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[\<- Shannon's Expansion and FPGAs](7.md)
---
# Verilog basics
## Introductory Verilog concepts
### Verilog
- A language to specify hardware
- Hardware Description Language (HDL)
- Has many features of other programming languages but it does \*not\* execute sequentially like most languages do
- Can specify structure or behavior
- Structure is like an actual circuit; netlist
- Behavior would be like logic equations or truth table
- Can be synthesized into a specific structure
### Verilog module
- HW is always described inside a module
- Like a schematic
- Always at least one output, and then however many inputs the circuit consumes
- Referred to as ports
- Taken together this is called the interface
- Variable names are used to make connections, either from ports to internal circuitry or between circuit elements inside the module
---
## Describing structure of 2:1 mux
- Example: Inputs x1,x2, and s, and output f
- This is the interface
- Need to describe what's happening inside
- Need to describe how gates are connected
- Use Verilog primitives to describe structure
### Interfaces for primitves
- AND (2-input example):
```
module and(y,x1,x2);
input x1,x2;
output y;
```
- OR (4-input example):
```
module or (y,x1,x2,x3,x4);
input x1,x2,x3,x4;
output y;
```
- NOT (always just one input):
```
module not (y,x);
input x;
output y;
```
### Structural example
- A gate-level implementation of a 2:1 mux
- Ports connected by name association
- E.g., "k" connects the output of the NOT gate to one of the inputs to the first AND gate
```
module example1(x1, x2, s, f);
input x1, x2, s;
output f;
wire k, g, h;
not(k, s);
and(g, k, x1);
and(h, s, x2);
or(f, g, h);
endmodule
```
- Note that the declaration of the wires in the above code is optional, but makes for good habit
- If not explicitly declared, `k`, `g`, and `h` would be implicitly considered wires by Verilog
---
## Behavioral description of 2:1 mux
### Behavioral code
- Uses syntax like our boolean algebra equations
- `&` instead of `*`
- `|` instead of `+`
- `~` instead of `!`
- Keyword of `assign` means the equation is always evaluated, just like logic gates work
```
module example3(x1, x2, s, f);
input x1, x2, s;
output f;
assign f = (~s & x1) | (s & x2);
endmodule
```
---
## Behavioral description of a more complex module
- Describe this circuit, with outputs f, g, h
- Assign statements don't have to be in the order listed
- Specifying the behavior of a wire
- Also don't have to specify intermediate nodes
```
module example4(x1, x2, x3, x4, f, g, h);
input x1, x2, x3, x4;
output f, g, h;
assign g = (x1 & x3) | (x2 & x4);
assign h = (x1 | ~x3) & (~x2 | x4);
assign f = g | h;
endmodule
```
---
## Hierarchy in Verilog
### Hierarchical design
- Block diagram of two modules working together
- Outputs of adder module connected to inputs of display module
### Code for the adder
- What is the truth table for a circuit that treats two inputs as numbers and adds them together?
```
//An adder module
module adder(a, b, s1, s0);
input a, b;
output s1, s0;
assign s1 = a & b;
assign s0 = a ^ b;
endmodule
```
- Note that `^` is XOR in Verilog
### Code for the display driver
- Useful to work out the truth table in order to figure out the equations
```
//A module for driving a 7-segment display
module display(s1, s0, a, b, c, d, e, f, g);
input s1, s0;
output a, b, c, d, e, f, g;
assign a = ~s0;
assign b = 1;
assign c = ~s1;
assign d = ~s0;
assign e = ~s0;
assign f = ~s1 & ~s0
assign g = s1 & ~s0
endmodule
```
### Instantiating and connecting
- Note the "wire" declarations for the internal (to this module) connections between adder and display
- Variables not part of interface definition
```
module adder_display (x, y, a, b, c, d, e, f, g);
input x, y;
output a, b, c, d, e, f, g;
wire w1, w0;
adder U1(x, y, w1, w0);
display U2(w1, w0, a, b, c, d, e, f, g);
endmodule
```
---
[Number systems and adders ->](9.md)
|
