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platform.add_clock_constraint does not work for instances with vivado #373

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@rroohhh

Description

@rroohhh

For clocks coming from an instance, platform.add_clock_constraint doesn't seem to work, as vivado is unable to find the net that the constraint is assigned to.

Running this example:

from nmigen import *
from nmigen_boards.zturn_lite_z010 import *

class Test(Elaboratable):
    def elaborate(self, plat):
        m = Module()

        fclk = Signal(4)

        m.submodules += Instance("PS7", o_FCLKCLK = fclk)

        clk = Signal()

        m.d.comb += clk.eq(fclk[0])

        m.domains += ClockDomain("sync")
        m.d.comb += ClockSignal().eq(clk)

        plat.add_clock_constraint(clk, 100_000_000)

        return m


plat = ZTurnLiteZ010Platform()

t = Test()

plat.build(t)

causes vivado to issue the following warning:

WARNING: [Vivado 12-507] No nets matched 'clk'. [/data/projects/nmigen_bugs/build/top.xdc:2]

And the subsequent timing report is empty.

Full build directory: build.zip

This is using nmigen commit ba79b0c.

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