal = 0.
If encoder signal = J, shift the distances by 8-J positions.
Example: if signal = 2, in0 is d[6], in1 is D[7], in2 is D[0], etc.
*/
module viterbi_distances
(Y2N,Y1N,Y0N,clk,res,in0,in1,in2,in3,in4,in5,in6,in7);
input clk,res,Y2N,Y1N,Y0N; output in0,in1,in2,in3,in4,in5,in6,in7;
reg [2:0] J,in0,in1,in2,in3,in4,in5,in6,in7; reg [2:0] d [7:0];
initial begin d[0]='b000;d[1]='b001;d[2]='b100;d[3]='b110;
d[4]='b111;d[5]='b110;d[6]='b100;d[7]='b001; end
always @(Y2N or Y1N or Y0N) begin
J[0]=Y0N;J[1]=Y1N;J[2]=Y2N;
J=8-J;in0=d[J];J=J+1;in1=d[J];J=J+1;in2=d[J];J=J+1;in3=d[J];
J=J+1;in4=d[J];J=J+1;in5=d[J];J=J+1;in6=d[J];J=J+1;in7=d[J];
end endmodule
11.12.3 Testing the System
/*****************************************************/
/* module viterbi_test_CDD */
/*****************************************************/
/* This is the top-level module, viterbi_test_CDD, that models the
communications link. It contains three modules: viterbi_encode,
viterbi_distances, and viterbi. There is no analog and no noise in
this version. The 2-bit message, X, is encoded to a 3-bit signal, Y.
In this module the message X is generated using a simple counter.
The digital 3-bit signal Y is transmitted, received with noise as an
analog signal (not modeled here), and converted to a set of eight
3-bit distance measures, in0, ..., in7. The distance measures form
the input to the Viterbi decoder that reconstructs the transmitted
signal Y, with an error signal if the measures are inconsistent.
CDD = counter input, digital transmission, digital reception */
module viterbi_test_CDD;
wire Error; // decoder out
wire [2:0] Y, Out; // encoder out, decoder out
reg [1:0] X; // encoder inputs
reg Clk, Res; // clock and reset
wire [2:0] in0,in1,in2,in3,in4,in5,in6,in7;
always #500 $display("t Clk X Y Out Error");
initial $monitor("%4g",$time,,Clk,,,,X,,Y,,Out,,,,Error);
initial $dumpvars; initial #3000 $finish;
always #50 Clk = ~Clk; initial begin Clk = 0;
ASICs... THE COURSE 11.12 A Viterbi Decoder 31
X = 3; // No special reason to start at 3.
#60 Res = 1;#10 Res = 0;end // Hit reset after inputs are stable.
always @(posedge Clk) #1 X = X + 1; // Drive the input with a
counter.
viterbi_encode v_1
(X[1],X[0],Y[2],Y[1],Y[0],Clk,Res);
viterbi_distances v_2
(Y[2],Y[1],Y[0],Clk,Res,in0,in1,in2,in3,in4,in5,in6,in7);
viterbi v_3
(in0,in1,in2,in3,in4,in5,in6,in7,Out,Clk,Res,Error);
endmodule
11.12.4 Verilog Decoder Model
/******************************************************/
/* module dff */
/******************************************************/
/* A D flip-flop module. */
module dff(D,Q,Clock,Reset); // N.B. reset is active-low.
output Q; input D,Clock,Reset;
parameter CARDINALITY = 1; reg [CARDINALITY-1:0] Q;
wire [CARDINALITY-1:0] D;
always @(posedge Clock) if (Reset !== 0) #1 Q = D;
always begin wait (Reset == 0); Q = 0; wait (Reset == 1); end
endmodule
/* Verilog code for a Viterbi decoder. The decoder assumes a rate
2/3 encoder, 8 PSK modulation, and trellis coding. The viterbi module
contains eight submodules: subset_decode, metric, compute_metric,
compare_select, reduce, pathin, path_memory, and output_decision.
The decoder accepts eight 3-bit measures of ||r-si||**2 and, after
an initial delay of thirteen clock cycles, the output is the best
estimate of the signal transmitted. The distance measures are the
Euclidean distances between the received signal r (with noise) and
each of the (in this case eight) possible transmitted signals s0 to
s7.
Original by Christeen Gray, University of Hawaii. Heavily modified
by MJSS; any errors are mine. Use freely. */
/******************************************************/
/* module viterbi */
32 SECTION 11 VERILOG HDL ASICS... THE COURSE
/******************************************************/
/* This is the top level of the Viterbi decoder. The eight input
signals {in0,...,in7} represent the distance measures, ||r-si||**2.
