Verilog_Overview.pdf

Verilog modeling* for
synthesis of ASIC designs
* for native speakers of VHDL
ELEC 4200
Victor P. Nelson

Hardware Description Languages
• Verilog – created in 1984 by Philip Moorby of Gateway Design
Automation (merged with Cadence)
• IEEE Standard 1364-1995/2001/2005
• Based on the C language
• Verilog-AMS – analog & mixed-signal extensions
• IEEE Std. 1800-2012 “System Verilog” – Unified hardware design, spec,
verification
• VHDL = VHSIC Hardware Description Language
(VHSIC = Very High Speed Integrated Circuits)
• Developed by DOD from 1983 – based on ADA language
• IEEE Standard 1076-1987/1993/2002/2008
• VHDL-AMS supports analog & mixed-signal extensions

HDLs in Digital System Design
• Model and document digital systems
• Behavioral model
• describes I/O responses & behavior of design
• Register Transfer Level (RTL) model
• data flow description at the register level
• Structural model
• components and their interconnections (netlist)
• hierarchical designs
• Simulation to verify circuit/system design
• Synthesis of circuits from HDL models
• using components from a technology library
• output is primitive cell-level netlist (gates, flip flops, etc.)

Verilog Modules
module small_block (a, b, c, o1, o2);
input a, b, c;
output o1, o2;
wire s;
assign o1 = s | c ; // OR operation
assign s = a & b ; // AND operation
assign o2 = s ^ c ; // XOR operation
endmodule
I/O port direction declarations
Logic functions
The module is the basic Verilog building block
Module name List of I/O signals (ports)
Internal wire (net) declarations
(Keywords in bold)

Lexical conventions
• Whitespaces include space, tab, and newline
• Comments use same format as C and C++:
// this is a one line comment to the end of line
/* this is another single line comment */
/* this is a multiple
line comment */
• Identifiers: any sequence of
• letters (a-z, A-Z), digits (0-9), $ (dollar sign) and _ (underscore).
• the first character must be a letter or underscore
Identifier_15, adder_register, AdderRegister
• Verilog is case sensitive (VHDL is case insensitive)
Bob, BOB, bob // three different identifiers in Verilog
• Semicolons are statement delimiters; Commas are list separators

Verilog module structure
module module_name (port list);
port and net declarations (IO plus wires and regs for internal nodes)
input, output, inout - directions of ports in the list
wire: internal “net” - combinational logic (needs a driver)
reg: data storage element (holds a value – acts as a “variable”)
parameter: an identifier representing a constant
functional description
endmodule

Module “ports”
• A port is a module input, output or both
module full_adder (ai, bi, cini, si, couti);
input ai, bi, cini; //declare direction and type
output si, couti; //default type is wire
• Verilog 2001: Signal port direction and data type can be combined
module dff (d, clk, q, qbar); //port list
input d, clk;
output reg q, qbar; // direction and type
• Verilog 2001: Can include port direction and data type in the port list (ANSI C format)
module dff (input d,
input clk,
output reg q, qbar);

Data types
• Nets connect components and are continuously assigned values
• wire is main net type (tri also used, and is identical)
• Variables store values between assignments
• reg is main variable type
• Also integer, real, time variables
• Scalar is a single value (usually one bit)
• Vector is a set of values of a given type
• reg [7:0] v1,v2; //8-bit vectors, MSB is highest bit #
• wire [1:4] v3; //4-bit vector, MSB is lowest bit #
• reg [31:0] memory [0:127]; //array of 128 32-bit values
• {v1,v2} // 16-bit vector: concatenate bits/vectors into larger vector

Logic values
• Logic values: 0, 1, x, z x = undefined state
z = tri-state/floating/high impedance
0 1 x z
0 0 x x 0
1 x 1 x 1
x x x x x
z 0 1 x z
wire
Multiple drivers
of one wire
A
B
A
B
State of the net
Analagous to VHDL
std_logic values
‘0’ ‘1’ ‘X’ ‘Z’

