ASICs...THE COURSE (1 WEEK)
1
LOGIC
SYNTHESIS
Key terms and concepts: logic synthesis converts an HDL behavioral model (Verilog or
VHDL) to a netlist (structural model) the same way a C compiler converts C code to machine
language ? a cell library is called the target library
12
2 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
12.1 A Logic-Synthesis Example
12.2 A Comparator/MUX
Key terms and concepts: synopsys_dc.setup ? script ? derived schematic ? analysis ?
elaboration ? logic optimization ? logic-mapping ? timing-analysis (timing engine)
A comparison of hand design with synthesis (using a 1.0 μm VLSI Technology cell
library)
Path delay/
ns
No. of
standard cells
No. of
transistors
Chip area/
mils2
Hand design 41.6 1,359 16,545 21,877
Synthesized design 36.3 1,493 11,946 18,322
// comp_mux.v
module comp_mux(a, b, outp);
input [2:0] a, b;
output [2:0] outp;
function [2:0] compare;
input [2:0] ina, inb;
begin
if (ina <= inb) compare = ina;
else compare = inb;
end
endfunction
assign outp = compare(a, b);
endmodule
Comparison of the comparator/MUX designs using a 1.0 μm standard-cell library
Delay /ns No. of standard cells No. of transistors Area /mils2
Hand design 4.3 12 116 68.68
Synthesized 2.9 15 66 46.43
a[1]b[1]
a[0]b[0]
outp[2]
outp[1]
outp[0]
a[2]b[2] 01
01
01
sel
ASICs... THE COURSE 12.2 A Comparator/MUX 3
`timescale 1ns / 10ps
module comp_mux_u (a, b, outp);
input [2:0] a; input [2:0] b;
output [2:0] outp;
supply1 VDD; supply0 VSS;
in01d0 u2 (.I(b[1]), .ZN(u2_ZN));
nd02d0 u3 (.A1(a[1]), .A2(u2_ZN), .ZN(u3_ZN));
in01d0 u4 (.I(a[1]), .ZN(u4_ZN));
nd02d0 u5 (.A1(u4_ZN), .A2(b[1]), .ZN(u5_ZN));
in01d0 u6 (.I(a[0]), .ZN(u6_ZN));
nd02d0 u7 (.A1(u6_ZN), .A2(u3_ZN), .ZN(u7_ZN));
nd02d0 u8 (.A1(b[0]), .A2(u3_ZN), .ZN(u8_ZN));
nd03d0 u9 (.A1(u5_ZN), .A2(u7_ZN), .A3(u8_ZN),
.ZN(u9_ZN));
in01d0 u10 (.I(a[2]), .ZN(u10_ZN));
nd02d0 u11 (.A1(u10_ZN), .A2(u9_ZN),
.ZN(u11_ZN));
nd02d0 u12 (.A1(b[2]), .A2(u9_ZN), .ZN(u12_ZN));
nd02d0 u13 (.A1(u10_ZN), .A2(b[2]), .ZN(u13_ZN));
nd03d0 u14 (.A1(u11_ZN), .A2(u12_ZN),
.A3(u13_ZN), .ZN(u14_ZN));
nd02d0 u15 (.A1(a[2]), .A2(u14_ZN), .ZN(u15_ZN));
in01d0 u16 (.I(u14_ZN), .ZN(u16_ZN));
nd02d0 u17 (.A1(b[2]), .A2(u16_ZN), .ZN(u17_ZN));
nd02d0 u18 (.A1(u15_ZN), .A2(u17_ZN),
.ZN(outp[2]));
nd02d0 u19 (.A1(a[1]), .A2(u14_ZN), .ZN(u19_ZN));
nd02d0 u20 (.A1(b[1]), .A2(u16_ZN), .ZN(u20_ZN));
nd02d0 u21 (.A1(u19_ZN), .A2(u20_ZN),
.ZN(outp[1]));
nd02d0 u22 (.A1(a[0]), .A2(u14_ZN), .ZN(u22_ZN));
nd02d0 u23 (.A1(b[0]), .A2(u16_ZN), .ZN(u23_ZN));
nd02d0 u24 (.A1(u22_ZN), .A2(u23_ZN),
.ZN(outp[0]));
endmodule
The comparator/MUX after logic synthesis, but before logic optimization
The structural netlist, comp_mux_u.v, and its derived schematic
outp[0]outp[2]outp[1]
a[0]b[0]b[2]a[1]a[2]a[1]
b[2] b[2]
a[2]
b[1]b[0]
b[1]
a[1]a[1] a[0]
logic cell namesINV (in01d0)
NAND2 (nd02d0)NAND3 (nd03d0)
4 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
`timescale 1ns / 10ps
module comp_mux_o (a, b, outp);
input [2:0] a; input [2:0] b;
output [2:0] outp;
supply1 VDD; supply0 VSS;
in01d0 B1_i1 (.I(a[2]),
.ZN(B1_i1_ZN));
in01d0 B1_i2 (.I(b[1]),
.ZN(B1_i2_ZN));
oa01d1 B1_i3 (.A1(a[0]),
.A2(B1_i4_ZN), .B1(B1_i2_ZN),
.B2(a[1]), .ZN(B1_i3_Z;
fn05d1 B1_i4 (.A1(a[1]), .B1(b[1]),
.ZN(B1_i4_ZN));
fn02d1 B1_i5 (.A(B1_i3_ZN),
.B(B1_i1_ZN), .C(b[2]),
.ZN(B1_i5_ZN));
mx21d1 B1_i6 (.I0(a[0]), .I1(b[0]),
.S(B1_i5_ZN), .Z(outp[0]));
mx21d1 B1_i7 (.I0(a[1]), .I1(b[1]),
.S(B1_i5_ZN), .Z(outp[1]));
mx21d1 B1_i8 (.I0(a[2]), .I1(b[2]),
.S(B1_i5_ZN), .Z(outp[2]));
endmodule
The comparator/MUX after logic synthesis and logic optimization with the default settings
The structural netlist, comp_mux_o.v, and its derived schematic
a[0]a[1]
a[1]b[1]
a[2]
b[2]
b[0] a[0] b[2] a[2] b[1] a[1]
outp[1]outp[2]outp[0]
b[1]
critical path
MAJ3(fn02d1)
OAI22(oa01d1)
NOR1-1(fn05d1)
MUX(mx21d1)
sel
01
INV(in01d0)
INV(in01d0)
0101
ASICs... THE COURSE 12.2 A Comparator/MUX 5
12.2.1 An Actel Version of the Comparator/MUX
Key terms and concepts: Actel ACT 2/3 FPGA architecture ? the symbols represent the
eight-input ACT 2/3 C-Module ? the logic synthesizer, in the technology-mapping step, decides
the connections to the inputs to the logic macro, CM8
`timescale 1 ns/100 ps
module comp_mux_actel_o (a, b, outp);
input [2:0] a, b; output [2:0] outp;
wire n_13, n_17, n_19, n_21, n_23, n_27, n_29,
n_31, n_62;
CM8 I_5_CM8(.D0(n_31), .D1(n_62), .D2(a[0]),
.D3(n_62), .S00(n_62), .S01(n_13), .S10(n_23),
.S11(n_21), .Y(outp[0]));
CM8 I_2_CM8(.D0(n_31), .D1(n_19), .D2(n_62),
.D3(n_62), .S00(n_62), .S01(b[1]), .S10(n_31),
.S11(n_17), .Y(outp[1]));
CM8 I_1_CM8(.D0(n_31), .D1(n_31), .D2(b[2]),
.D3(n_31), .S00(n_62), .S01(n_31), .S10(n_31),
.S11(a[2]), .Y(outp[2]));
VCC VCC_I(.Y(n_62));
CM8 I_4_CM8(.D0(a[2]), .D1(n_31), .D2(n_62),
.D3(n_62), .S00(n_62), .S01(b[2]), .S10(n_31),
.S11(a[1]), .Y(n_19));
CM8 I_7_CM8(.D0(b[1]), .D1(b[2]), .D2(n_31),
.D3(n_31), .S00(a[2]), .S01(b[1]), .S10(n_31),
.S11(a[1]), .Y(n_23));
CM8 I_9_CM8(.D0(n_31), .D1(n_31), .D2(a[1]),
.D3(n_31), .S00(n_62), .S01(b[1]), .S10(n_31),
.S11(b[0]), .Y(n_27));
CM8 I_8_CM8(.D0(n_29), .D1(n_62), .D2(n_31),
.D3(a[2]), .S00(n_62), .S01(n_27), .S10(n_31),
.S11(b[2]), .Y(n_13));
CM8 I_3_CM8(.D0(n_31), .D1(n_31), .D2(a[1]),
.D3(n_31), .S00(n_62), .S01(a[2]), .S10(n_31),
.S11(b[2]), .Y(n_17));
CM8 I_6_CM8(.D0(b[2]), .D1(n_31), .D2(n_62),
.D3(n_62), .S00(n_62), .S01(a[2]), .S10(n_31),
.S11(b[0]), .Y(n_21));
CM8 I_10_CM8(.D0(n_31), .D1(n_31), .D2(b[0]),
.D3(n_31), .S00(n_62), .S01(n_31), .S10(n_31),
.S11(a[2]), .Y(n_29));
GND GND_I(.Y(n_31));
endmodule
The Actel version of the comparator/MUX after logic optimization
The structural netlist, comp_mux_actel_o_adl_e.v, and its derived schematic
b[2]a[2]b[0] a[2]b[2] a[1]
a[2]b[2]a[1]
outp[2]
b[1]
a[2]b[2]
outp[1]
a[0]
b[1]b[1]b[0] a[1]
b[2]a[2]b[1]a[1]
a[2] b[2]b[0]a[2]
