This commit adds an option `fs_per_delta=` to `Simulator.write_vcd()`.
Specifying a positive integer value for it causes the simulator to
offset value change times by that many femtoseconds for each delta
cycle after the last timeline advancement.
This option is only suitable for debugging. If the timeline is advanced
by less than the combined duration of expanded delta cycles, an error
similar to the following will be raised:
vcd.writer.VCDPhaseError: Out of order timestamp: 62490
Typically `fs_per_delta=1` is best, since it allows thousands of delta
cycles to be expanded without risking a VCD phase error, but bigger
values can be used for an exaggerated visual effect.
Also, the VCD writer is changed to use 1 fs as the timebase instead of
1 ps. This change is largely invisible to designers, resulting only in
slightly larger VCD files due to longer timestamps.
Since the `fs_per_delta=` option is per VCD writer, it is possible to
simultaneously dump two VCDs, one with and one without delta cycle
expansion:
with sim.write_vcd("sim.vcd"), sim.write_vcd("sim.d.vcd", fs_per_delta=1):
sim.run()
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