Numeric Constants
• Numbers/Vectors: (bit width)‘(radix)(digits)
Verilog: VHDL: Note:
4’b1010 “1010” or B“1010” 4-bit binary value
12’ha5c X“0a5c” 12-bit hexadecimal value
6’o71 O“71” 6-bit octal value
8’d255 255 8-bit decimal value
255 255 32-bit decimal value (default)
16’bZ x”ZZZZ” 16-bit floating value
6’h5A x”5A“ 6-bit value,upper bits truncated
10’h55 10-bit value, zero fill left bits
10’sh55 10-bit signed-extended value
-16’d55 16-bit negative decimal (-55)

Equating symbols to constants
• Use ‘define to create global constants (across modules)
‘define WIDTH 128
‘define GND 0
module (input [WIDTH-1:0] dbus)
…
• Use parameter to create local constants (within a module)
module StateMachine ( )
parameter StateA = 3’b000; parameter StateB = 3’b001;
…
always @(posedge clock)
begin
if (state == StateA)
state <= StateB; //state transition

Verilog module examples
// Structural model of a full adder
module fulladder (si, couti, ai, bi, cini);
input ai, bi, cini;
output si, couti;
wire d,e,f,g;
xor (d, ai, bi);
xor (si, d, cini);
and (e, ai, bi);
and (f, ai, cini);
and (g, bi, cini);
or (couti, e, f, g);
endmodule
// Dataflow model of a full adder
input ai, bi, cini;
output si, couti;
assign si = ai ^ bi ^ cini;
// ^ is the XOR operator in Verilog
assign couti = ai & bi | ai & cini | bi & cini;
// & is the AND operator and | is OR
endmodule
// Behavioral model of a full adder
input ai, bi, cini;
output si, couti;
assign {couti,si} = ai + bi + cini;
endmodule
Gate
instances
Continuous
driving of a
net

Operators (in increasing order of precedence*):
|| logical OR
&& logical AND
| bitwise OR ~| bitwise NOR
^ bitwise XOR ~^ bitwise XNOR
& bitwise AND ~& bitwise NAND
== logical equality !== logical inequality
< less than <= less than or equal
also > greater than >= greater than or equal
<< shift left >> shift right
+ addition - subtraction
* multiply / divide % modulus
*Note that: A & B | C & D is equivalent to: (A & B) | (C & D)
A * B + C * D is equivalent to: (A * B) + (C * D)
Preferred forms - emphasizing precedence

Unary operators:
Examples:
! logical negation
~ bitwise negation ~4’b0101 is 4’b1010
& reduction AND & 4’b1111 is 1’b1
~& reduction NAND ~& 4’b1111 is 1’b0
| reduction OR | 4’b0000 is 1’b0
~| reduction NOR ~| 4’b0000 is 1’b1
^ reduction XOR ^ 4’b0101 is 1’b0
~^ reduction XNOR ~^4’b0101 is 1’b1
reduction operator
is applied to bits of a
vector, returning a
one-bit result

Combining statements
// Wire declaration and subsequent signal assignment
wire a;
assign a = b | (c & d);
// Equivalent to:
wire a = b | (c & d);

Examples: 2-to-1 multiplexer
// function modeled by its “behavior”
module MUX2 (A,B,S,Z);
input A,B,S; //input ports
output Z; //output port
always //evaluate block continuously
begin
if (S == 0) Z = A; //select input A
else Z = B; //select input B
end
endmodule
// function modeled as a logic expression
output Z; //output port
assign Z = (~S & A) | (S & B); //continuous evaluation
endmodule
A, B, Z could
also be vectors
(of equal # bits)
Using conditional operator:
assign Z = (S == 0) ? A : B;
True/false
condition
if true : if false

Multi-bit signals (vectors)
// Example: 2-to-1 MUX with 4-bit input/output vectors
module MUX2ARR(A,B,S,Z);
input [3:0] A,B; // whitespace before & after array declaration
input S;
output [3:0] Z; // little-endian form, MSB = bit 3 (left-most)
reg [0:3] G; // big-endian form, MSB = bit 0 (left-most)
always
begin
if (S == 0) G = A; //Select 4-bit A as value of G
else G = B; //Select 4-bit B as value of G
end
assign Z = G;
endmodule
A,B,Z,G analagous to
VHDL std_logic_vector