outp[0]
VCC
GNDn_31
n_62
D0D2D3D1S01S10S00S11
Y
CM8 Actel ACT2/3C-Module
6 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
12.3 Inside a Logic Synthesizer
Key terms and concepts: The logic synthesizer parses the Verilog and builds an internal data
structure (CDFG) ? logic minimization finds a minimum cover ? synthesized network ? logic
optimization uses factoring, substitution, and elimination ? technology-decomposition builds
a generic network ? technology-mapping (logic-mapping) matches pieces of the network with
the logic cells ? we imply A ? the logic synthesizer has to infer B ? we must write HDL code so
A=B
12.4 Synthesis of the Viterbi Decoder
12.4.1 ASIC I/O
Key terms and concepts: inference of I/O cells ? directives for special pads (clock buffers) ? pull-
up resistor, slew rate ? no standards ? no accepted way to set these parameters from an HDL ?
generic technology-independent I/O models ? instantiate I/O cells directly from a library
// asPadBidir #(W, N, S, L, P) I (Pad, toCore, frCore, OEN) //1
// W = width, integer (default=1) //2
// N = pin number string, e.g. "1:3,5:8" //3
// S = strength = {2, 4, 8, 16} in mA drive //4
// L = level = {cmos, ttl, schmitt} (default = cmos) //5
// P = pull-up resistor = {down, float, none, up} //6
// Vxx = {Vss, Vdd} //7
module PadTri (Pad, I, Oen); // active-low output enable //1
parameter width = 1, pinNumbers = "", \strength = 1, //2
level = "CMOS", externalVdd = 5; //3
output [width-1:0] Pad; input [width-1:0] I; input Oen; //4
assign #1 Pad = (Oen ? {width{1'bz}} : I); //5
endmodule //6
module PadBidir (C, Pad, I, Oen); // active-low output enable //1
parameter width = 1, pinNumbers = "", \strength = 1, //2
level = "CMOS", pull = "none", externalVdd = 5; //3
output [width-1:0] C; inout [width-1:0] Pad; //4
input [width-1:0] I; input Oen; //5
assign #1 Pad = Oen ? {width{1'bz}} : I; assign #1 C = Pad; //6
endmodule //7
ASICs... THE COURSE 12.4 Synthesis of the Viterbi Decoder 7
Logic maps for the comparator/MUX
(a) If the input b is less than a, then sel is '1'. If a=b, then sel = 'x' (don’t care)
(b) A cover for sel.
0 0 0 0x 0 x 1x 0 x 1
1 1 1 1
0 0 0 00 0 0 00 0 0 0
0 0 0 0
1 1 1 11 1 1 11 1 1 1
1 1 1 1
0 0 0 0x 0 x 1x 0 x 1
1 1 1 1
00 0100
0111
10
sel 11 10
a[2]b[2]=00
00 01a[0]b[0]a[1]b[1] 00
0111
10
sel 11 10
a[2]b[2]=10
00 01a[0]b[0]a[1]b[1] 00
0111
10
sel 11 10
a[2]b[2]=11
00 01a[0]b[0]a[1]b[1] 00
0111
10
sel 11 10
a[2]b[2]=01
a2 b2'
a0
a[0]b[0]a[1]b[1]
b0
b1 a1a2' b2' a2' b2
a2 b2
(a)
(b) 25
2531
556
47 6
3
4
1
2
sel = a1*!b1*!b2 + a0*!b1*!b2 + a0*a1*!b2 + a1*!b1*a2 + a0*!b1*a2 + a0*a1*a2 + a2*!b1 = 1 + 2 + 3 + 4 + 5 + 6 + 7
2
8 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
12.4.2 Flip-Flops
Key terms and concepts: synthesis tools cannot handle two waitstatements
module dff(D,Q,Clock,Reset); // N.B. reset is active-low //1
output Q; input D,Clock,Reset; //2
parameter CARDINALITY = 1; reg [CARDINALITY-1:0] Q; //3
wire [CARDINALITY-1:0] D; //4
always @(posedge Clock) if (Reset!==0) #1 Q=D; //5
always begin wait (Reset==0); Q=0; wait (Reset==1); end //6
endmodule //7
module dff(D, Q, Clk, Rst); // new flip-flop for Viterbi decoder //1
parameter width = 1, reset_value = 0; input [width - 1 : 0] D; //2
output [width - 1 : 0] Q; reg [width - 1 : 0] Q; input Clk, Rst; //3
initial Q <= {width{1'bx}}; //4
always @ ( posedge Clk or negedge Rst ) //5
if ( Rst == 0 ) Q <= #1 reset_value; else Q <= #1 D; //6
endmodule //7
12.4.3 The Top-Level Model
Key terms and concepts: top-level Viterbi decoder ? generic input, output, power, and clock I/O
cells from the standard-component library
/* This is the top-level module, viterbi_ASIC.v */ //1
module viterbi_ASIC //2
(padin0, padin1, padin2, padin3, padin4, padin5, padin6, padin7, //3
padOut, padClk, padRes, padError); //4
input [2:0] padin0, padin1, padin2, padin3, //5
padin4, padin5, padin6, padin7; //6
input padRes, padClk; output padError; output [2:0] padOut; //7
wire Error, Clk, Res; wire [2:0] Out; // core //8
wire padError, padClk, padRes; wire [2:0] padOut; //9
wire [2:0] in0,in1,in2,in3,in4,in5,in6,in7; // core //10
wire [2:0] //11
padin0, padin1,padin2,padin3,padin4,padin5,padin6,padin7; //12
// Do not let the software mess with the pads. //13
//compass dontTouch u* //14
asPadIn #(3,"1,2,3") u0 (in0, padin0); //15
asPadIn #(3,"4,5,6") u1 (in1, padin1); //16
asPadIn #(3,"7,8,9") u2 (in2, padin2); //17
asPadIn #(3,"10,11,12") u3 (in3, padin3); //18
asPadIn #(3,"13,14,15") u4 (in4, padin4); //19
ASICs... THE COURSE 12.4 Synthesis of the Viterbi Decoder 9
asPadIn #(3,"16,17,18") u5 (in5, padin5); //20
asPadIn #(3,"19,20,21") u6 (in6, padin6); //21
asPadIn #(3,"22,23,24") u7 (in7, padin7); //22
asPadVdd #("25","both") u25 (vddb); //23
asPadVss #("26","both") u26 (vssb); //24
asPadClk #("27") u27 (Clk, padClk); //25
asPadOut #(1,"28") u28 (padError, Error); //26
asPadin #(1,"29") u29 (Res, padRes); //27
asPadOut #(3,"30,31,32") u30 (padOut, Out); //28
// Here is the core module: //29
viterbi v_1 //30
The core logic of the Viterbi decoder . Bus names are abbreviated (label m_out0-3 denotes
four buses: m_out0, m_out1, m_out2, and m_out3)
compute_metric
errormetricreduce
compare_selectout0-3p0_0-p3_3 m_in0-3
ACS3ACS1ACS2ACS0
ACS0 p0-3path_memory
subset_decode sout2sout3sout1sout0
sout0sout1sout2sout3
ACS0ACS2
ACS1ACS3
out[2:0]
pathin
output_decision
ACS1ACS2ACS3
path0[11:0]
path0[11:0]
resetclk
in0-7
s0-3
m_out0-3
control
s0-3
reset clk
clk
p0_0-p3_3
10 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
(in0,in1,in2,in3,in4,in5,in6,in7,Out,Clk,Res,Error); //31
endmodule //32
ASICs... THE COURSE 12.5 Verilog and Logic Synthesis 11
12.5 Verilog and Logic Synthesis
Key terms and concepts: top-down design approach ? stubs contain a minimum of code
module MyChip_ASIC()
// behavioral "always", etc. ...
