Timescale Directive

Part of the Fundamentals section of Coddy's Verilog journey. Lesson 70 of 90.

The timescale directive sets the time unit and time precision for simulation. It tells the simulator how to interpret delay values like #10.

In previous lessons, we used delays like #10 without specifying what "10" means. Is it 10 nanoseconds? 10 picoseconds? The timescale directive answers this question.

Syntax:

`timescale time_unit / time_precision

Part	Meaning	Example
`time_unit`	The basic unit for delays	`1ns`
`time_precision`	How precise simulation should be	`1ps`

The backtick ` before timescale means it is a compiler directive.

Example

`timescale 1ns / 1ps

time_unit = 1ns → #10 means 10 nanoseconds
time_precision = 1ps → Simulation rounds to the nearest picosecond

Common Timescale Values

Time Unit	Precision	Use Case
`1ns / 1ns`	Nanosecond	Simple simulations
`1ns / 1ps`	Picosecond	More precise
`1ps / 1ps`	Picosecond	High-speed designs
`1us / 1ns`	Microsecond	Slow simulations

How It Affects Delays

`timescale 1ns / 1ps

initial begin
  #10 a = 1;   // Waits 10 nanoseconds
end

Without timescale, some simulators assume a default (often 1ns). It is best to always specify it.

Important Rules

Rule	Explanation
Must be outside modules	Placed before any module definition
Backtick symbol	`timescale not `'timescale`
Time unit ≥ time precision	`1ns / 1ps` is fine, `1ps / 1ns` is not
Affects all delays in the file	General delays, gate delays, assignment delays

Challenge

What to do:

Add the missing timescale directive to make the delays represent 10ns, 20ns, and 30ns.

Try it yourself

// TODO: Add timescale directive (1ns / 1ps)

module timescale_challenge;
  
  initial begin
    $display("Time %0t", $time);
    #10 $display("Time %0t (10ns = 10000ps)", $time);
    #10 $display("Time %0t (20ns = 20000ps)", $time);
    #10 $display("Time %0t (30ns = 30000ps)", $time);
    $finish;
  end
endmodule