Examples: 4-to-1 multiplexer
module MUX2 (A,B,C,D,S,Z1,Z2);
input A,B,C,D; //mux inputs
input [1:0] S; //mux select inputs
output Z; //mux output
always //evaluate block whenever there are changes in S,A,B,C,D
begin //if-else form
if (S == 2’b00) Z1 = A; //select input A for S=00
else if (S == 2’b01) Z1 = B; //select input B for S=01
else if (S == 2’b10) Z1= C; //select input C for S=10
else if (S == 2’b11) Z1 = D; //select input D for S=11
else Z1 = x; //otherwise unknown output
end
//assign statement using the conditional operator (in lieu of always block)
assign Z2 = (S == 2’b00) ? A: //select A for S=00
(S == 2’b01) ? B: //select B for S=01
(S == 2’b10) ? C: //select C for S=10
(S == 2’b11) ? D: //select D for S=11
x; //otherwise default to x
endmodule
//equivalent case statement form
case (S)
2’b00: Z1 = A;
2’b01: Z1 = B;
2’b10: Z1 = C;
2’b11: Z1 = D;
default: Z1 = x;
endcase
Synthesis may insert latches
when defaults not specified.

Hierarchical structure of 4-to-1 MUX
(using the previous 2-to-1 MUX)
module MUX4 (A,B,c,d,S0,S1,Z);
input A,B,c,d,S0,S1;
output Z;
wire z1,z2;
MUX2 M1(A,B,S0,z1); //instance M1 of MUX2
MUX2 M2(c,d,S0,z2); //instance M2 of MUX2
MUX2 M3(.S(S1), .Z(Z), .A(z1),.B(z2)); //connect signal to port: .port(signal)
endmodule
Define MUX2 module in Verilog source before compiling MUX4 module
A
B
c
d
S1
S0
Z
z1
z2
// more descriptive, less error-prone

Procedural statements and blocks
• A procedure can be an: always block, initial block, function, task
• Define functionality in an algorithmic manner
• Insert multiple procedural statements between begin .. end keywords
• A block contains one or more “procedural statements”
• initial block
• Executes immediately at start of simulation
• Executes one time only
• Used primarily to initialize simulation values (rather than for synthesis)
• always block
• Executes as an “infinite loop”
• Executes immediately at start of simulation
• Executes again whenever “enabled”
• Enablement can result from time delay, signal change, signal state, etc.
See previous adder/multiplexer examples.

Example: generating a clock
wire clk;
initial //execute once – at start of simulation
begin
clk <= 0; //initial state of clk
reset <= 0; //initial state of reset line
#10 reset <= 1; //delay until time 10, and set reset to 1
#10 reset <= 0; //delay until time 20, and set reset back to 0
end
always //execute as infinite loop, beginning at start of simulation
begin
#10 clk <= ~clk; //suspend loop for 10 time units, toggle clk, and repeat
end
If a block contains a single procedural statement, begin-end can be omitted.

module bin_2_7seg (seg7, hexval);
input [2:0] hexval;
output [6:0] seg7;
reg [6:0] seg7;
always @(hexval) begin //any change in hexval initiates execution
case (hexval)
3'b000: seg7 = 7'b1000000; //0
3'b001: seg7 = 7'b1111001; //1
3'b010: seg7 = 7'b0100100; //2
3'b011: seg7 = 7'b0110000; //3
3'b100: seg7 = 7'b0011001; //4
3'b101: seg7 = 7'b0010010; //5
3'b110: seg7 = 7'b0000010; //6
3'b111: seg7 = 7'b1111000; //7
endcase
end
endmodule
ELEC 4200 Lab 2: Binary to Seven-Segment Display Driver