SecondLevelStub1 port mapping
SecondLevelStub2 port mapping
... endmodule
module SecondLevelStub1() ... assign Output1 = ~Input1; endmodule
module SecondLevelStub2() ... assign Output2 = ~Input2;
endmodule
12.5.1 Verilog Modeling
Key terms and concepts: synthesizable ? synthesis policy ? modeling style ? functionally
identical, or functionally equivalent
12.5.2 Delays in Verilog
Key terms and concepts: Synthesis tools ignore delay values
module Step_Time(clk, phase); //1
input clk; output [2:0] phase; reg [2:0] phase; //2
always @(posedge clk) begin //3
phase <= 4'b0000; //4
phase <= #1 4'b0001; phase <= #2 4'b0010; //5
phase <= #3 4'b0011; phase <= #4 4'b0100; //6
end //7
endmodule //8
module Step_Count (clk_5x, phase); //1
input clk_5x; output [2:0] phase; reg [2:0] phase; //2
always@(posedge clk_5x) //3
case (phase) //4
0:phase = #1 1; 1:phase = #1 2; 2:phase = #1 3; 3:phase = #1 4; //5
default: phase = #1 0; //6
endcase //7
endmodule //8
12.5.3 Blocking and Nonblocking Assignments
Key terms and concepts: race condition (or a race)
12 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
module race(clk, q0); input clk, q0; reg q1, q2;
always @(posedge clk) q1 = #1 q0; always @(posedge clk) q2 = #1 q1;
endmodule
module no_race_1(clk, q0, q2); input clk, q0; output q2; reg q1, q2;
always @(posedge clk) begin q2 = q1; q1 = q0; end
endmodule
module no_race_2(clk, q0, q2); input clk, q0; output q2; reg q1, q2;
always @(posedge clk) q1 <= #1 q0; always @(posedge clk) q2 <= #1 q1;
endmodule
12.5.4 Combinational Logic in Verilog
Key terms and concepts: level-sensitive sensitivity list ? continuous assignment statements
also imply combinational logic
module And_Always(x, y, z); input x,y; output z; reg z;
always @(x or y) z <= x & y; // combinational logic method 1
endmodule
module And_Assign(x, y, z); input x,y; output z; wire z;
assign z <= x & y; // combinational logic method 2 = method 1
endmodule
module And_Or (a,b,c,z); input a,b,c; output z; reg [1:0]z;
always @(a or b or c) begin z[1]<= &{a,b,c}; z[2]<= |{a,b,c}; end
endmodule
module Parity (BusIn, outp); input[7:0] BusIn; output outp; reg outp;
always @(BusIn) if (^Busin == 0) outp = 1; else outp = 0;
endmodule
module And_Bad(a, b, c); input a, b; output c; reg c;
always@(a) c <= a & b; // b is missing from this sensitivity list
endmodule
ASICs... THE COURSE 12.5 Verilog and Logic Synthesis 13
module CL_good(a, b, c); input a, b; output c; reg c;
always@(a or b)
begin c = a + b; d = a & b; e = c + d; end // c, d: LHS before RHS
endmodule
module CL_bad(a, b, c); input a, b; output c; reg c;
always@(a or b)
begin e = c + d; c = a + b; d = a & b; end // c, d: RHS before LHS
endmodule
// The complement of this function is too big for synthesis.
module Achilles (out, in); output out; input [30:1] in;
assign out = in[30]&in[29]&in[28] | in[27]&in[26]&in[25]
| in[24]&in[23]&in[22] | in[21]&in[20]&in[19]
| in[18]&in[17]&in[16] | in[15]&in[14]&in[13]
| in[12]&in[11]&in[10] | in[9] & in[8]&in[7]
| in[6] & in[5]&in[4] | in[3] & in[2]&in[1];
endmodule
12.5.5 Multiplexers In Verilog
Key terms and concepts: We imply a MUX using a case or if statement ? metalogical values or
simbits (such as 'x') are not “real” ? avoid using casexand casezstatements ? if you need
to “remember” a value, this implies sequential logic
module Mux_21a(sel, a, b, z); input sel, a , b; output z; reg z;
always @(a or b or sel)
begin case(sel) 1'b0: z <= a; 1'b1: z <= b; end
endmodule
module Mux_x(sel, a, b, z); input sel, a, b; output z; reg z;
always @(a or b or sel)
begin case(sel) 1'b0: z <= 0; 1'b1: z <= 1; 1'bx: z <= 'x'; end
endmodule
module Mux_21b(sel, a, b, z); input sel, a, b; output z; reg z;
always @(a or b or sel) begin if (sel) z <= a else z <= b; end
endmodule
14 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
module Mux_Latch(sel, a, b, z); input sel, a, b; output z; reg z;
always @(a or sel) begin if (sel) z <= a; end
endmodule
module Mux_81(InBus, sel, OE, OutBit); //1
input [7:0] InBus; input [2:0] Sel; //2
input OE; output OutBit; reg OutBit; //3
always @(OE or sel or InBus) //4
begin //5
if (OE == 1) OutBit = InBus[sel]; else OutBit = 1'bz; //6
end //7
endmodule //8
12.5.6 The Verilog Case Statement
Key terms and concepts: exhaustive ? compiler directive ? synthesis directive ?
pseudocomment ? an 'x' (synthesis don’t care value) gives the synthesizer flexibility in
optimization ? priority encoder
module case8_oneHot(oneHot, a, b, c, z); //1
input a, b, c; input [2:0] oneHot; output z; reg z; //2
always @(oneHot or a or b or c) //3
begin case(oneHot) //synopsys full_case //4
3'b001: z <= a; 3'b010: z <= b; 3'b100: z <= c; //5
default: z <= 1'bx; endcase //6
end //7
endmodule //8
module case8_priority(oneHot, a, b, c, z); //1
input a, b, c; input [2:0] oneHot; output z; reg z; //2
always @(oneHot or a or b or c) begin //3
case(1'b1) //synopsys parallel_case //4
oneHot[0]: z <= a; //5
oneHot[1]: z <= b; //6
oneHot[2]: z <= c; //7
default: z <= 1'bx; endcase //8
end //9
endmodule //10
ASICs... THE COURSE 12.5 Verilog and Logic Synthesis 15
12.5.7 Decoders In Verilog
Key terms and concepts: the synthesizer infers a three-state buffer from an assignment of 'z'
module Decoder_4To16(enable, In_4, Out_16); // 4-to-16 decoder //1
input enable; input [3:0] In_4; output [15:0] Out_16; //2
reg [15:0] Out_16; //3
always @(enable or In_4) //4
begin Out_16 = 16'hzzzz; //5
if (enable == 1) //6
begin Out_16 = 16'h0000; Out_16[In_4] = 1; end //7
end //8
endmodule //9
if (enable === 1) // can't make logic to check for enable = x or z
12.5.8 Priority Encoder in Verilog
Key terms and concepts: The logic synthesizer must be able to unroll a loop in aforstatement.