The other input signals are clk and reset. The output signals are
out and error. */
module viterbi
(in0,in1,in2,in3,in4,in5,in6,in7,
out,clk,reset,error);
input [2:0] in0,in1,in2,in3,in4,in5,in6,in7;
output [2:0] out; input clk,reset; output error;
wire sout0,sout1,sout2,sout3;
wire [2:0] s0,s1,s2,s3;
wire [4:0] m_in0,m_in1,m_in2,m_in3;
wire [4:0] m_out0,m_out1,m_out2,m_out3;
wire [4:0] p0_0,p2_0,p0_1,p2_1,p1_2,p3_2,p1_3,p3_3;
wire ACS0,ACS1,ACS2,ACS3;
wire [4:0] out0,out1,out2,out3;
wire [1:0] control;
wire [2:0] p0,p1,p2,p3;
wire [11:0] path0;
subset_decode u1(in0,in1,in2,in3,in4,in5,in6,in7,
s0,s1,s2,s3,sout0,sout1,sout2,sout3,clk,reset);
metric u2(m_in0,m_in1,m_in2,m_in3,m_out0,
m_out1,m_out2,m_out3,clk,reset);
compute_metric u3(m_out0,m_out1,m_out2,m_out3,s0,s1,s2,s3,
p0_0,p2_0,p0_1,p2_1,p1_2,p3_2,p1_3,p3_3,error);
compare_select u4(p0_0,p2_0,p0_1,p2_1,p1_2,p3_2,p1_3,p3_3,
out0,out1,out2,out3,ACS0,ACS1,ACS2,ACS3);
reduce u5(out0,out1,out2,out3,
m_in0,m_in1,m_in2,m_in3,control);
pathin u6(sout0,sout1,sout2,sout3,
ACS0,ACS1,ACS2,ACS3,path0,clk,reset);
path_memory u7(p0,p1,p2,p3,path0,clk,reset,
ACS0,ACS1,ACS2,ACS3);
output_decision u8(p0,p1,p2,p3,control,out);
endmodule
/******************************************************/
/* module subset_decode */
/******************************************************/
ASICs... THE COURSE 11.12 A Viterbi Decoder 33
/* This module chooses the signal corresponding to the smallest of
each set {||r-s0||**2,||r-s4||**2}, {||r-s1||**2, ||r-s5||**2},
{||r-s2||**2,||r-s6||**2}, {||r-s3||**2,||r-s7||**2}. Therefore
there are eight input signals and four output signals for the
distance measures. The signals sout0, ..., sout3 are used to control
the path memory. The statement dff #(3) instantiates a vector array
of 3 D flip-flops. */
module subset_decode
(in0,in1,in2,in3,in4,in5,in6,in7,
s0,s1,s2,s3,
sout0,sout1,sout2,sout3,
clk,reset);
input [2:0] in0,in1,in2,in3,in4,in5,in6,in7;
output [2:0] s0,s1,s2,s3;
output sout0,sout1,sout2,sout3;
input clk,reset;
wire [2:0] sub0,sub1,sub2,sub3,sub4,sub5,sub6,sub7;
dff #(3) subout0(in0, sub0, clk, reset);
dff #(3) subout1(in1, sub1, clk, reset);
dff #(3) subout2(in2, sub2, clk, reset);
dff #(3) subout3(in3, sub3, clk, reset);
dff #(3) subout4(in4, sub4, clk, reset);
dff #(3) subout5(in5, sub5, clk, reset);
dff #(3) subout6(in6, sub6, clk, reset);
dff #(3) subout7(in7, sub7, clk, reset);
function [2:0] subset_decode; input [2:0] a,b;
begin
subset_decode = 0;
if (a<=b) subset_decode = a; else subset_decode = b;
end
endfunction
function set_control; input [2:0] a,b;
begin
if (a<=b) set_control = 0; else set_control = 1;
end
endfunction
assign s0 = subset_decode (sub0,sub4);
34 SECTION 11 VERILOG HDL ASICS... THE COURSE
assign s1 = subset_decode (sub1,sub5);
assign s2 = subset_decode (sub2,sub6);
assign s3 = subset_decode (sub3,sub7);
assign sout0 = set_control(sub0,sub4);
assign sout1 = set_control(sub1,sub5);
assign sout2 = set_control(sub2,sub6);
assign sout3 = set_control(sub3,sub7);
endmodule
/******************************************************/
/* module compute_metric */
/******************************************************/
/* This module computes the sum of path memory and the distance for
each path entering a state of the trellis. For the four states,
there are two paths entering it; therefore eight sums are computed
in this module. The path metrics and output sums are 5 bits wide.
