Verilog timescale directive specifies time unit and precision for simulations.
Verilog $timeformat
system function specifies %t
format specifier reporting style in display statements like $display
and $strobe
.
Syntax
$timeformat(<unit_number>, <precision>, <suffix_string>, <minimum field width>);
- unit_number is the smallest time precision out of all
`timescale
directives used in source code - precision represents the number of fractional digits for the current timescale
- suffix_string is an option to display the scale alongside the real time values
Unit number | Time unit |
---|---|
-3 | 1ms |
-6 | 1us |
-9 | 1ns |
-12 | 1ps |
-15 | 1fs |
Example #1: 1ns/1ps
Here is an example of how $timeformat
affects the format of time unit display.
`timescale 1ns/1ps
module tb;
bit a;
initial begin
// Wait for some time - note that because precision is 1/1000 of
// the main scale (1ns), this delay will be truncated by the 3rd
// position
#10.512351;
// Display current time with default timeformat parameters
$display("[T=%0t] a=%0b", $realtime, a);
// Change timeformat parameters and display again
$timeformat(-9, 2, " ns");
$display("[T=%0t] a=%0b", $realtime, a);
// Remove the space in suffix, and extend fractional digits to 5
$timeformat(-9, 5, "ns");
$display("[T=%0t] a=%0b", $realtime, a);
// Here suffix is wrong, it should not be "ns" because we are
// setting display in "ps" (-12)
$timeformat(-12, 3, " ns");
$display("[T=%0t] a=%0b", $realtime, a);
// Correct the suffix to ps
$timeformat(-12, 2, " ps");
$display("[T=%0t] a=%0b", $realtime, a);
end
endmodule
Simulation Log
xcelium> run [T=10512] a=0 [T=10.51 ns] a=0 [T=10.51200ns] a=0 [T=10512.000 ns] a=0 [T=10512.00 ps] a=0 xmsim: *W,RNQUIE: Simulation is complete.
Example #2: 1ns/100ps
Here is the same example from above with a different timescale.
`timescale 1ns/100ps
Simulation Log
xcelium> run [T=105] a=0 [T=10.50 ns] a=0 [T=10.50000ns] a=0 [T=10500.000 ns] a=0 [T=10500.00 ps] a=0 xmsim: *W,RNQUIE: Simulation is complete.
Example #3: 100ns/1ns
`timescale 100ns/1ns
#1 represents 100ns and hence #10 yields 1000ns
Simulation Log
xcelium> run [T=1051] a=0 [T=1051.00 ns] a=0 [T=1051.00000ns] a=0 [T=1051000.000 ns] a=0 [T=1051000.00 ps] a=0 xmsim: *W,RNQUIE: Simulation is complete.