module Pri_Encoder32 (InBus, Clk, OE, OutBus); //1
input [31:0]InBus; input OE, Clk; output [4:0]OutBus; //2
reg j; reg [4:0]OutBus; //3
always@(posedge Clk) //4
begin //5
if (OE == 0) OutBus = 5'bz ; //6
else //7
begin OutBus = 0; //8
for (j = 31; j >= 0; j = j - 1) //9
begin if (InBus[j] == 1) OutBus = j; end //10
end //11
end //12
endmodule //13
12.5.9 Arithmetic in Verilog
Key terms and concepts: make room for the carry bit when you add two numbers in Verilog ?
resource allocation ? resource sharing ? multiplication assumes nets are unsigned
module Adder_8 (A, B, Z, Cin, Cout); //1
input [7:0] A, B; input Cin; output [7:0] Z; output Cout; //2
assign {Cout, Z} = A + B + Cin; //3
endmodule //4
module Adder_16 (A, B, Sum, Cout); //1
input [15:0] A, B; output [15:0] Sum; output Cout; //2
16 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
reg [15:0] Sum; reg Cout; //3
always @(A or B) {Cout, Sum} = A + B + 1; // One adder not two! //4
endmodule //5
module Add_A (sel, a, b, c, d, y); //1
input a, b, c, d, sel; output y; reg y; //2
always@(sel or a or b or c or d) // One or two adders? //3
begin if (sel == 0) y <= a + b; else y <= c + d; end //4
endmodule //5
module Add_B (sel, a, b, c, d, y); //1
input a, b, c, d, sel; output y; reg t1, t2, y; //2
always@(sel or a or b or c or d) begin // One adder not two! //3
if (sel == 0) begin t1 = a; t2 = b; end // Temporary //4
else begin t1 = c; t2 = d; end // variables. //5
y = t1 + t2; end //6
endmodule //7
module Multiply_unsigned (A, B, Z); //1
input [1:0] A, B; output [3:0] Z; //2
assign Z <= A * B; //3
endmodule //4
module Multiply_signed (A, B, Z); //1
input [1:0] A, B; output [3:0] Z; //2
// 00 -> 00_00 01 -> 00_01 10 -> 11_10 11 -> 11_11 //3
assign Z = { { 2{A[1]} }, A} * { { 2{B[1]} }, B}; //4
endmodule //5
12.5.10 Sequential Logic in Verilog
Key terms and concepts: edges (posedge or negedge) in the sensitivity list of an always
statement imply a clocked storage element ? however, an alwaysstatement does not have to
be edge-sensitive to imply sequential logic ? all sequential logic cells must be initialized ?
template ? synthesis style guide
always@(posedge clock) Q_flipflop = D; // A flip-flop.
always@(clock or D) if (clock) Q_latch = D; // A latch.
always@(posedge clock or negedge reset) // names mean nothing,
always@(posedge day or negedge year) // which is the reset?
ASICs... THE COURSE 12.5 Verilog and Logic Synthesis 17
always@(posedge clk or negedge reset) begin // Template for reset:
if (reset == 0) Q = 0; // initialize,
else Q = D; // normal clocking
end
module Counter_With_Reset (count, clock, reset); //1
input clock, reset; output count; reg [7:0] count; //2
always @ (posedge clock or negedge reset) //3
if (reset == 0) count = 0; else count = count + 1; //4
endmodule //5
module DFF_MasterSlave (D, clock, reset, Q); // D type flip-flop //1
input D, clock, reset; output Q; reg Q, latch; //2
always @(posedge clock or posedge reset) //3
if (reset == 1) latch = 0; else latch = D; // the master. //4
always @(latch) Q = latch; // the slave. //5
endmodule //6
12.5.11 Component Instantiation in Verilog
Key terms and concepts: HDL description is technology-independent (CMOS, FPGA, TTL,
GaAs) ? the only way to use a particular cell is to use structural Verilog and hand instantiation
? dont_touch ? soft models or standard components ? DesignWare
//Compass dontTouch my_inv_8x or // synopsys dont_touch
INVD8 my_inv_8x(.I(a), .ZN(b) );
module Count4(clk, reset, Q0, Q1, Q2, Q3); //1
input clk, reset; output Q0, Q1, Q2, Q3; wire Q0, Q1, Q2, Q3; //2
// Q , D , clk, reset //3
asDff dff0( Q0, ~Q0, clk, reset); // The asDff is a //4
asDff dff1( Q1, ~Q1, Q0, reset); // standard component, //5
asDff dff2( Q2, ~Q2, Q1, reset); // unique to one set of tools. //6
asDff dff3( Q3, ~Q3, Q2, reset); //7
endmodule //8
module asDff (D, Q, Clk, Rst); //1
parameter width = 1, reset_value = 0; //2
input [width-1:0] D; output [width-1:0] Q; reg [width-1:0] Q; //3
input Clk,Rst; initial Q = {width{1'bx}}; //4
always @ ( posedge Clk or negedge Rst ) //5
18 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
if ( Rst==0 ) Q <= #1 reset_value; else Q <= #1 D; //6
endmodule //7
12.5.12 Datapath Synthesis in Verilog
Key terms and concepts: Datapath synthesis ? Synopsys VHDL DesignWare ? compiler direc-
tives ? X-BLOX ? LPM (library of parameterized modules) ? RPM (relationally placed modules)
? thinking like the hardware
module DP_csum(A1,B1,Z1); input [3:0] A1,B1; output Z1; reg [3:0] Z1;
always@(A1 or B1) Z1 <= A1 + B1;//Compass adder_arch cond_sum_add
endmodule
module DP_ripp(A2,B2,Z2); input [3:0] A2,B2; output Z2; reg [3:0] Z2;
always@(A2 or B2) Z2 <= A2 + B2;//Compass adder_arch ripple_add
endmodule
module DP_sub_A(A,B,OutBus,CarryIn); //1
input [3:0] A, B ; input CarryIn ; //2
output OutBus ; reg [3:0] OutBus ; //3
always@(A or B or CarryIn) OutBus <= A - B - CarryIn ; //4
endmodule //5
module DP_sub_B (A, B, CarryIn, Z) ; //1
input [3:0] A, B, CarryIn ; output [3:0] Z; reg [3:0] Z; //2
always@(A or B or CarryIn) begin //3
case (CarryIn) //4
1'b1 : Z <= A - B - 1'b1; //5
default : Z <= A - B - 1'b0; endcase //6
end //7
endmodule //8
12.6 VHDL and Logic Synthesis
Key terms and concepts: IEEE VHDL nine-value system ? You can use '1', 'H', '0', and 'L'
in any manner ? Some synthesis tools do not accept 'U' ? You can use logic states 'Z', 'X',
'W', and '-' in assignments in any manner ? 'Z' is synthesized to three-state logic ? 'X', 'W',
ASICs... THE COURSE 12.6 VHDL and Logic Synthesis 19
and '-' are treated as unknown or don’t care values ? The IEEE synthesis packages provide the
STD_MATCH function for comparisons
12.6.1 Initialization and Reset
Key terms and concepts: a VHDL process with a sensitivity list synthesizes to clocked logic
with a reset
process (signal_1, signal_2) begin
if (signal_2'EVENT and signal_2 = '0')
then -- Insert initialization and reset statements.
elsif (signal_1'EVENT and signal_1 = '1')
then -- Insert clocking statements.
