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lib.cdc: specify maximum input delay in seconds.

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Since we use hertz elsewhere, this provides for easy conversions.

Also, cast the delay to string before applying it in xilinx_7series,
to avoid stripping the fractional digits.

Closes #234.
This commit is contained in:
whitequark 2019-09-24 12:30:02 +00:00
parent b43d2d36e8
commit 0858b8bf6c
2 changed files with 4 additions and 4 deletions
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4
nmigen/lib/cdc.py
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@ -39,7 +39,7 @@ class FFSynchronizer(Elaboratable):
Number of synchronization stages between input and output. The lowest safe number is 2,
with higher numbers reducing MTBF further, at the cost of increased latency.
max_input_delay : None or float
Maximum delay from the input signal's clock to the first synchronization stage.
Maximum delay from the input signal's clock to the first synchronization stage, in seconds.
If specified and the platform does not support it, elaboration will fail.
Platform override
@ -122,7 +122,7 @@ class ResetSynchronizer(Elaboratable):
Number of synchronization stages between input and output. The lowest safe number is 2,
with higher numbers reducing MTBF further, at the cost of increased deassertion latency.
max_input_delay : None or float
Maximum delay from the input signal's clock to the first synchronization stage.
Maximum delay from the input signal's clock to the first synchronization stage, in seconds.
If specified and the platform does not support it, elaboration will fail.
Platform override

4
nmigen/vendor/xilinx_7series.py vendored
View file

@ -392,7 +392,7 @@ class Xilinx7SeriesPlatform(TemplatedPlatform):
if ff_sync._max_input_delay is None:
flops[0].attrs["nmigen.vivado.false_path"] = "TRUE"
else:
flops[0].attrs["nmigen.vivado.max_delay"] = ff_sync._max_input_delay
flops[0].attrs["nmigen.vivado.max_delay"] = str(ff_sync._max_input_delay * 1e9)
for i, o in zip((ff_sync.i, *flops), flops):
m.d[ff_sync._o_domain] += o.eq(i)
m.d.comb += ff_sync.o.eq(flops[-1])
@ -407,7 +407,7 @@ class Xilinx7SeriesPlatform(TemplatedPlatform):
if reset_sync._max_input_delay is None:
flops[0].attrs["nmigen.vivado.false_path"] = "TRUE"
else:
flops[0].attrs["nmigen.vivado.max_delay"] = reset_sync._max_input_delay
flops[0].attrs["nmigen.vivado.max_delay"] = str(reset_sync._max_input_delay * 1e9)
for i, o in zip((0, *flops), flops):
m.d.reset_sync += o.eq(i)
m.d.comb += [