Applications 3

Behavioral descriptions, continuous assignment, conditional instructions, use of blocks always for describing combinational circuits.

Structural descriptions, logic gates, verilog gate primitives.

Exercise 1

gate-level description of the 1-bit half adder

Use data flow description (a simpler kind of behavioral description that uses logic expressions). Logic expressions employ only logic operators.

operator	function
~	not
&	and
|	or
^	xor

One assign instruction for each output. Concurrent assignments.

assign c[0] = a ^ b;
assign c[1] = a & b;

Use gate-level description (a structural description with gate primitives). Verilog gate primitives: not, and, nand, or, nor, xor and xnor. Except for the not gate, all other primitive gates may have 2, 3 or more inputs.

They are all instantiated using the same template: gate primitive myGate(outputWire, inputWire1, inputWire2 ...) as in the following examples:

not g1(dout, din); is a not gate with dout at output and din connected at its input.

nand g2(dout, d1, d2); is a nand gate with dout at output and two wires, d1 and d2, connected to its inputs.

nand g3(dout, d1, d2, d3, d4); is a nand gate with dout at output and for inputs, connected to d1, d2, d3 and d4.

Using gate primitives, a possible structural description of the half-adder may be:

xor g1(c[0], a, b);
and g2(c[1], a, b);

Write a simple testbench for the half-adder and simulate both descriptions, data-flow and gate-level with primitives.

Exercise 2

2 to 1 multiplexer, behavioral description

always block. Used for behavioral descriptions. Template:

always @(sensitivity list) begin
  block of instructions
end

For combinational circuits the safest way to declare the sensitivity list is to use the wildcard (*). It stands for all variables that appear on RHS expressions and in evaluation expressions.

always @(*) begin
  if(s)
    c = in[1];
  else
    c = in[0];
end

Exercise 3

The same 2 to 1 multiplexer, described at the gate level

Exercise 4

4 to 1 multiplexer

case instruction. Selects between multiple cases. Template

case(selectVariable)
  value1 : instruction block 1
  value2 : instruction block 2
   a.s.o.
endcase

value1, value2 a.s.o. are integer values of the selectVariable

case(s)
  0: c = in[0];
  1: c = in[1];
  2: c = in[2];
  3: c = in[3];
endcase