1. Hardware Description Languages
Languages used to describe hardware designs
Can be used for synthesis or simulation
Synthesis is manufacturing an Application Specific Integrated Circuit (ASIC) or mapping to a Field Programmable Gate Array (FPGA)
Performance/Area/Power are a priority
Simulation is for the purpose of checking functionality
Simulation must match the real world
In this class we are doing simulation, not synthesis

2. Procedural Design vs. Structural Design
Hardware Design
Procedural Design vs. Structural Design
c = a + b
f = d + e
if (g)
out = c;
else
out = f;
Procedural design describes functionality as a sequence of steps.
Structural design describes an interconnection of components. + a b d e
out g

3. Structural Description, Modules
module T_FF (q, clock, reset); .
endmodule
Represents a physical component, can be instantiated many times
I/O ports declared at the top
Typically represents a physical component
Can be structurally connected to other components
Cannot be invoked like a function

4. Instances TFF is instantated within ripple_carry_counter
module ripple_carry_counter(q, clk, reset);
output [3:0] q;
input clk, reset;
//4 instances of the module TFF are created.
TFF tff0(q[0],clk, reset);
TFF tff1(q[1],q[0], reset);
TFF tff2(q[2],q[1], reset);
TFF tff3(q[3],q[2], reset);
endmodule
module TFF(q, clk, reset);
output q;
input clk, reset;
wire d;
DFF dff0(q, d, clk, reset);
not n1(d, q);
endmodule
TFF is instantated within ripple_carry_counter
DFF and not are instantiated within TFF
Structural interconnect is established through instantiation

5. Testbench (Stimulus Block)
// Control the reset
initial
begin
reset = 1'b1;
#15 reset = 1'b0;
#180 reset = 1'b1;
#10 reset = 1'b0;
#20 $stop;
end
// Monitor the outputs
$monitor($time, " Output q = %d", q);
endmodule
module stimulus;
reg clk;
reg reset;
wire[3:0] q;
// instantiate the design block
ripple_carry_counter r1(q, clk, reset);
// Control the clock
initial clk = 1'b0;
always #5 clk = ~clk;
The testbench generates the input stimulus
Observation of data is often included in the testbench