end if;
end process;
12.6.2 Combinational Logic Synthesis in VHDL
Key terms and concepts: a level-sensitive process has a sensitivity list with signals that are not
tested for event attributes ('EVENT or 'STABLE, for example) ? combinational logic uses a level-
sensitive process or a concurrent assignment statement ? some synthesizers do not allow a
signal inside a level-sensitive process unless the signal is in the sensitivity list
entity And_Bad is port (a, b: in BIT; c: out BIT); end And_Bad;
architecture Synthesis_Bad of And_Bad is
begin process (a) -- this should be process (a, b)
begin c <= a and b;
end process;
end Synthesis_Bad;
12.6.3 Multiplexers in VHDL
Key terms and concepts: multiplexers can be synthesized using an (exhaustive) case statement
(avoid the reserved word 'select') ? a concurrent signal assignment is equivalent
20 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
entity Mux4 is port
(i: BIT_VECTOR(3 downto 0); sel: BIT_VECTOR(1 downto 0); s: out BIT);
end Mux4;
architecture Synthesis_1 of Mux4 is
begin process(sel, i) begin
case sel is
when "00" => s <= i(0); when "01" => s <= i(1);
when "10" => s <= i(2); when "11" => s <= i(3);
end case;
end process;
end Synthesis_1;
architecture Synthesis_2 of Mux4 is
begin with sel select s <=
i(0) when "00", i(1) when "01", i(2) when "10", i(3) when "11";
end Synthesis_2;
library IEEE; use ieee.std_logic_1164.all;
entity Mux8 is port
(InBus : in STD_LOGIC_VECTOR(7 downto 0);
Sel : in INTEGER range 0 to 7;
OutBit : out STD_LOGIC);
end Mux8;
architecture Synthesis_1 of Mux8 is
begin process(InBus, Sel)
begin OutBit <= InBus(Sel);
end process;
end Synthesis_1;
12.6.4 Decoders in VHDL
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --2
entity Decoder is port (enable : in BIT; --3
Din: STD_LOGIC_VECTOR (2 downto 0); --4
Dout: out STD_LOGIC_VECTOR (7 downto 0)); --5
end Decoder; --6
ASICs... THE COURSE 12.6 VHDL and Logic Synthesis 21
architecture Synthesis_1 of Decoder is --7
begin --8
with enable select Dout <= --9
STD_LOGIC_VECTOR --10
(UNSIGNED' --11
(shift_left --12
("00000001", TO_INTEGER (UNSIGNED(Din)) --13
) --14
) --15
) --16
when '1', --17
"11111111" when '0', "00000000" when others; --18
end Synthesis_1; --19
library IEEE; --1
use IEEE.NUMERIC_STD.all; use IEEE.STD_LOGIC_1164.all; --2
entity Concurrent_Decoder is port ( --3
enable : in BIT; --4
Din : in STD_LOGIC_VECTOR (2 downto 0); --5
Dout : out STD_LOGIC_VECTOR (7 downto 0)); --6
end Concurrent_Decoder; --7
architecture Synthesis_1 of Concurrent_Decoder is --8
begin process (Din, enable) --9
variable T : STD_LOGIC_VECTOR(7 downto 0); --10
begin --11
if (enable = '1') then --12
T := "00000000"; T( TO_INTEGER (UNSIGNED(Din))) := '1'; --13
Dout <= T ; --14
else Dout <= (others => 'Z'); --15
end if; --16
end process; --17
end Synthesis_1; --18
12.6.5 Adders in VHDL
Key terms and concepts: To add two n-bit numbers and keep the overflow bit, we need to assign
to a signal with more bits
library IEEE; --1
use IEEE.NUMERIC_STD.all; use IEEE.STD_LOGIC_1164.all; --2
entity Adder_1 is --3
port (A, B: in UNSIGNED(3 downto 0); C: out UNSIGNED(4 downto 0)); --4
end Adder_1; --5
architecture Synthesis_1 of Adder_1 is --6
22 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
begin C <= ('0' & A) + ('0' & B); --7
end Synthesis_1; --8
12.6.6 Sequential Logic in VHDL
Key terms and concepts: Sensitivity to an edge implies sequential logic in VHDL ? Either: (1) no
sensitivity list with a wait until statement (2) a sensitivity list and test for 'EVENT plus a
specific level ? any signal assigned in an edge-sensitive process statement should be reset—
but be careful to distinguish between asynchronous and synchronous resets
library IEEE; use IEEE.STD_LOGIC_1164.all; entity DFF_With_Reset is
port(D, Clk, Reset : in STD_LOGIC; Q : out STD_LOGIC);
end DFF_With_Reset;
architecture Synthesis_1 of DFF_With_Reset is
begin process(Clk, Reset) begin
if (Reset = '0') then Q <= '0'; -- asynchronous reset
elsif rising_edge(Clk) then Q <= D;
end if;
end process;
end Synthesis_1;
architecture Synthesis_2 of DFF_With_Reset is
begin process begin
wait until rising_edge(Clk);
-- This reset is gated with the clock and is synchronous:
if (Reset = '0') then Q <= '0'; else Q <= D; end if;
end process;
end Synthesis_2;
Key terms and concepts: sequential logic results when we have to “remember” something
between successive executions of a process statement. This occurs when a process
statement contains one or more of the following situations (1) A signal is read but is not in the
ASICs... THE COURSE 12.6 VHDL and Logic Synthesis 23
sensitivity list of a process statement (2) A signal or variable is read before it is updated (3) A
signal is not always updated (4) There are multiple wait statements
Not all of the models that we could write using the above constructs will be synthesizable. Any
models that do use one or more of these constructs and that are synthesizable will result in
sequential logic.