Enabling a procedural block with a clock
@ (posedge CLK) wait for rising edge of CLK (0->1, 0->X, X->1)
@ (negedge CLK) wait for falling edge of CLK (1->0, 1->X, X->0)
@ (CLK) wait for either edge of CLK
//Example: simple rising-edge triggered flip-flop:
always @ (posedge CLK) //wait for rising CLK edge
begin
Q <= D; //Q changes on rising edge
end
//Example: falling-edge triggered flip-flop with sync preset and clock enable:
always @ (negedge CLK)
begin
if (PR == 1) Q <= 1; //synchronous set
else if (CE == 1) Q <= D; //clock enable
end
Analagous to VHDL
process with CLK in
sensitivity list

DFF example – with asynchronous reset
module dff (q,d,clk,reset)
input d,clk,reset;
output q;
reg q; //”reg” since q stores the flip flop state
//can combine above two lines: output reg q;
always @(posedge clk or posedge reset) //sensitive to clk or reset change
if (reset)
q <= 1’b0; //load prevented if reset active
else
q <= d; //load if rising clk edge when reset not active
endmodule

DFF example – with synchronous reset
module dff (q,d,clk,reset)
output reg q; //”reg” since q stores the flip flop state
input d, clk, reset;
always @(posedge clk) //sensitive to rising edge of clk
if (reset) //reset takes precedence over load
q <= 1’b0;
else //load if reset not active
q <= d;
endmodule

DFF-based register with asynchronous reset
module dreg (q,d,clk,reset)
input clk,reset;
input [31:0] d; //32-bit input
output reg [31:0] q; //32-bit register state
always @(posedge clk or posedge reset) //react to clk or reset change
if (reset) //reset takes precedence over clock
q <= 0; //load 32-bit constant 0
else //rising clk edge while reset=0
q <= d; //load 32-bit input
endmodule
//Use in RTL models

D latch (level sensitive)
// q and d could also be vectors
module d_latch (q,d,en)
input d,en;
output reg q; //q output holds a value
always @(en or d) //wait until en or d change
if (en) q <= d; //q becomes d while en=1
endmodule

Sensitivity list for combinational logic
output reg Z; //output port
always @(S or A or B) //evaluate block on any change in S,A,B
begin
if (S == 0) Z = A; //select input A
else Z = B; //select input B
end
endmodule
Alternative:
always @(*)
Where * indicates any
signal that might affect
another within this block.
All inputs to
the combinational
logic function

Timing control and delays**
‘timescale 1ns/10ps //time units/precision (s,ns,ps,fs), multiplier=1,10,100
Intra-assignment:
x = #5 y; //Equivalent to the following
hold = y; //capture y at t = 0
#5; //delay until t = 5
x = hold; //update x
Delayed assignment:
#5 x = y; //Equivalent to the following
#5; //delay from t = 0 until t = 5
x = y; //copy value of y to x
** Delays are ignored by synthesis tools

Blocking vs non-blocking assignments
• Blocking statements ( x = y; )
• Executed in order listed, delaying execution of next statement as specified
• Effects of one statement take effect before next statement executed
• Will not block execution of statements in parallel blocks
• Use for modeling combinational logic
• Non-blocking statements ( x <= y; )
• Schedule assignments without blocking other statements
• Execute next statement without waiting for first statement to execute
• Use for modeling sequential logic
//Blocking example
A = B; //block until after A changes
C = A + 1; //use “new” A value from above
//Non-blocking example
A <= B; //schedule A change and continue
C <= A + 1; //use “old” A value – not the new one

Blocking vs non-blocking examples
// Blocking example
x = 0; // x changes at t = 0
a = 1; // a changes at t = 0
#10 c = 3; // delay until t=10, then c changes
#15 d = 4; // delay until t=25, then d changes
e = 5; // e changes at t = 25
//Non-blocking example
x = 0; //execute at t = 0
a = 1; //execute at t = 0
c <= #15 3; //evaluate at t = 0, schedule c to change at t = 15
d <= #10 4; //evaluate at t = 0, schedule d to change at t = 10
c <= c + 1; //evaluate at t = 0, schedule c to change at t = 0