The output sum is bounded and should never be greater than 5 bits
for a valid input signal. The overflow from the sum is the error
output and indicates an invalid input signal.*/
module compute_metric
(m_out0,m_out1,m_out2,m_out3,
s0,s1,s2,s3,p0_0,p2_0,
p0_1,p2_1,p1_2,p3_2,p1_3,p3_3,
error);
input [4:0] m_out0,m_out1,m_out2,m_out3;
input [2:0] s0,s1,s2,s3;
output [4:0] p0_0,p2_0,p0_1,p2_1,p1_2,p3_2,p1_3,p3_3;
output error;
assign
p0_0 = m_out0 + s0,
p2_0 = m_out2 + s2,
p0_1 = m_out0 + s2,
p2_1 = m_out2 + s0,
p1_2 = m_out1 + s1,
p3_2 = m_out3 + s3,
p1_3 = m_out1 + s3,
p3_3 = m_out3 + s1;
function is_error; input x1,x2,x3,x4,x5,x6,x7,x8;
begin
if (x1||x2||x3||x4||x5||x6||x7||x8) is_error = 1;
ASICs... THE COURSE 11.12 A Viterbi Decoder 35
else is_error = 0;
end
endfunction
assign error = is_error(p0_0[4],p2_0[4],p0_1[4],p2_1[4],
p1_2[4],p3_2[4],p1_3[4],p3_3[4]);
endmodule
/******************************************************/
/* module compare_select */
/******************************************************/
/* This module compares the summations from the compute_metric
module and selects the metric and path with the lowest value. The
output of this module is saved as the new path metric for each
state. The ACS output signals are used to control the path memory of
the decoder. */
module compare_select
(p0_0,p2_0,p0_1,p2_1,p1_2,p3_2,p1_3,p3_3,
out0,out1,out2,out3,
ACS0,ACS1,ACS2,ACS3);
input [4:0] p0_0,p2_0,p0_1,p2_1,p1_2,p3_2,p1_3,p3_3;
output [4:0] out0,out1,out2,out3;
output ACS0,ACS1,ACS2,ACS3;
function [4:0] find_min_metric; input [4:0] a,b;
begin
if (a <= b) find_min_metric = a; else find_min_metric = b;
end
endfunction
function set_control; input [4:0] a,b;
begin
if (a <= b) set_control = 0; else set_control = 1;
end
endfunction
assign out0 = find_min_metric(p0_0,p2_0);
assign out1 = find_min_metric(p0_1,p2_1);
assign out2 = find_min_metric(p1_2,p3_2);
assign out3 = find_min_metric(p1_3,p3_3);
36 SECTION 11 VERILOG HDL ASICS... THE COURSE
assign ACS0 = set_control (p0_0,p2_0);
assign ACS1 = set_control (p0_1,p2_1);
assign ACS2 = set_control (p1_2,p3_2);
assign ACS3 = set_control (p1_3,p3_3);
endmodule
/******************************************************/
/* module path */
/******************************************************/
/* This is the basic unit for the path memory of the Viterbi
decoder. It consists of four 3-bit D flip-flops in parallel. There
is a 2:1 mux at each D flip-flop input. The statement dff #(12)
instantiates a vector array of 12 flip-flops. */
module path(in,out,clk,reset,ACS0,ACS1,ACS2,ACS3);
input [11:0] in; output [11:0] out;
input clk,reset,ACS0,ACS1,ACS2,ACS3; wire [11:0] p_in;
dff #(12) path0(p_in,out,clk,reset);
function [2:0] shift_path; input [2:0] a,b; input control;
begin
if (control == 0) shift_path = a; else shift_path = b;
end
endfunction
assign p_in[11:9] = shift_path(in[11:9],in[5:3],ACS0);
assign p_in[ 8:6] = shift_path(in[11:9],in[5:3],ACS1);
assign p_in[ 5:3] = shift_path(in[8: 6],in[2:0],ACS2);
assign p_in[ 2:0] = shift_path(in[8: 6],in[2:0],ACS3);
endmodule
/******************************************************/
/* module path_memory */
/******************************************************/
/* This module consists of an array of memory elements (D
flip-flops) that store and shift the path memory as new signals are
added to the four paths (or four most likely sequences of signals).
