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.

Cheat sheet

The `timescale directive sets the time unit and time precision for simulation. It must be placed before any module definition.

`timescale time_unit / time_precision

`timescale 1ns / 1ps

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

time_unit — the basic unit for delays (e.g., #10 = 10ns)
time_precision — how precisely simulation rounds (must be ≤ time unit)

Timescale	Use Case
`1ns / 1ns`	Simple simulations
`1ns / 1ps`	More precise
`1ps / 1ps`	High-speed designs
`1us / 1ns`	Slow simulations

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