Transcript Intro to Verilog

GWU – ECE 2140
Spring 2012
Revised by Scott Trocchia
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What are FPGAs? What’s inside them?
Brief Verilog History
What is Verilog?
Levels of Verilog Coding
Modules
Lots of Operators
Verilog Modeling
◦ Always blocks
◦ Continuous assignments
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FPGA =
Field-Programmable Gate Array
◦ Include space for lots of logic gates
◦ Can be programmed…
 …and reprogrammed ~100,000 times
◦ Benefits include:
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Cheap
Easy to program (barring errors)
Short amount of time for commercialization
etc.
A: This
Let’s go back to 1984…
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A world without Verilog…
◦ … was boring
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In 1984, the language was created by
Gateway Design Automation, Inc.
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Hardware Description Language
◦ Not meant to be understood for your operating
system (Windows, Mac, Linux)
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Used to describe digital systems, such as
◦ Register (memory)
◦ CPU
◦ Network switch
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Built-in functions: not, and, nand, nor, or,
xor, xnor, buf, …
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Behavioral level
◦ A functional representation
◦ How does my circuit work?
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Register-Transfer Level (RTL)
◦ How is data transferred from inputs to outputs?
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Gate Level
◦ What gates are contained within my circuit?
module half_adder(x, y, sum, carry);
module half_adder(A, B, Sum, C_out);
input x;
input y;
output sum;
output carry;
input A;
input B;
output Sum;
output C_out;
assign sum = x ^ y;
assign carry = x & y;
xor(Sum, A, B);
and(C_out, A, B);
endmodule
endmodule
module Multiplexer(In, Select, Out);
input [3:0] In;
input [1:0] Select;
output reg Out;
always @ (*) begin
case (Select)
2'b00: Out <=
2'b01: Out <=
2'b10: Out <=
2'b11: Out <=
endcase
end
endmodule
In[0];
In[1];
In[2];
In[3];
Verilog is case-sensitive,
so be mindful!
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reg – register – stores a value
wire – used for connecting logic
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Number representation
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◦ Typical format is:
(#bits)’(RADIX)(number)
 RADIX: b=binary, h=hex, d=decimal
◦ Example: 4’b1110 = 4’hE = 4’d14
◦ Negative numbers in 2’s complement
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Arithmetic
Logical
Relational
Equality
Reduction
Shift
Concatenation
Conditional
Symbol Arithmetic
operation
*
Multiply
/
Divide
+
Add
Subtract
%
Modulus
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Relational operators
◦ Same as C
◦ a < b, a > b, a <= b, a >= b
◦ 1-bit result: 0 if false, 1 if true
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Equality operators
◦ a == b
◦ a != b
◦ Compared bit-by-bit
◦ 1-bit result: 0 (false), 1 (true)
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Logical operators
operator Description
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!
Logical
negation
&&
Logical and
||
Logical or
Bit-wise operators
operator
Description
~
NOT
&
AND
|
OR
^
XOR
^~ or ~^
XNOR
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Shift
– Left shift <<
– Right shift >>
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Concatenation
– {2’b10, 2’b01} equals 4’b1001
– reg
A =
B =
C =
C =
A,B,C;
1'b0;
2'b11;
{B,A};
{A,B};
Will these operations give different results?
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Conditional Operators
◦ Conditional_expression ? True_expr : false expr
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Example:
◦ x = 0; q = 0
◦ (if x < 0) ? (q = 0) : (q = 1);
Part 2
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Executes loop over and over
Can only assign to registers in always blocks
2 types
– Level triggered - latch
– Edge triggered – flip-flop
always @ (posedge Clk) begin
if (Reset) begin
data <= 0;
end
else begin
data <= q_in;
end
end
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Always blocks can be used for combinational
logic too
always @ (*) begin
case (Select)
2'b00: out <= A;
2'b01: out <= B;
2'b10: out <= C;
2'b11: out <= D;
endcase
end
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Sequential assignment inside block
◦ Remember, this is HARDWARE
◦ Example:
 assume A=B=0 before always block executes
always @ (posedge Clk) begin
A <= 1;
B <= A;
end
 What will B equal after always block execution?
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Can only be assigned to wires
assign A = B^Y;
assign C = Sel ? TrueVal : FalseVal;
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???
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http://en.wikipedia.org/wiki/Fieldprogrammable_gate_array
http://www.asicworld.com/verilog/intro1.html#Introduction
http://en.wikipedia.org/wiki/Verilog
“Verilog – Representation of Number Literals”,
http://web.engr.oregonstate.edu/~traylor/ec
e474/lecture_verilog/beamer/verilog_number
_literals.pdf