This module instantiates 11 instances of the path module. */
module path_memory
(p0,p1,p2,p3,
path0,clk,reset,
ACS0,ACS1,ACS2,ACS3);
output [2:0] p0,p1,p2,p3; input [11:0] path0;
ASICs... THE COURSE 11.12 A Viterbi Decoder 37
input clk,reset,ACS0,ACS1,ACS2,ACS3;
wire [11:0]out1,out2,out3,out4,out5,out6,out7,out8,out9,out10,out11;
path x1 (path0,out1 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x2 (out1, out2 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x3 (out2, out3 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x4 (out3, out4 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x5 (out4, out5 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x6 (out5, out6 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x7 (out6, out7 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x8 (out7, out8 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x9 (out8, out9 ,clk,reset,ACS0,ACS1,ACS2,ACS3),
x10(out9, out10,clk,reset,ACS0,ACS1,ACS2,ACS3),
x11(out10,out11,clk,reset,ACS0,ACS1,ACS2,ACS3);
assign p0 = out11[11:9];
assign p1 = out11[ 8:6];
assign p2 = out11[ 5:3];
assign p3 = out11[ 2:0];
endmodule
/******************************************************/
/* module pathin */
/******************************************************/
/* This module determines the input signal to the path for each of
the four paths. Control signals from the subset decoder and compare
select modules are used to store the correct signal. The statement
dff #(12) instantiates a vector array of 12 flip-flops. */
module pathin
(sout0,sout1,sout2,sout3,
ACS0,ACS1,ACS2,ACS3,
path0,clk,reset);
input sout0,sout1,sout2,sout3,ACS0,ACS1,ACS2,ACS3;
input clk,reset; output [11:0] path0;
wire [2:0] sig0,sig1,sig2,sig3; wire [11:0] path_in;
dff #(12) firstpath(path_in,path0,clk,reset);
function [2:0] subset0; input sout0;
begin
if(sout0 == 0) subset0 = 0; else subset0 = 4;
end
endfunction
38 SECTION 11 VERILOG HDL ASICS... THE COURSE
function [2:0] subset1; input sout1;
begin
if(sout1 == 0) subset1 = 1; else subset1 = 5;
end
endfunction
function [2:0] subset2; input sout2;
begin
if(sout2 == 0) subset2 = 2; else subset2 = 6;
end
endfunction
function [2:0] subset3; input sout3;
begin
if(sout3 == 0) subset3 = 3; else subset3 = 7;
end
endfunction
function [2:0] find_path; input [2:0] a,b; input control;
begin
if(control==0) find_path = a; else find_path = b;
end
endfunction
assign sig0 = subset0(sout0);
assign sig1 = subset1(sout1);
assign sig2 = subset2(sout2);
assign sig3 = subset3(sout3);
assign path_in[11:9] = find_path(sig0,sig2,ACS0);
assign path_in[ 8:6] = find_path(sig2,sig0,ACS1);
assign path_in[ 5:3] = find_path(sig1,sig3,ACS2);
assign path_in[ 2:0] = find_path(sig3,sig1,ACS3);
endmodule
/******************************************************/
/* module metric */
/******************************************************/
/* The registers created in this module (using D flip-flops) store
the four path metrics. Each register is 5 bits wide. The statement
dff #(5) instantiates a vector array of 5 flip-flops. */
ASICs... THE COURSE 11.12 A Viterbi Decoder 39
module metric
(m_in0,m_in1,m_in2,m_in3,
m_out0,m_out1,m_out2,m_out3,
clk,reset);
input [4:0] m_in0,m_in1,m_in2,m_in3;
output [4:0] m_out0,m_out1,m_out2,m_out3;
input clk,reset;
dff #(5) metric3(m_in3, m_out3, clk, reset);
dff #(5) metric2(m_in2, m_out2, clk, reset);
dff #(5) metric1(m_in1, m_out1, clk, reset);
dff #(5) metric0(m_in0, m_out0, clk, reset);
endmodule
/******************************************************/
/* module output_decision */
/******************************************************/
/* This module decides the output signal based on the path that
corresponds to the smallest metric. The control signal comes from
the reduce module. */
module output_decision(p0,p1,p2,p3,control,out);
input [2:0] p0,p1,p2,p3; input [1:0] control; output [2:0] out;
function [2:0] decide;
input [2:0] p0,p1,p2,p3; input [1:0] control;
begin
if(control == 0) decide = p0;
else if(control == 1) decide = p1;