Example of blocking/non-blocking delays
initial begin
a = 1; b = 0; //block until after change at t=0
#1 b = 1; //delay until t=1; then block until b=1
c = #1 1; //block until t=2, c=val from t=1
#1; //delay to t=3
d = 1; //block until change at t=3
e <= #1 1; //non-blocking, update e at t=4
#1 f <= 1; //delay to t=4, non-blocking f update
g <= 1; //still t=4, non-blocking g update
end
Results:
t a b c d e f g
0 1 0 x x x x x
1 1 1 x x x x x
2 1 1 1 x x x x
3 1 1 1 1 x x x
4 1 1 1 1 1 1 1

Example
• Blocking: (a-b end up with same value – race condition)
a = b; //change a NOW
b = a; //change b to new a value
• Non-blocking: (a-b swap values)
a <= b; //read b at t=0, schedule a to change
b <= a; //read a at t=0, schedule b to change
D Q D Q
a b

Anext
Bnext Cnext
A_r B_r C_r
+1 +1
A B C
Clk
wire Anext, Bnext, Cnext;
reg A_r, B_r, C_r;
always @(posedge Clk) begin
A_r <= Anext; //load FFs
B_r <= Bnext; //after delay
C_r <= Cnext;
end;
//Comb logic with blocking stmts
assign Bnext = A_r + 1;
assign Cnext = B_r + 1;
wire Anext;
reg A_r, B_r, C_r;
always @(posedge Clk) begin
A_r <= Anext; //A_r to change later
B_r <= A_r + 1; //use “old” A_r
C_r <= B_r + 1; //use “old” B_r
end;
// Registers react to clock concurrently
// Adders modeled as combinational logic
// Registers react to clock concurrently
// “old values” at rising clock time, before outputs change
Non-blocking
examples

Arithmetic operations for RTL modeling
• Verilog recognizes standard arithmetic operators
• Synthesis tools will generate arithmetic circuits
module Adder_8 (A, B, Z, Cin, Cout);
input [7:0] A, B; //8-bit inputs
input Cin; //carry input bit
output [7:0] Z; //8-bit sum
output Cout; //carry output bit
assign {Cout, Z} = A + B + Cin; //extra output bit for carry
endmodule
The size of the operation is that of the largest operand (input or output).
In this example, the result is 9 bits, which is the size of {Cout,Z}.

Alternate adder example
module Adder_16 (A, B, Z, Cin, Cout);
input [31:0] A, B; //32-bit inputs
input Cin; //carry input bit
output [31:0] Z; //32-bit sum
output Cout; //carry output bit
wire [33:0] Temp; //34-bit temporary result
assign Temp = {1’b0,A,Cin} + {1’b0,B,1’b1}; //1 added to bit 1 if Cin=1
assign {Cout, Z} = Temp[33:1]; //Cout=bit 33, Sum=bits 32:1
endmodule

Optimizing circuits (1)
//synthesis tool infers two adders from the following
module Add1 (sel, a, b, c, d, y);
input a, b, c, d, sel; output reg y;
always @(sel or a or b or c or d) begin
if (sel == 0) //mux selects output
y <= a + b; //adder 1
else
y <= c + d; //adder 2
end
endmodule

Optimizing circuits (2)
//synthesis tool infers a single adder from the following
//indicate that a mux selects adder inputs
module Add2 (sel, a, b, c, d, y);
input a, b, c, d, sel; output y; reg t1, t2, y;
always @(sel or a or b or c or d) begin
if (sel == 0) //muxes select adder inputs
begin t1 = a; t2 = b; end
else
begin t1 = c; t2 = d; end
y = t1 + t2; //adder circuit
end
endmodule
Note use of blocking statements
to ensure desired adder inputs