12.6.7 Instantiation in VHDL
Key terms and concepts: to help hand instantiate a component generate a structural netlist
`timescale 1ns/1ns //1
module halfgate (myInput, myOutput); //2
input myInput; output myOutput; wire myOutput; //3
assign myOutput = ~myInput; //4
endmodule //5
library IEEE; use IEEE.STD_LOGIC_1164.all; --1
library COMPASS_LIB; use COMPASS_LIB.COMPASS.all; --2
--compass compile_off -- synopsys etc. --3
use COMPASS_LIB.COMPASS_ETC.all; --4
--compass compile_on -- synopsys etc. --5
entity halfgate_u is --6
--compass compile_off -- synopsys etc. --7
generic ( --8
myOutput_cap : Real := 0.01; --9
INSTANCE_NAME : string := "halfgate_u" ); --10
--compass compile_on -- synopsys etc. --11
port ( myInput : in Std_Logic := 'U'; --12
myOutput : out Std_Logic := 'U' ); --13
end halfgate_u; --14
architecture halfgate_u of halfgate_u is --15
component in01d0 --16
port ( I : in Std_Logic; ZN : out Std_Logic ); end component; --17
begin --18
u2: in01d0 port map ( I => myInput, ZN => myOutput ); --19
end halfgate_u; --20
--compass compile_off -- synopsys etc. --21
library cb60hd230d; --22
configuration halfgate_u_CON of halfgate_u is --23
for halfgate_u --24
for u2 : in01d0 use configuration cb60hd230d.in01d0_CON --25
generic map ( --26
24 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
ZN_cap => 0.0100 + myOutput_cap, --27
INSTANCE_NAME => INSTANCE_NAME&"/u2" ) --28
port map ( I => I, ZN => ZN); --29
end for; --30
end for; --31
end halfgate_u_CON; --32
--compass compile_on -- synopsys etc. --33
component ASDFF
generic (WIDTH : POSITIVE := 1;
RESET_VALUE : STD_LOGIC_VECTOR := "0" );
port (Q : out STD_LOGIC_VECTOR (WIDTH-1 downto 0);
D : in STD_LOGIC_VECTOR (WIDTH-1 downto 0);
CLK : in STD_LOGIC;
RST : in STD_LOGIC );
end component;
library IEEE, COMPASS_LIB; --1
use IEEE.STD_LOGIC_1164.all; use COMPASS_LIB.STDCOMP.all; --2
entity Ripple_4 is --3
port (Trig, Reset: STD_LOGIC; QN0_5x: out STD_LOGIC; --4
Q : inout STD_LOGIC_VECTOR(0 to 3)); --5
end Ripple_4; --6
architecture structure of Ripple_4 is --7
signal QN : STD_LOGIC_VECTOR(0 to 3); --8
component in01d1 --9
port ( I : in Std_Logic; ZN : out Std_Logic ); end component; --10
component in01d5 --11
port ( I : in Std_Logic; ZN : out Std_Logic ); end component; --12
begin --13
--compass dontTouch inv5x -- synopsys dont_touch etc. --14
-- Named association for hand-instantiated library cells: --15
inv5x: IN01D5 port map( I=>Q(0), ZN=>QN0_5x ); --16
inv0 : IN01D1 port map( I=>Q(0), ZN=>QN(0) ); --17
inv1 : IN01D1 port map( I=>Q(1), ZN=>QN(1) ); --18
inv2 : IN01D1 port map( I=>Q(2), ZN=>QN(2) ); --19
inv3 : IN01D1 port map( I=>Q(3), ZN=>QN(3) ); --20
-- Positional association for standard components: --21
-- Q D Clk Rst --22
d0: asDFF port map(Q (0 to 0), QN(0 to 0), Trig, Reset); --23
d1: asDFF port map(Q (1 to 1), QN(1 to 1), Q(0), Reset); --24
d2: asDFF port map(Q (2 to 2), QN(2 to 2), Q(1), Reset); --25
ASICs... THE COURSE 12.6 VHDL and Logic Synthesis 25
d3: asDFF port map(Q (3 to 3), QN(3 to 3), Q(2), Reset); --26
end structure; --27
`timescale 1ns / 10ps //1
module ripple_4_u (trig, reset, qn0_5x, q); //2
input trig; input reset; output qn0_5x; inout [3:0] q; //3
wire [3:0] qn; supply1 VDD; supply0 VSS; //4
in01d5 inv5x (.I(q[0]),.ZN(qn0_5x)); //5
in01d1 inv0 (.I(q[0]),.ZN(qn[0])); //6
in01d1 inv1 (.I(q[1]),.ZN(qn[1])); //7
in01d1 inv2 (.I(q[2]),.ZN(qn[2])); //8
in01d1 inv3 (.I(q[3]),.ZN(qn[3])); //9
dfctnb d0(.D(qn[0]),.CP(trig),.CDN(reset),.Q(q[0]),.QN(\d0.QN )); //10
dfctnb d1(.D(qn[1]),.CP(q[0]),.CDN(reset),.Q(q[1]),.QN(\d1.QN )); //11
dfctnb d2(.D(qn[2]),.CP(q[1]),.CDN(reset),.Q(q[2]),.QN(\d2.QN )); //12
dfctnb d3(.D(qn[3]),.CP(q[2]),.CDN(reset),.Q(q[3]),.QN(\d3.QN )); //13
endmodule //14
12.6.8 Shift Registers and Clocking in VHDL
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --2
entity SIPO_1 is port ( --3
Clk : in STD_LOGIC; --4
SI : in STD_LOGIC; -- serial in --5
PO : buffer STD_LOGIC_VECTOR(3 downto 0)); -- parallel out --6
end SIPO_1; --7
architecture Synthesis_1 of SIPO_1 is --8
begin process (Clk) begin --9
if (Clk = '1') then PO <= SI & PO(3 downto 1); end if; --10
end process; --11
end Synthesis_1; --12
module sipo_1_u (clk, si, po); //1
input clk; input si; output [3:0] po; //2
supply1 VDD; supply0 VSS; //3
dfntnb po_ff_b0 (.D(po[1]),.CP(clk),.Q(po[0]),.QN(\po_ff_b0.QN)); //4
dfntnb po_ff_b1 (.D(po[2]),.CP(clk),.Q(po[1]),.QN(\po_ff_b1.QN)); //5
dfntnb po_ff_b2 (.D(po[3]),.CP(clk),.Q(po[2]),.QN(\po_ff_b2.QN)); //6
dfntnb po_ff_b3 (.D(si),.CP(clk),.Q(po[3]),.QN(\po_ff_b3.QN )); //7
endmodule //8
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --2
26 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
entity SIPO_R is port ( --3
clk : in STD_LOGIC ; res : in STD_LOGIC ; --4
SI : in STD_LOGIC ; PO : out STD_LOGIC_VECTOR(3 downto 0)); --5
end; --6
architecture Synthesis_1 of SIPO_R is --7
signal PO_t : STD_LOGIC_VECTOR(3 downto 0); --8
begin --9
process (PO_t) begin PO <= PO_t; end process; --10
process (clk, res) begin --11
if (res = '0') then PO_t <= (others => '0'); --12
elsif (rising_edge(clk)) then PO_t <= SI & PO_t(3 downto 1); --13
end if; --14
end process; --15
end Synthesis_1; --16
12.6.9 Adders and Arithmetic Functions
Key terms and concepts: to perform BIT_VECTOR or STD_LOGIC_VECTOR arithmetic you have
three choices: (1) Use a vendor-supplied package (2) Convert to SIGNED (or UNSIGNED) and
use the IEEE standard synthesis packages (3) Use overloaded functions in packages or
functions that you define yourself
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --2
entity Adder4 is port ( --3
in1, in2 : in BIT_VECTOR(3 downto 0) ; --4
mySum : out BIT_VECTOR(3 downto 0) ) ; --5
end Adder4; --6
architecture Behave_A of Adder4 is --7
function DIY(L,R: BIT_VECTOR(3 downto 0)) return BIT_VECTOR is --8
variable sum:BIT_VECTOR(3 downto 0);variable lt,rt,st,cry: BIT; --9
begin cry := '0'; --10
for i in L'REVERSE_RANGE loop --11
lt := L(i); rt := R(i); st := lt xor rt; --12
sum(i):= st xor cry; cry:= (lt and rt) or (st and cry); --13
end loop; --14
return sum; --15
end; --16
begin mySum <= DIY (in1, in2); -- do it yourself (DIY) add --17
end Behave_A; --18
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --2
ASICs... THE COURSE 12.6 VHDL and Logic Synthesis 27
entity Adder4 is port ( --3
in1, in2 : in UNSIGNED(3 downto 0) ; --4
mySum : out UNSIGNED(3 downto 0) ) ; --5
end Adder4; --6
architecture Behave_B of Adder4 is --7
begin mySum <= in1 + in2; -- This uses an overloaded '+'. --8
end Behave_B; --9
12.6.10 Adder/Subtracter and Don’t Cares
Key terms and concepts: whether to use simple code or more complex code that more
accurately describes the hardware?