else if(control == 2) decide = p2;
else decide = p3;
end
endfunction
assign out = decide(p0,p1,p2,p3,control);
endmodule
/******************************************************/
/* module reduce */
/******************************************************/
/* This module reduces the metrics after the addition and compare
operations. This algorithm selects the smallest metric and subtracts
it from all the other metrics. */
40 SECTION 11 VERILOG HDL ASICS... THE COURSE
module reduce
(in0,in1,in2,in3,
m_in0,m_in1,m_in2,m_in3,
control);
input [4:0] in0,in1,in2,in3;
output [4:0] m_in0,m_in1,m_in2,m_in3;
output [1:0] control; wire [4:0] smallest;
function [4:0] find_smallest;
input [4:0] in0,in1,in2,in3; reg [4:0] a,b;
begin
if(in0 <= in1) a = in0; else a = in1;
if(in2 <= in3) b = in2; else b = in3;
if(a <= b) find_smallest = a;
else find_smallest = b;
end
endfunction
function [1:0] smallest_no;
input [4:0] in0,in1,in2,in3,smallest;
begin
if(smallest == in0) smallest_no = 0;
else if (smallest == in1) smallest_no = 1;
else if (smallest == in2) smallest_no = 2;
else smallest_no = 3;
end
endfunction
assign smallest = find_smallest(in0,in1,in2,in3);
assign m_in0 = in0 - smallest;
assign m_in1 = in1 - smallest;
assign m_in2 = in2 - smallest;
assign m_in3 = in3 - smallest;
assign control = smallest_no(in0,in1,in2,in3,smallest);
endmodule
11.13 Other Verilog Features
Key terms and concepts: system tasks and functions are part of the IEEE standard
ASICs... THE COURSE 11.13 Other Verilog Features 41
11.13.1 Display Tasks
Key terms and concepts: display system tasks ? $display(format works like C) ? $write
? $strobe
module test_display; // display system tasks:
initial begin $display ("string, variables, or expression");
/* format specifications work like printf in C:
%d=decimal %b=binary %s=string %h=hex %o=octal
%c=character %m=hierarchical name %v=strength %t=time format
%e=scientific %f=decimal %g=shortest
examples: %d uses default width %0d uses minimum width
%7.3g uses 7 spaces with 3 digits after decimal point */
// $displayb, $displayh, $displayo print in b, h, o formats
// $write, $strobe, $monitor also have b, h, o versions
$write("write"); // as $display, but without newline at end of line
$strobe("strobe"); // as $display, values at end of simulation cycle
$monitor(v); // disp. @change of v (except v= $time,$stime,$realtime)
$monitoron; $monitoroff; // toggle monitor mode on/off
end endmodule
11.13.2 File I/O Tasks
Key terms and concepts: file I/O system tasks ? $fdisplay ? $fopen ? $fclose ?
multichannel descriptor ? 32 flags ? channel 0 is the standard output (screen) and is always
open ? $readmemb and $readmemh read a text file into a memory ? file may contain only spaces,
new lines, tabs, form feeds, comments, addresses, and binary ($readmemb) or hex
($readmemh)
module file_1; integer f1, ch; initial begin f1 = $fopen("f1.out");
if(f1==0) $stop(2); if(f1==2)$display("f1 open");
ch = f1|1; $fdisplay(ch,"Hello"); $fclose(f1); end endmodule
42 SECTION 11 VERILOG HDL ASICS... THE COURSE
> vlog file_1.v
> vsim -c file_1
# Loading work.file_1
VSIM 1> run 10
# f1 open
# Hello
VSIM 2> q
> more f1.out
Hello
>
mem.dat
@2 1010_1111 @4 0101_1111 1010_1111 // @address in hex
x1x1_zzzz 1111_0000 /* x or z is OK */
module load; reg [7:0] mem[0:7]; integer i; initial begin
$readmemb("mem.dat", mem, 1, 6); // start_address=1, end_address=6
for (i= 0; i<8; i=i+1) $display("mem[%0d] %b", i, mem[i]);
end endmodule
> vsim -c load
# Loading work.load
VSIM 1> run 10
# ** Warning: $readmem (memory mem) file mem.dat line 2:
# More patterns than index range (hex 1:6)
# Time: 0 ns Iteration: 0 Instance:/
# mem[0] xxxxxxxx
# mem[1] xxxxxxxx
# mem[2] 10101111
# mem[3] xxxxxxxx
# mem[4] 01011111
# mem[5] 10101111
# mem[6] x1x1zzzz
# mem[7] xxxxxxxx
VSIM 2> q
>
11.13.3 Timescale, Simulation, and Timing-Check Tasks
Key terms and concepts: timescale tasks: $printtimescale and $timeformat ?
simulation control tasks: $stop and $finish ? timing-check tasks ? edge specifiers ?
