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A Brief Intro to Verilog
Brought to you by: Sat Garcia
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Meet your 141(L) TA
 Sat Garcia
sat@cs.ucsd.edu
2nd Year Ph.D. Student
Office Hours: (Tentative)
 Place: EBU3b B225 (basement)
 Monday: 3-4pm
 Wednesday: 11am-Noon
 Please come to my office hours. I get lonely there
by myself!
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What is Verilog?
 Verilog is:
A hardware design language (HDL)
Tool for specifying hardware circuits
Syntactically, a lot like C or Java
An alternative to VHDL (and more widely used)
What you'll be using in 141L
HELLA COOL!*
* If you are totally into hardware design languages
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Verilog in the Design Process
Behavioral
Algorithm
Register
Transfer Level
Gate Level
Manual
Logic Synthesis
Auto Place + Route
Test
Results
Simulate
Test
Results
Simulate
Test
Results
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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Ways To Use Verilog
 Structural Level
 Lower level
 Has all the details in it (which gates to use, etc)
 Is always synthesizable
 Functional Level
 Higher Level
 Easier to write
 Gate level, RTL level, high-level behavioral
 Not always synthesizable
 We’ll be sticking with functional mostly
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Data Types in Verilog
 Basic type: bit vector
Values: 0, 1, X (don't care), Z (high
impedence)
 Bit vectors expressed in multiple ways:
binary: 4'b11_10 ( _ is just for readability)
hex: 16'h034f
decimal: 32'd270
other formats but these are the most useful
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Data types (continued)
 Connect things together with: wire
Single wire:
 wire my_wire;
“Array” of wires
 wire[7:0] my_wire;
 Why not wire[0:7]?
 For procedural assignments, we'll use reg
Again, can either have a single reg or an array
 reg[3:0] accum; // 4 bit “reg”
reg is not necessarily a hardware register
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A simple example (comb. circuit)
 Let's design a 1 bit full adder
FA
ba
s
cin
cout
module FA( input a, b, cin,
output s, cout);
assign s = a ^ b ^ c;
assign cout = (a & b) | (a & cin) | (b & cin);
endmodule
 Ok, but what if we want more than 1 bit FA?
*** Note: red means new concept, blue and
green are just pretty colors :-p
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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A 4-bit Full Adder
 We can use 1 bit FA
to build a 4 bit full
adder
module 4bitFA( input [3:0] A, B, input cin,
output [3:0] S, output cout);
wire c0, c1, c2;
FA fa0(A[0],B[0],cin,S[0],c0); // implicit binding
FA fa1(.a(A[1]), .b(B[1]), .cin(c0), .s(S[1]), .cout(c1)); // explicit binding
FA fa2(A[2],B[2],c1,S[2],c2);
FA fa3(A[3],B[3],c2,S[3],cout);
endmodule
FA FA FA FA
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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Testing the adder
`timescale 1ns/1ns // Add this to the top of your file to set time scale
module testbench();
reg [3:0] A, B;
reg C0;
wire [3:0] S;
wire C4;
4bitFA uut (.B(B), .A(A), .cin(C0), .S(S), .cout(C4)); // instantiate adder
initial // initial blocks run only at the beginning of simulation (only use in testbenches)
begin
$monitor($time,"A=%b,B=%b, c_in=%b, c_out=%b, sum = %b\n",A,B,C0,C4,S);
end
initial
begin
A = 4'd0; B = 4'd0; C0 = 1'b0;
#50 A = 4'd3; B = 4'd4; // wait 50 ns before next assignment
#50 A = 4'b0001; B = 4'b0010; // don’t use #n outside of testbenches
end
endmodule
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Verilog RTL Operators
 Avoid using %, **, and / because you'll run
into problems when trying to synthesis
~ & | ^ ^~Bitwise
== != ===
!===
Equality
> < >= <=Relational
! && ||Logical
+ - * / % **Arithmetic
& ~& | ~| ^
^~
Reduction
>> << >>> <<<Shift
{ }Concatenation
?:Conditional
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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A simple D flip flop (seq. circuit)
 For sequential circuits, use always blocks
 Always blocks (and assign) are executed in
parallel!
module DFF( input clk, d,
output q, q_bar);
reg q, q_bar;
always @ (posedge clk) // triggered on the rising edge of the clock
begin
q <= d; // non-blocking assignment (LHS not updated until later)
q_bar <= ~d;
/* q_bar <= ~q will not function correctly! */
end
endmodule
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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Always blocks in comb. circuits
 Can use continual assignment AND always
blocks for combinational circuits
 Our 1-bit adder using always block
module FA( input a, b, cin,
output s, cout);
reg s, cout; // when using always block, LHS must be reg type
always @ ( a or b or cin ) // for comb circuits, sensitive to ALL inputs
begin
s = a ^ b ^ cin; // use blocking assignment here (LHS immediately)
cout = (a & b) | (a & cin) | (b & cin);
end
endmodule
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Quick Note on blocking vs. non-
blocking
 Order of blocking statements matter
 These are not the same
 Order of non-blocking statements doesn’t
 These are the same
 Use non-blocking with sequential, blocking with
combintational
c = a + b;
d = c + e;
d = c + e;
c = a + b;
c <= a + b;
d <= c + e;
d <= c + e;
c <= a + b;
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Tips for maintaining
synthesizability
 Only leaf modules should have functionality
 All other modules are strictly structural, i.e., they only wire together
sub-modules
 Use only positive-edge triggered flip-flops for state
 Do not assign to the same variable from more than one
always block
 Separate combinational logic from sequential logic
 Avoid loops like the plague
 Use for and while loops only for test benches
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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Another Example (4 input MUX)
 We can use case
statements within
an always block
module mux4( input a, b, c, d,
input [1:0] sel,
output out );
reg out;
always @( * )
begin
case ( sel )
2’d0 : out = a;
2’d1 : out = b;
2’d2 : out = c;
2’d3 : out = d;
default : out = 1’bx;
endcase
end
endmodule
Adapted from Arvind & Asanovic’s MIT 6.375 lecture
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Finite State Machines (FSMs)
 Useful for designing many different types of
circuits
 3 basic components:
 Combinational logic (next state)
 Sequential logic (store state)
 Output logic
 Different encodings for state:
 Binary (min FF’s), Gray, One hot (good for FPGA),
One cold, etc
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A simple FSM in Verilog
module simple_fsm( input clk, start,
output restart);
reg [1:0] state, next_state;
parameter S0 = 2’b00, S1 = 2’b01, S2 = 2’b10; // binary encode
always @ (*)
begin : next_state_logic
case ( state )
S0: begin
if ( start ) next_state = S1;
else next_state = S0;
end
S1: begin next_state = S2; end
S2: begin
if (restart) next_state = S0;
else next_state = S2;
end
default: next_state = S0;
endcase
end // continued to the right
// continued from left
always @ (posedge clk)
begin: state_assignment
state <= next_state;
end
endmodule
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Tips on FSMs
 Don’t forget to handle the default case
 Use two different always blocks for next state
and state assignment
 Can do it in one big block but not as clear
 Outputs can be a mix of combin. and seq.
 Moore Machine: Output only depends on state
 Mealy Machine: Output depends on state and inputs
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Next step: design your own HW
 Now that you have the basics…
Check out MIT’s 6.375 course webpage
 Thanks to Asanovic & Arvind for slides
 Lectures 2 and 3 on Verilog
 http://csg.csail.mit.edu/6.375/handouts.html
Try making some simple circuits
Beware when Googling for “verilog tutorial”
 A lot of code out there isn’t synthesizable