Conditional statements
• if-else constructs
• like C, except that instead of open and close brackets { … } use keywords begin …
end to group multiple assignments associated with a given condition
• begin … end are not needed for single assignments
• case constructs
• similar to C switch statements, selecting one of multiple options based on values of a
single selection signal
• for (i = 0; I < 10; i = i + 1) statements
• repeat (count) statements //repeat statements “count” times
• while (abc) statements //repeat statements while abc “true” (non-0)

if-else example: 4-to-1 multiplexer
module MUX4 (A,B,C,D,S0,S1,Z);
input A,B,C,D,S0,S1;
output Z;
wire Z1,Z2;
always
begin
if ((S1 == 1’b0) && (S0 == 1’b0)) Z = A;
else if ((S1 == 1’b0) && (S0 == 1’b1)) Z = B;
else if ((S1 == 1’b1) && (S0 == 1’b0)) Z = C;
else Z = D;
end
endmodule

module tri_asgn (source, ce, wrclk, selector, result) ;
input [3:0] source ;
input ce, wrclk ;
input [1:0] selector ;
output reg result ;
reg [3:0] intreg;
tri result_int ; //tri (tri-state) is same as wire
// combine net declaration and assignment
wire [1:0] sel = selector;
// select one “intreg” bit to drive “result”
assign // (condition) ? true-result : false-result
result_int = (sel == 2’b00) ? intreg[0] : 1’bZ ,
result_int = (sel == 2’b11) ? intreg[3] : 1’bZ ;
//“if” statement
always @(posedge wrclk)
begin
if (ce)
begin
intreg = source ;
result = result_int ;
end
end
endmodule
Example:
tri-state bus driver

module MooreFSM (CLK,X,Z);
input CLK,X;
output Z;
reg [1:0] CS;
parameter SA = 2’b00 // define state A with binary value 00
parameter SB = 2’b01 // define state B with binary value 01
parameter SC = 2’b10 // define state C with binary value 10
// State transitions
always @ (posedge CLK)
begin
if (CS == SA) // IF-ELSE form
begin
if (X == 0) CS = SC;
else CS = SB;
end
else if (CS == SB)
begin
if (X == 0) CS = SA;
else CS = SC;
end
else
begin
if (X == 0) CS = SB;
else CS = SA;
end
end
SB
1
SC
1
SA
0
0
1
1
X=1
0 0
// Moore model output
always @ (CS)
begin //CASE statement form
case (CS) // CASE (selector)
SA: begin
Z = 1’b0;
end
SB: begin
Z = 1’b1;
end
SC: begin
Z = 1’b1;
end
endcase
end
endmodule
FSM modeling styles

/* ELEC 4200 Lab 4 – Moore model finite state machine */
module MooreFSM (RST, EN, Clock, OUT, C1, C0);
input RST, EN, Clock; output C1, C0;
output [3:0] OUT; reg [3:0] OUT;
parameter S0 = 4'b0001; parameter S1 = 4'b0010;
parameter S2 = 4'b0100; parameter S3 = 4'b1000;
always @(posedge Clock) begin
if (RST == 1) begin
OUT = S0; //reset to S0
end
else if (EN == 1) begin //state changes
case (OUT)
S0: OUT = S1;
S1: OUT = S2;
S2: OUT = S3;
S3: OUT = S0;
endcase
end
end
assign C1 = OUT[3] | OUT[2]; //Encode outputs
assign C0 = OUT[1] | OUT[3];
endmodule

/* ELEC 4200 Lab 5 – Universal 8-bit register/counter */
module Counter(CLK, RST, CE, M, Din, Q);
input CLK, RST, CE;
input [1:0] M;
input [7:0] Din;
output reg [7:0] Q;
always @(posedge CLK) begin
if (RST) begin
Q = 8'h00; //reset
end
else if (CE) begin //clock enable
if (M == 2'b01) Q = Q << 1; //shift
else if (M == 2'b10) Q = Q + 1; //count
else if (M == 2'b11) Q = Din; //load
end
end
endmodule

for loop – similar to C construct
// 32-bit full adder
always
begin
for (n=0; n<32; n++) // ripple carry form
begin
sum[n] = Ain[n] ^ Bin[n] ^ carry[n];
carry[n+1] = (Ain[n] & Bin[n]) | (Ain[n] & carry[n]) | (Bin[n] & carry[n]);
end
end