ASICs... THE COURSE 11.13 Other Verilog Features 43
'edge [01, 0x, x1] clock'is equivalent to 'posedge clock' ? edge transitions with
'z' are treated the same as transitions with 'x'? notifier register (changed when a timing-
check task detects a violation)
edge_control_specifier ::= edge [edge_descriptor {, edge_descriptor}]
edge_descriptor ::= 01 | 0x | 10 | 1x | x0 | x1
// timescale tasks:
module a; initial $printtimescale(b.c1); endmodule
module b; c c1 (); endmodule
`timescale 10 ns / 1 fs
module c_dat; endmodule
`timescale 1 ms / 1 ns
module Ttime; initial $timeformat(-9, 5, " ns", 10); endmodule
/* $timeformat [ ( n, p, suffix , min_field_width ) ] ;
units = 1 second ** (-n), n = 0->15, e.g. for n = 9, units = ns
p = digits after decimal point for %t e.g. p = 5 gives 0.00000
suffix for %t (despite timescale directive)
min_field_width is number of character positions for %t */
Timing-check system task parameters
Timing task argu-
ment Description of argument Type of argument
reference_event to establish reference time module input or inout
(scalar or vector net)
data_event signal to check against
reference_event
module input or inout
(scalar or vector net)
limit time limit to detect timing violation on
data_event
constant expression
or specparam
threshold largest pulse width ignored by timing check
$width
constant expression
or specparam
notifier flags a timing violation (before -> after):
x->0, 0->1, 1->0, z->z
register
44 SECTION 11 VERILOG HDL ASICS... THE COURSE
module test_simulation_control; // simulation control system tasks:
initial begin $stop; // enter interactive mode (default parameter 1)
$finish(2); // graceful exit with optional parameter as follows:
// 0 = nothing 1 = time and location 2 = time, location, and
statistics
end endmodule
module timing_checks (data, clock, clock_1,clock_2); //1
input data,clock,clock_1,clock_2; reg tSU,tH,tHIGH,tP,tSK,tR; //2
specify // timing check system tasks: //3
/* $setup (data_event, reference_event, limit [, notifier]); //4
violation = (T_reference_event)-(T_data_event) < limit */ //5
$setup(data, posedge clock, tSU); //6
/* $hold (reference_event, data_event, limit [, notifier]); //7
violation = //8
(time_of_data_event)-(time_of_reference_event) < limit */ //9
$hold(posedge clock, data, tH); //10
/* $setuphold (reference_event, data_event, setup_limit, //11
hold_limit [, notifier]); //12
parameter_restriction = setup_limit + hold_limit > 0 */ //13
$setuphold(posedge clock, data, tSU, tH); //14
/* $width (reference_event, limit, threshold [, notifier]); //15
violation = //16
threshold < (T_data_event) - (T_reference_event) < limit //17
reference_event = edge //18
data_event = opposite_edge_of_reference_event */ //19
$width(posedge clock, tHIGH); //20
/* $period (reference_event, limit [, notifier]); //21
violation = (T_data_event) - (T_reference_event) < limit //22
reference_event = edge //23
data_event = same_edge_of_reference event */ //24
$period(posedge clock, tP); //25
/* $skew (reference_event, data_event, limit [, notifier]); //26
violation = (T_data_event) - (T_reference_event) > limit */ //27
$skew(posedge clock_1, posedge clock_2, tSK); //28
/* $recovery (reference_event, data_event, limit, [, notifier]); //29
violation = (T_data_event) - (T_reference_event) < limit */ //30
$recovery(posedge clock, posedge clock_2, tR); //31
/* $nochange (reference_event, data_event, start_edge_offset, //32
end_edge_offset [, notifier]); //33
reference_event = posedge | negedge //34
ASICs... THE COURSE 11.13 Other Verilog Features 45
violation = change while reference high (posedge)/low (negedge) //35
+ve start_edge_offset moves start of window later //36
+ve end_edge_offset moves end of window later */ //37
$nochange (posedge clock, data, 0, 0); //38
endspecify endmodule //39
primitive dff_udp(q, clock, data, notifier);
output q; reg q; input clock, data, notifier;
table // clock data notifier:state: q
r 0 ? : ? : 0 ;
r 1 ? : ? : 1 ;
n ? ? : ? : - ;
? * ? : ? : - ;
? ? * : ? : x ; endtable // notifier
endprimitive
`timescale 100 fs / 1 fs
module dff(q, clock, data); output q; input clock, data; reg
notifier;
dff_udp(q1, clock, data, notifier); buf(q, q1);
specify
specparam tSU = 5, tH = 1, tPW = 20, tPLH = 4:5:6, tPHL = 4:5:6;
(clock *> q) = (tPLH, tPHL);
$setup(data, posedge clock, tSU, notifier); // setup: data to clock
$hold(posedge clock, data, tH, notifier); // hold: clock to data
$period(posedge clock, tPW, notifier); // clock: period
endspecify
endmodule
11.13.4 PLA Tasks
Key terms and concepts: The PLA modeling tasks model two-level logic ? eqntott logic
equations ? array format ('1' or '0' in personality array) ? espresso input plane format ?