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --2
entity Adder_Subtracter is port ( --3
xin : in UNSIGNED(15 downto 0); --4
clk, addsub, clr: in STD_LOGIC; --5
result : out UNSIGNED(15 downto 0)); --6
end Adder_Subtracter; --7
architecture Behave_A of Adder_Subtracter is --8
signal addout, result_t: UNSIGNED(15 downto 0); --9
begin --10
result <= result_t; --11
with addsub select --12
addout <= (xin + result_t) when '1', --13
(xin - result_t) when '0', --14
(others => '-') when others; --15
process (clr, clk) begin --16
if (clr = '0') then result_t <= (others => '0'); --17
elsif rising_edge(clk) then result_t <= addout; --18
end if; --19
end process; --20
end Behave_A; --21
architecture Behave_B of Adder_Subtracter is --1
signal result_t: UNSIGNED(15 downto 0); --2
begin --3
result <= result_t; --4
process (clr, clk) begin --5
if (clr = '0') then result_t <= (others => '0'); --6
elsif rising_edge(clk) then --7
case addsub is --8
when '1' => result_t <= (xin + result_t); --9
when '0' => result_t <= (xin - result_t); --10
28 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
when others => result_t <= (others => '-'); --11
end case; --12
end if; --13
end process; --14
end Behave_B; --15
12.7 Finite-State Machine Synthesis
Key terms and concepts: synthesis of a finite-state machine (FSM) ? let the logic synthesizer
operate on the state machine as random logic ? use directives to guide the logic synthesis tool to
improve or modify state assignment ? use a special state-machine compiler ? FSM encoding
options ? Adjacent encoding ( Gray codes) ? One-hot encoding ? Random encoding ? User-
specified encoding (keep explicit state assignment) ? Moore encoding (useful for FSMs that
require fast outputs)
12.7.1 FSM Synthesis in Verilog
Key terms and concepts: FSM paired processes ? one process synthesizes to sequential logic
and the second process synthesizes to combinational logic ? pseudocomments to define the
states and state vector
`define resSt 0 //1
`define S1 1 //2
`define S2 2 //3
`define S3 3 //4
module StateMachine_1 (reset, clk, yOutReg); //5
input reset, clk; output yOutReg; //6
reg yOutReg, yOut; reg [1:0] curSt, nextSt; //7
always @(posedge clk or posedge reset) //8
begin:Seq //Compass statemachine oneHot curSt //9
if (reset == 1) //10
begin yOut = 0; yOutReg = yOut; curSt = `resSt; end //11
else begin //12
case (curSt) //13
`resSt:yOut = 0;`S1:yOut = 1;`S2:yOut = 1;`S3:yOut = 1; //14
default:yOut = 0; //15
endcase //16
yOutReg = yOut; curSt = nextSt; // ... update the state. //17
end //18
ASICs... THE COURSE 12.7 Finite-State Machine Synthesis 29
end //19
always @(curSt or yOut) // Assign the next state: //20
begin:Comb //21
case (curSt) //22
`resSt:nextSt = `S3; `S1:nextSt = `S2; //23
`S2:nextSt = `S1; `S3:nextSt = `S1; //24
default:nextSt = `resSt; //25
endcase //26
end //27
endmodule //28
module StateMachine_2 (reset, clk, yOutReg); //1
input reset, clk; output yOutReg; reg yOutReg, yOut; //2
parameter [1:0] //synopsys enum states //3
resSt = 2'b00, S1 = 2'b01, S2 = 2'b10, S3 = 2'b11; //4
reg [1:0] /* synopsys enum states */ curSt, nextSt; //5
//synopsys state_vector curSt //6
always @(posedge clk or posedge reset) begin //7
if (reset == 1) //8
begin yOut = 0; yOutReg = yOut; curSt = resSt; end //9
else begin //10
case (curSt) resSt:yOut = 0;S1:yOut = 1;S2:yOut = 1;S3:yOut = 1; //11
default:yOut = 0; endcase //12
yOutReg = yOut; curSt = nextSt; end //13
end //14
always @(curSt or yOut) begin //15
case (curSt) //16
resSt:nextSt = S3; S1:nextSt = S2; S2:nextSt = S1; S3:nextSt = S1;//17
default:nextSt = S1; endcase //18
end //19
endmodule //20
parameter [3:0] //synopsys enum states
resSt = 4'b0000, S1 = 4'b0010, S2 = 4'b0100, S3 = 4'b1000;
30 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
12.7.2 FSM Synthesis in VHDL
Key terms and concepts: Moore state machine ? Mealy state machine ? An FSM compiler
extracts a state machine
library IEEE; use IEEE.STD_LOGIC_1164.all; --1
entity SM1 is --2
port (aIn, clk : in Std_logic; yOut: out Std_logic); --3
end SM1; --4
architecture Moore of SM1 is --5
type state is (s1, s2, s3, s4); --6
signal pS, nS : state; --7
begin --8
process (aIn, pS) begin --9
case pS is --10
when s1 => yOut <= '0'; nS <= s4; --11
when s2 => yOut <= '1'; nS <= s3; --12
when s3 => yOut <= '1'; nS <= s1; --13
when s4 => yOut <= '1'; nS <= s2; --14
end case; --15
end process; --16
process begin --17
-- synopsys etc. --18
--compass Statemachine adj pS --19
wait until clk = '1'; pS <= nS; --20
end process; --21
end Moore; --22
library IEEE; use IEEE.STD_LOGIC_1164.all; --1
entity SM2 is --2
port (aIn, clk : in Std_logic; yOut: out Std_logic); --3
end SM2; --4
architecture Mealy of SM2 is --1
type state is (s1, s2, s3, s4); --2
signal pS, nS : state; --3
begin --4
process(aIn, pS) begin --5
case pS is --6
when s1 => if (aIn = '1') --7
then yOut <= '0'; nS <= s4; --8
else yOut <= '1'; nS <= s3; --9
end if; --10
when s2 => yOut <= '1'; nS <= s3; --11
when s3 => yOut <= '1'; nS <= s1; --12
ASICs... THE COURSE 12.8 Memory Synthesis 31
when s4 => if (aIn = '1') --13
then yOut <= '1'; nS <= s2; --14
else yOut <= '0'; nS <= s1; --15
end if; --16
end case; --17
end process; --18
process begin --19
wait until clk = '1' ; --20
--Compass Statemachine oneHot pS --21
pS <= nS; --22
end process; --23
end Mealy; --24
12.8 Memory Synthesis
Key terms and concepts: approaches to memory synthesis: (1) Random logic using flip-flops or
latches (2) Register files in datapaths (3) RAM standard components (4) RAM compilers
12.8.1 Memory Synthesis in Verilog
Key terms and concepts: Verilog memory array ? an array of latches or flip-flops
reg [31:0] MyMemory [3:0]; // a 4 x 32-bit register
module RAM_1(A, CEB, WEB, OEB, INN, OUTT); //1
input [6:0] A; input CEB,WEB,OEB; input [4:0]INN; //2
output [4:0] OUTT; //3
reg [4:0] OUTT; reg [4:0] int_bus; reg [4:0] memory [127:0]; //4
always@(negedge CEB) begin //5
if (CEB == 0) begin //6
if (WEB == 1) int_bus = memory[A]; //7
else if (WEB == 0) begin memory[A] = INN; int_bus = INN; end //8
else int_bus = 5'bxxxxx; //9
end //10
end //11
always@(OEB or int_bus) begin //12
case (OEB) 0 : OUTT = int_bus; //13
default : OUTT = 5'bzzzzz; endcase //14
32 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
end //15
endmodule //16
memory[i + 1] = memory[i]; // needs two clock cycles
pointer = memory[memory[i]]; // needs two clock cycles
pc = memory[addr1]; memory[addr2] = pc + 1; // not on the same cycle
12.8.2 Memory Synthesis in VHDL
Key terms and concepts: VHDL multidimensional arrays ? array of latches ? standard-cell RAM
type memStor is array(3 downto 0) of integer; -- This is OK.
subtype MemReg is STD_LOGIC_VECTOR(15 downto 0); -- So is this.
type memStor is array(3 downto 0) of MemReg;
-- other code...
