Verilog
Section 3.10
Section 4.5
Keywords
• Keywords are predefined lowercase
identifiers that define the language
constructs
– Key example of keywords: module,
endmodule, input, output, and wire.
assign
• The assignment is said to be sensitive to
the variables in the RHS expression
because anytime a variable in the RHS
changes during the simulation, the RHS
expression is reevaluated and the
result is used to update the LHS.
Semicolon
• Each statement must end with a
semicolon (;)
Bitwise Logic Operation
• Bitwise means 1 bit at a time
Bitwise logic operator
Verilog
AND
a&b
OR
a|b
XOR
a^b
INVERT
~a
NAND
~(a&b)
NOR
~(a|b)
XNOR
!(a^b)
wire
You can think of a wire as a wire in a
circuit where actual voltages
Could be measured.
Wire example
Use & for AND operation
Use tilda (~) for the INVERT operation
Use | for the OR operation
Waveform
Using Verilog Primitives
• Verilog also has keywords such as and
or and not.
The output of a primitive
must be listed first.
Gate Delays
• In Verilog, the propagation delay of a
gate is specified in terms of time units
and is specified by the symbol #.
• `timescale 1ns/100ps
– The first number specifies the unit of
measurement for time delays.
– The second number specifies the
precisions for which the delays are
rounded off.
Gate Delay
E is not defined until after 1 ns.
Gate Delay
E is not defined until 1 ns.
W is not defined until 2 ns.
This means that D is not defined until 3 ns.
Binary Addition Example
Derivation of ∑ (ES112 Slides)
B
A
∑
0
0
0
1
0
1
0
1
1
1
1
0
Question: What primitive best implements ∑?
• Inputs: A, B
• Outputs: ∑=𝐴𝐵 + 𝐴𝐵 = 𝐴 ⊕ 𝐵
Derivation of Carry Out
(ES112 Slides)
B
A
Co
0
0
0
1
0
0
0
1
0
1
1
1
Question: What primitive best implements Co?
• Inputs: A, B
• Outputs: Co =A∙B
Implementation of a Half-Adder
Limitation of a Half Adder
A half-adder does not account for carry-in.
Truth Table for a Full Adder
carry-in
Karnaugh Map For the Sum Bit
(ES112 Review)
𝑆 = 𝑥 ′ 𝑦 ′ 𝑧 + 𝑥 ′ 𝑦𝑧 ′ + 𝑥𝑦 ′ 𝑧 ′ + 𝑥𝑦𝑧
=𝑧 ′ 𝑥 ′ 𝑦 + 𝑥𝑦 ′ + 𝑧 𝑥 ′ 𝑦 ′ + 𝑥𝑦
=𝑧 ′ 𝑥 ′ 𝑦 + 𝑥𝑦 ′ + 𝑧 𝑥 ′ 𝑦 + 𝑥𝑦 ′ ′
=𝑧 ⊕ (𝑥 ⊕ 𝑦)
Karnaugh Map For the Carry-Out Bit
(ES112 Review)
C = 𝑥 ′ 𝑦𝑧 + 𝑥𝑦 ′ 𝑧 + 𝑥𝑦 = 𝑧 𝑥 ⊕ 𝑦 + 𝑥𝑦
Implementation of a Full Adder
(carry-in)
C = 𝑧 ⊕ (𝑥 ⊕ 𝑦)
𝑆 = 𝑧 𝑥 ⊕ 𝑦 + 𝑥𝑦
Schematic of a Full Adder
Half-adder
Half-adder(not including the bubble)
Build a Verilog Representation of
a Full Adder Circuit
Build a half adder circuit
Build a test bench for the adder circuit
Assemble a full adder circuit
Build a test bench circuit to test the full
–adder
• Write the code to implement the
adder circuit on FPGA
•
•
•
•
Build a Half-Adder Circuit
(Figure 4.5)
Build a Test Bench in Verilog
Ideas: (page 112 of the textbook)
1. reg
2. Initial statement
3. Assign value to a single bit
4. $finish
1’b0=one binary digit with a value of 0
1’b1=one binary digit with a value of 1
Initial, $finish
• inital: keyword used with a set of
statements that begin executing when
simulation is initialized.
• $finish: specifies the termination of
simulation.
Block statement
A block statement consists of several
statements that are executed in sequence
from top to bottom.
Build a Full-Adder Circuit
M1
M2
w1
w2
w3
Full-Adder Top Level Circuit
Build a FPGA Top Level Circuit
(x)
(y)
(z)
(s)
(c)
See gates2.pdf (available from the course website) for reference