while loop – execute until while expression not true
reg [15:0] buffer [0:7];
integer k;
…
begin
k = 8;
while (k) //store data at posedge of next 8 clocks
begin
k = k - 1; //copy data to words 7-0 of buffer
@(posedge clock) buffer[k] = data;
end
end

repeat loop – repeat a fixed #times
parameter cycles = 8; // repeat loop counter for below
reg [15:0] buffer [0:7];
integer k;
…
begin
k = 0;
repeat (cycles) //store data at posedge of next 8 clocks
begin //fill buffer words 0-7 with data
@(posedge clock) buffer[k] = data;
k = k + 1;
end
end

Memory models
• Memory is an array of registers
reg [7:0] accumulator; //8-bit register
reg mem1bit [ 0:1023]; //array of bits
reg [7:0] membyte [0:1023] //array of bytes
mem1bit[511] - refers to one bit of memory
membyte[511] - refers to one byte of memory
accumulator[5] – refers to bit 5 of the accumulator register
accumulator[3:0] – refers to lower half of the accumulator register
• Additional dimensions: reg [7:0] mem [0..127][0..63]

/* Lab 6 - Register file */
`timescale 1ns / 1ns
module RegFile (ReadAddress, WriteAddress,WE, DataIn, DataOut);
input [3:0] ReadAddress, WriteAddress;
input [7:0] DataIn;
input WE;
output [7:0] DataOut;
reg [7:0] RF [0:15]; //16 8-bit registers
assign DataOut = RF[ReadAddress]; //continuous read
always @(WE) begin
if (WE)
RF[WriteAddress] = DataIn; //write register
end
endmodule

//-----------------------------------------------------------------------------
// Title Lab 6 test bench : RegFile_tb
//-----------------------------------------------------------------------------
`timescale 1ns / 1ns
module RegFile_tb;
//Internal signals declarations:
reg [3:0]ra;
reg [3:0]wa;
reg we;
reg [7:0]din;
wire [7:0]dout;
// Unit Under Test port map
RegFile UUT (
.ReadAddress(ra),
.WriteAddress(wa),
.WE(we),
.DataIn(din),
.DataOut(dout));
//continued on next slide

//testbench continued – stimulus for inputs
initial begin
we = 0;
ra = 4'b0000;
din = 8'd0;
for (wa = 4'h0; wa <= 4'hf; wa = wa + 1) begin //16 write operations
din = din + 5;
#5 we = 1; //we pulse = 5ns
#5 we = 0; //we period = 10ns
end
din = 8’d0; //expected read-back value
for (ra = 4'h0; ra <= 4'hf; ra = ra + 1) begin //read the 16 registers
din = din + 5; //next value that was written
#10; //read time 10ns
if (dout !== din) $display (“ERROR dout not correct. ”);
end
$finish;
end
endmodule

Producing a clock signal
initial x = 0; //set initial value
always begin //block is repeated (assume t=0 initially)
#25 x = 1; //delay to t=25, then continue by assigning x=1
#10 x = 0; //delay to t=35, then continue by assigning x=0
#5; //delay to t=40, then continue
end
40
25 10 5

Example – D flip flop
module example
reg Q, Clk;
wire D;
assign D = 1; //D=1 for this example
always @(posedge Clk) Q = D; //normal flip flop clocking
initial Clk = 0; //initial state of Clk reg
always #10 Clk = ~Clk; //toggle clock for period of 20
initial begin
#50;
$finish; //simulation control – end simulation
end
always begin
$display(“T=“,%2g, $time,” D=“,D,” Clk =“,Clk,” Q=“,Q); //generate output listing every 10 time units
#10;
end
endmodule

Verilog built-in primitive gates
• Verilog has 8 gate types that are primitive components:
and, or, nand, nor, xor, xnor, not, buf
• Format:
gate INSTANCE_NAME (Z,I1,I2,…IN); // list output first, followed by inputs
module carry_out(A,B,Cin,Cout)
input A,B,Cin;
output Cout;
wire w1,w2,w3;
and A1 (w1,A,B); //primitive and gate instances
and A2 (w2,A,Cin);
and A3 (w3,B,Cin);
or O1 (Cout,w1,w2,w3); //primitive or gate instance
endmodule

