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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
+
+![diagram](8.1.png)
+
+### 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, x1);
+	or(f, g, h);
+
+endmodule
+```
+
+![diagram](8.2.png)
+
+- 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
+```
+
+![diagram](8.3.png)
+
+---
+
+## Behavioral description of a more complex module
+
+- Describe this circuit, with outputs f, g, h
+
+![diagram](8.4.png)
+
+- 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
+
+![diagram](8.5.png)
+
+### Code for the adder
+
+- What is the truth table for a circuit that treats two inputs as numbers and adds them together?
+
+![diagram](8.6.png)
+
+```
+//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
+```