plane format allows '1', '0', '?' or 'z' (either may be used for don’t care) in personality
array
b1 = a1 & a2; b2 = a3 & a4 & a5 ; b3 = a5 & a6 & a7;
array.dat
1100000
46 SECTION 11 VERILOG HDL ASICS... THE COURSE
0011100
0000111
module pla_1 (a1,a2,a3,a4,a5,a6,a7,b1,b2,b3);
input a1, a2, a3, a4, a5, a6, a7 ; output b1, b2, b3;
reg [1:7] mem[1:3]; reg b1, b2, b3;
initial begin
$readmemb("array.dat", mem);
#1; b1=1; b2=1; b3=1;
$async$and$array(mem,{a1,a2,a3,a4,a5,a6,a7},{b1,b2,b3});
end
initial $monitor("%4g",$time,,b1,,b2,,b3);
endmodule
b1 = a1 & !a2; b2 = a3; b3 = !a1 & !a3; b4 = 1;
module pla_2; reg [1:3] a, mem[1:4]; reg [1:4] b;
initial begin
$async$and$plane(mem,{a[1],a[2],a[3]},{b[1],b[2],b[3],b[4]});
mem[1] = 3'b10?; mem[2] = 3'b??1; mem[3] = 3'b0?0; mem[4] = 3'b???;
#10 a = 3'b111; #10 $displayb(a, " -> ", b);
#10 a = 3'b000; #10 $displayb(a, " -> ", b);
#10 a = 3'bxxx; #10 $displayb(a, " -> ", b);
#10 a = 3'b101; #10 $displayb(a, " -> ", b);
end endmodule
111 -> 0101
000 -> 0011
xxx -> xxx1
101 -> 1101
11.13.5 Stochastic Analysis Tasks
Key terms and concepts: The stochastic analysis tasks model queues
module stochastic; initial begin // stochastic analysis system tasks:
/* $q_initialize (q_id, q_type, max_length, status) ;
q_id is an integer that uniquely identifies the queue
ASICs... THE COURSE 11.13 Other Verilog Features 47
q_type 1=FIFO 2=LIFO
max_length is an integer defining the maximum number of entries */
$q_initialize (q_id, q_type, max_length, status) ;
/* $q_add (q_id, job_id, inform_id, status) ;
job_id = integer input
inform_id = user-defined integer input for queue entry */
$q_add (q_id, job_id, inform_id, status) ;
/* $q_remove (q_id, job_id, inform_id, status) ; */
$q_remove (q_id, job_id, inform_id, status) ;
/* $q_full (q_id, status) ;
status = 0 = queue is not full, status = 1 = queue full */
$q_full (q_id, status) ;
/* $q_exam (q_id, q_stat_code, q_stat_value, status) ;
q_stat_code is input request as follows:
1=current queue length 2=mean inter-arrival time 3=max. queue length
4=shortest wait time ever
5=longest wait time for jobs still in queue 6=ave. wait time in queue
Status values for the stochastic analysis tasks.
Status value Meaning
0 OK
1 queue full, cannot add
2 undefined q_id
3 queue empty, cannot remove
4 unsupported q_type, cannot create queue
5 max_length <= 0, cannot create queue
6 duplicate q_id, cannot create queue
7 not enough memory, cannot create queue
48 SECTION 11 VERILOG HDL ASICS... THE COURSE
q_stat_value is output containing requested value */
$q_exam (q_id, q_stat_code, q_stat_value, status) ;
end endmodule
11.13.6 Simulation Time Functions
Key terms and concepts: The simulation time functions return the time
module test_time; initial begin // simulation time system functions:
$time ;
// returns 64-bit integer scaled to timescale unit of invoking module
$stime ;
// returns 32-bit integer scaled to timescale unit of invoking module
$realtime ;
// returns real scaled to timescale unit of invoking module
end endmodule
11.13.7 Conversion Functions
Key terms and concepts: The conversion functions for reals handle real numbers:
module test_convert; // conversion functions for reals:
integer i; real r; reg [63:0] bits;
initial begin #1 r=256;#1 i = $rtoi(r);
#1; r = $itor(2 * i) ; #1 bits = $realtobits(2.0 * r) ;
#1; r = $bitstoreal(bits) ; end
initial $monitor("%3f",$time,,i,,r,,bits); /*
$rtoi converts reals to integers w/truncation e.g. 123.45 -> 123
$itor converts integers to reals e.g. 123 -> 123.0
$realtobits converts reals to 64-bit vector
$bitstoreal converts bit pattern to real
Real numbers in these functions conform to IEEE Std 754. Conversion
rounds to the nearest valid number. */
endmodule
ASICs... THE COURSE 11.13 Other Verilog Features 49
module test_real;wire [63:0]a; driver d (a); receiver r (a);
initial $monitor("%3g",$time,,a,,d.r1,,r.r2); endmodule
module driver (real_net);
output real_net; real r1; wire [64:1] real_net = $realtobits(r1);
initial #1 r1 = 123.456; endmodule
module receiver (real_net);
input real_net; wire [64:1] real_net; real r2;
initial assign r2 = $bitstoreal(real_net);
endmodule
11.13.8 Probability Distribution Functions
Key terms and concepts: probability distribution functions ? $random ? uniform ? normal ?