Lists of assign/gate instance statements
• Can specify a comma-separated list of gates of one type
• Likewise for “assign” statements
module carry_out(A,B,Cin,Cout)
input A,B,Cin;
output Cout;
wire w1,w2,w3,w4,w5;
and A1 (w1,A,B), // list of three and gate instances
A2 (w2,A,Cin),
A3 (w3,B,Cin);
assign w4 = w1 & w2, // list of two assign statements
Cout = w4 & w3;
endmodule

Specifying delays
• Net delays:
assign #8 a = b & c; //a changes 8 time units after b/c change
wire #8 a = b & c; //equivalent to the following statement pair
wire a;
assign #8 a = b & c;
//above also equivalent to the following statement pair
wire #8 a; //8 units of delay to any assignment to net a
assign a = b & c;
Logic gate delays:
nand #5 N1(out1,in1,in2); //delay of 5 for any change at gate output
nand #(3,5) N2(out2,in1,in2); //output rising delay=3, falling delay=5
nand #(3,5,7) N3(out3,in1,in2); //rising delay=3, falling delay=5, delay to hi-Z=7

Allow for process variations and loading
• Triplet of delay values: (minimum : typical : maximum)
// triplet of net delays
#(1.1 : 1.4 : 1.5) assign delay_a = a;
// triplet of nand gate rise, fall times
nand #(1:2:3, 2:3:4) N1(out1, in1, in2);
// 3 triplets of buffer delays, for rise, fall and change to hi-Z state
buf #(1:2:3, 2:3:4, 4:5:6) B1(out2, in3);

Specify block for pin-to-pin delays
module DFF (Q, clk, D, pre, clr);
input clk,D,pre,clr; output Q;
DFlipFlop(Q, clk, D) ; //previously-defined D flip flop module
specify specparam
tPLH_clk_Q = 3, tPHL_clk_Q = 2.9;
tPLH_set_Q = 1.2, tPHL_set_Q = 1.1;
(clk => Q) = (tPLH_clk_Q,tPHL_clk_Q); //=> clk to Q (rise,fall)
(pre,clr *> Q) = (tPLH_set_Q,tPHL_set_Q); //*> each input to each output
end specify
endmodule

Verilog simulation commands
$finish //stop simulation
$time //current simulation time
$display(“Some text to be printed to listing window”);
$monitor(“T=“, $time, ” A=“, A, “ B=“, B); //print text & signals to list window
T=0 A=1 B=x //similar to C printf statement
T=5 A=0 B=1
…
$dumpvars //

Signal strengths
• Verilog allows a signal strength to be associated with a logic state
Strength Abbreviation Value Meaning
Supply Su 7 Power supply
Strong St 6 Default gate drive (default)
Pull Pu 5 Pull up/down
Large La 4 Large capacitance
Weak We 3 Weak drive
Medium Me 2 Medium capacitance
Small Sm 1 Small capacitance
High Z Hi 0 High impedance
(Similar to IEEE 1164 std_logic for VHDL)
Examples: strong1, pull1, supply0, Su1, Su0, etc.

User-defined primitive gates (combinational)
• Define own primitive “gates” via a truth table
primitive Adder (Sum, InA, InB);
output Sum; input InA, InB;
table
// inputs : output
00 : 0;
01: 1;
10: 1;
11 : 0;
endtable
endprimitive

User-defined primitive gates (sequential)
• Include a “state” in the primitive truth table between input & output
primitive Dlatch (Q, Clock, Data);
output Q; reg Q; input Clock, Data;
table
// inputs : present state : output (next state)
1 0 : ? : 0 ; // ? Represents 0, 1, or x
1 1 : b : 1 ; // b Represents 0 or 1 input
1 1 : x : 1 ; // can combine with previous line
0 1 : ? : - ; // - represents no change in output
endtable
endprimitive

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