[libre-riscv-dev] buffered pipeline

Luke Kenneth Casson Leighton lkcl at lkcl.net
Thu Mar 14 09:29:09 GMT 2019

On Thu, Mar 14, 2019 at 8:40 AM Jacob Lifshay <programmerjake at gmail.com>

> I'm using the nmigen.back.pysim.Simulator class, which is apparently the
> recommended solution for simulation. It supports executing multiple
> generators simultaneously, you just have to watch out for checking values
> immediately after a clock edge, it needs a slight delay before it
> calculates the combinatorial values.
> It's used like:
> m = MyModule()
> with Simulator(test_stage, vcd_file=open("test.vcd", "w"),
> gtkw_file=open("test.gtkw", "w"), traces=[m.sig1, m.sig2]) as sim:
>     clock_period = 1e-6
>     sim.add_clock(clock_period)
>     def process() -> Generator[Any, Any, None]:
>         yield m.sig1.eq(1) # set m.sig1
>         yield #wait for clock edge
>         yield Delay(1e-7) # delay 100ns
>         sig2_value = yield m.sig2 # read sig2 as an int
>         print(sig2_value)
>         yield # clock some more
>     sim.add_sync_process(process)
>     sim.run()
ah interesting, you're using the Simulation class directly rather than the
run_simulation function which is the usual way.

actually... looking at compat/sim/__init__.py, it *is* basically exactly
the code above!  sim.add_clock is called, sim.add_sync_process is called,
sim.run is called:

run_simulation however takes an array (or dictionary) of functions
[generators], and adds all of them.

mostly, people leave the clock and so on alone, trusting that the defaults
will be sufficient.

yes, you can't grab even combinatorial logic created-and-outputted on the
exact same clock cycle as its inputs change, there has to be some
propagation delay.  the usual design/behaviour is to store the
combinatorial block in a sync (clocked) signal.

it would be *extremely* bad to write HDL/code that relies on anything other
than that pattern.  to rely on specific gate propagation delays would be a
nightmare to synthesise (even if it was possible) or even put into an FPGA.


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