exponential ? poisson ? chi_square ? t ? erlang
module probability; // probability distribution functions: //1
/* $random [ ( seed ) ] returns random 32-bit signed integer //2
seed = register, integer, or time */ //3
reg [23:0] r1,r2; integer r3,r4,r5,r6,r7,r8,r9; //4
integer seed, start, \end , mean, standard_deviation; //5
integer degree_of_freedom, k_stage; //6
initial begin seed=1; start=0; \end =6; mean=5; //7
standard_deviation=2; degree_of_freedom=2; k_stage=1; #1; //8
r1 = $random % 60; // random -59 to 59 //9
r2 = {$random} % 60; // positive value 0-59 //10
r3=$dist_uniform (seed, start, \end ) ; //11
r4=$dist_normal (seed, mean, standard_deviation) ; //12
r5=$dist_exponential (seed, mean) ; //13
r6=$dist_poisson (seed, mean) ; //14
r7=$dist_chi_square (seed, degree_of_freedom) ; //15
r8=$dist_t (seed, degree_of_freedom) ; //16
r9=$dist_erlang (seed, k_stage, mean) ; end //17
initial #2 $display ("%3f",$time,,r1,,r2,,r3,,r4,,r5); //18
initial begin #3; $display ("%3f",$time,,r6,,r7,,r8,,r9); end //19
/* All parameters are integer values. //20
Each function returns a pseudo-random number //21
e.g. $dist_uniform returns uniformly distributed random numbers //22
mean, degree_of_freedom, k_stage //23
50 SECTION 11 VERILOG HDL ASICS... THE COURSE
(exponential, poisson, chi-square, t, erlang) > 0. //24
seed = inout integer initialized by user, updated by function //25
start, end ($dist_uniform) = integer bounding return values */ //26
endmodule //27
2.000000 8 57 0 4 9
3.000000 7 3 0 2
11.13.9 Programming Language Interface
Key terms and concepts: The C language Programming Language Interface (PLI) allows you
to access the internal Verilog data structure ? three generations of PLI routines ? task/function
(TF) routines (or utility routines) ? access (ACC) routines access delay and logic values ? Verilog
Procedural Interface (VPI) routines are a superset of the TF and ACC routines
11.14 Summary
Key terms and concepts: concurrent processes and sequential execution ? difference between a
reg and a wire ? scalars and vectors ? arithmetic operations on reg and wire ? data slip ?
delays and events
ASICs... THE COURSE 11.14 Summary 51
Verilog on one page
Verilog feature Example
Comments a = 0; // comment ends with newline
/* This is a multiline or block
comment */
Constants: string and numeric parameter BW = 32 // local, BW
`define G_BUS 32 // global, `G_BUS
4'b2 1'bx
Names (case-sensitive, start with letter or '_') _12name A_name $BAD NotSame notsame
Two basic types of logic signals: wire and reg wire myWire; reg myReg;
Use continuous assignment statement with wire assign myWire = 1;
Use procedural assignment statement with reg always myReg = myWire;
Buses and vectors use square brackets reg [31:0] DBus; DBus[12] = 1'bx;
We can perform arithmetic on bit vectors reg [31:0] DBus; DBus = DBus + 2;
Arithmetic is performed modulo 2n reg [2:0] R; R = 7 + 1; // now R = 0
Operators: as in C (but not ++ or --)
Fixed logic-value system 1, 0, x (unknown), z (high-impedance)
Basic unit of code is the module module bake (chips, dough, cookies);
input chips, dough; output cookies;
assign cookies = chips & dough;
endmodule
Ports input or input/output ports are wire
output ports are wire or reg
Procedures happen at the same time
and may be sensitive to an edge, posedge,
negedge, or to a level.
always @rain sing; always @rain
dance;
always @(posedge clock) D = Q; //
flop
always @(a or b) c = a & b; // and
gate
Sequential blocks model repeating things:
always: executes forever
initial: executes once only at start of simula-
tion
initial born;
always @alarm_clock begin : a_day
metro=commute; bulot=work;
dodo=sleep;
end
Functions and tasks function ... endfunction
task ... endtask
Output $display("a=%f",a);$dumpvars;$monitor(a)
Control simulation $stop; $finish // sudden/gentle halt
Compiler directives `timescale 1ns/1ps // units/resolution
Delay #1 a = b; // delay then sample b
a = #1 b; // sample b then delay
