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shuttler: remove sdm constants

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occheung 2023-10-04 16:29:55 -07:00 committed by Sébastien Bourdeauducq
parent 4f3e58db52
commit 43926574da
1 changed files with 8 additions and 5 deletions
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13
artiq/gateware/shuttler.py
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@ -100,12 +100,15 @@ class SigmaDeltaModulator(Module):
self.x = Signal(x_width)
self.y = Signal(y_width)
# The SDM cannot represent any sample >0x7ffc with pulse modulation
# Allowing pulse modulation on values >0x7ffc may overflow the
# accumulator, so the DAC code becomes 0x2000 -> -10.V.
# SDM can at most output the max DAC code `Replicate(1, y_width-1)`,
# which represents the sample of value
# `Replicate(1, y_width-1) << (x_width-y_width)`.
#
# If the input sample exceeds such limit, SDM may overflow.
x_capped = Signal(x_width)
self.comb += If((self.x & 0xfffc) == 0x7ffc,
x_capped.eq(0x7ffc),
max_dac_code = Replicate(1, (y_width-1))
self.comb += If(self.x[x_width-y_width:] == max_dac_code,
x_capped.eq(Cat(Replicate(0, x_width-y_width), max_dac_code)),
).Else(
x_capped.eq(self.x),
)