signal Mem1 : memStor;
library IEEE; --1
use IEEE.STD_LOGIC_1164.all; --2
package RAM_package is --3
constant numOut : INTEGER := 8; --4
constant wordDepth: INTEGER := 8; --5
constant numAddr : INTEGER := 3; --6
subtype MEMV is STD_LOGIC_VECTOR(numOut-1 downto 0); --7
type MEM is array (wordDepth-1 downto 0) of MEMV; --8
end RAM_package; --9
library IEEE; --10
use IEEE.STD_LOGIC_1164.all; use IEEE.NUMERIC_STD.all; --11
use work.RAM_package.all; --12
entity RAM_1 is --13
port (signal A : in STD_LOGIC_VECTOR(numAddr-1 downto 0); --14
signal CEB, WEB, OEB : in STD_LOGIC; --15
signal INN : in MEMV; --16
signal OUTT : out MEMV); --17
end RAM_1; --18
architecture Synthesis_1 of RAM_1 is --19
signal i_bus : MEMV; -- RAM internal data latch --20
signal mem : MEM; -- RAM data --21
begin --22
process begin --23
wait until CEB = '0'; --24
ASICs... THE COURSE 12.9 The Multiplier 33
if WEB = '1' then i_bus <= mem(TO_INTEGER(UNSIGNED(A))); --25
elsif WEB = '0' then --26
mem(TO_INTEGER(UNSIGNED(A))) <= INN; --27
i_bus <= INN; --28
else i_bus <= (others => 'X'); --29
end if; --30
end process; --31
process(OEB, int_bus) begin -- control output drivers: --32
case (OEB) is --33
when '0' => OUTT <= i_bus; --34
when '1' => OUTT <= (others => 'Z'); --35
when others => OUTT <= (others => 'X'); --36
end case; --37
end process; --38
end Synthesis_1; --39
12.9 The Multiplier
Key terms and concepts: warnings and errors during elaboration
Sum <= X xor Y xor Cin after TS;
Warning: AFTER clause in a waveform element is not supported
port (A, B : in BIT_VECTOR (7 downto 0); Sel : in BIT := '0'; Y : out
BIT_VECTOR (7 downto 0));
Warning: Default values on interface signals are not supported
port (X:BIT_VECTOR; F:out BIT );
Error: An index range must be specified for this data type
begin assert (D'LENGTH <= Q'LENGTH)
report "D wider than output Q" severity Failure;
34 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
Warning: Assertion statements are ignored
Error: Statements in entity declarations are not supported
if CLR = '1' then St := (others => '0'); Q <= St after TCQ;
Error: Illegal use of aggregate with the choice "others": the
derived subtype of an array aggregate that has a choice "others" must
be a constrained array subtype
signal SRA, SRB, ADDout, MUXout, REGout: BIT_VECTOR(7 downto 0);
Warning: Name is reserved word in VHDL-93: sra
signal Zero, Init, Shift, Add, Low: BIT := '0'; signal High: BIT :=
'1';
Warning: Initial values on signals are only for simulation and
setting the value of undriven signals in synthesis. A synthesized
circuit can not be guaranteed to be in any known state when the power
is turned on.
12.9.1 Messages During Synthesis
Key terms and concepts: error and warning messages during synthesis
These unused instances are being removed: in full_adder_p_dup8: u5,
u2, u3, u4
These unused instances are being removed: in dffclr_p_dup1: u2
architecture Behave of DFFClr is --1
signal Qi : BIT; --2
begin QB <= not Qi; Q <= Qi; --3
process (CLR, CLK) begin --4
if CLR = '1' then Qi <= '0' after TRQ; --5
elsif CLK'EVENT and CLK = '1' then Qi <= D after TCQ; --6
end if; --7
end process; --8
end; --9
A1:Adder8 port map(A=>SRB,B=>REGout,Cin=>Low,Cout=>OFL,Sum=>ADDout);
Cout <= (X and Y) or (X and Cin) or (Y and Cin) after TC;
ASICs... THE COURSE 12.10 The Engine Controller 35
12.10 The Engine Controller
Key terms and concepts: warnings and errors during optimization ? unassigned or uninitialized
variables
Warning: Made latches to store values on: net d(4), d(5), d(6), d(7),
d(8), d(9), d(10), d(11), in module fifo_control
case sel is
when "01" => D <= D_1 after TPD; r1 <= '1' after TPD;
when "10" => D <= D_2 after TPD; r2 <= '1' after TPD;
when "00" => D(3) <= f1 after TPD; D(2) <= f2 after TPD;
D(1) <= e1 after TPD; D(0) <= e2 after TPD; -- Bad!
when others => D <= "ZZZZZZZZZZZZ" after TPD;
end case;
when "00" => D(3) <= f1 after TPD; D(2) <= f2 after TPD; -- Write
D(1) <= e1 after TPD; D(0) <= e2 after TPD; -- to
D(11 downto 4) <= "ZZZZZZZZ" after TPD; -- all bits.
12.11 Performance-Driven Synthesis
Key terms and concepts: use of directives and pseudocomments ? timing arcs (or timing paths)
? a pathcluster (a group of circuit nodes) ? required time for a signal to reach the output nodes
(the end set) ? arrival time of the signals at all the inputs ? constrained delay ? timing constraint
? slack ?the timing constraint is met or violated
12.12 Optimization of the Viterbi Decoder
Key terms and concepts: set the environment using worst-case conditions ? die temperature of
25°C (fastest logic) to 120°C (slowest logic) ? power supply voltage of VDD =5.5V (fastest logic)
to VDD =4.5V (slowest logic) ? worst process (slowest logic) to best process (fastest logic)
36 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
12.13 Summary
Key terms and concepts: A logic synthesizer may contain over 500,000 lines of code ? danger of
the “garbage in, garbage out” syndrome ? “What do I expect to see at the output?” ? “Does the
output make sense?” ? the worst thing you can do is write and simulate a huge amount of code,
read it into the synthesis tool, and try and optimize it all at once with the default settings ? inter-
connect delay is increasingly dominant ? it is important to begin physical design as early as
possible ? ideally floorplanning and logic synthesis should be completed at the same time
Timing constraints
(a) A pathcluster
(b) Defining constraints
a[1]b[1]
a[0]b[0]
outp[2]outp[1]
outp[0]
a[2]b[2]
arrival time=0nstiming arcs
pathclusterconstraint
comparator/MUX
start set + end set = pathclusterstart set end set
all inputs pathclusterconstraint
outp[0]// comp_mux.vmodule comp_mux(a, b, outp); input [2:0] a, b; output [2:0] outp;function [2:0] compare; input [2:0] ina, inb;
begin if (ina <= inb) compare = ina; else compare = inb;end
endfunction assign outp = compare(a, b);endmodule > set pathcluster pc1> set requiredTime 2 outp[0] outp[1] outp[2]
-pathcluster pc1> set arrivalTime 0 * -pathcluster pc1
required time =2ns
example:
(a) (b)
ASICs... THE COURSE 12.13 Summary 37
`timescale 1ns / 10ps
module comp_mux_o (a, b, outp);
input [2:0] a; input [2:0] b;
output [2:0] outp;
supply1 VDD; supply0 VSS;
mx21d1 B1_i1 (.I0(a[0]), .I1(b[0]),
.S(B1_i6_ZN), .Z(outp[0]));
oa03d1 B1_i2 (.A1(B1_i9_ZN), .A2(a[2]),
.B1(a[0]), .B2(a[1]), .C(B1_i4_ZN),
.ZN(B1_i2_ZN));
nd02d0 B1_i3 (.A1(a[1]), .A2(a[0]),
.ZN(B1_i3_ZN));
nd02d0 B1_i4 (.A1(b[1]), .A2(B1_i3_ZN),
.ZN(B1_i4_ZN));
mx21d1 B1_i5 (.I0(a[1]), .I1(b[1]),
.S(B1_i6_ZN), .Z(outp[1]));
oa04d1 B1_i6 (.A1(b[2]), .A2(B1_i7_ZN),
.B(B1_i2_ZN), .ZN(B1_i6_ZN));
in01d0 B1_i7 (.I(a[2]), .ZN(B1_i7_ZN));
an02d1 B1_i8 (.A1(b[2]), .A2(a[2]),
.Z(outp[2]));
in01d0 B1_i9 (.I(b[2]), .ZN(B1_i9_ZN));
endmodule
The comparator/MUX example after logic optimization with timing constraints
The structural netlist, comp_mux_o2.v, and its derived schematic
b[2]b[1]
a[1]a[0]
b[2]
a[2]a[2]a[0]a[1]
a[1]b[1]b[2]a[2]a[0]b[0]
outp[2] outp[1]outp[0]1 0 1 0
INV(in01d0)OAI221
(oa03d0)
OAI21(oa04d0)
AND2(an02d1) MUX(mx21d1)
NAND2(nd02d0) INV(in01d0)
NAND2(nd02d0)
38 SECTION 12 LOGIC SYNTHESIS ASICS... THE COURSE
