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Unusual Display Behaviors #583

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BracketMaster opened this issue Jan 27, 2021 · 4 comments
Closed

Unusual Display Behaviors #583

BracketMaster opened this issue Jan 27, 2021 · 4 comments
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@BracketMaster
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Sometimes display will trigger twice(when it should trigger once) or once(when
it should trigger twice).

Below, I present a case where display should fire twice in state two:

from nmigen import Module, Display, Signal

m = Module()

sig = Signal()

with m.FSM(name="simple"):
    with m.State("one"):
        m.d.comb += Display("one")
        m.next = "two"
    with m.State("two"):
        m.d.comb += Display("two")
        m.next = "two"

from nmigen.sim import Simulator, Tick

sim = Simulator(m)
sim.add_clock(1e-6)

def process():
    yield Tick()

sim.add_sync_process(process)
sim.run()

evaluating this produces:

$ python3 test.py
one
two

where we expect:

$ python3 test.py
one
two
two

A counterexample:

By changing the following

    with m.State("two"):
        m.d.comb += Display("two")
        m.next = "two"

to

    with m.State("two"):
        m.d.comb += Display("two")
        m.next = "one"

we observe in the expected behavior of

$ python3 test.py
one
two
one
@BracketMaster
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after further investigation, it seems that using Display within sync is more reliable

@whitequark
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This is generally expected behavior for comb Display.

@BracketMaster
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Alright. I think I agree.

@whitequark
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Once there is a sink for Display-produced events I think you will be able to post-process them to make sure only one fires during a given simulation time instant.

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@whitequark @BracketMaster