migen.genlib: Improve "first word fall through" docs. #152
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* Moved "first word fall through" `fwft` to be part of _FIFOInterface. * Added documentation what the hell `fwft` stands for. * Checks the compatibility of the `fwft` argument given to the FIFO.
| @@ -48,7 +52,8 @@ class _FIFOInterface: | |||
| Acknowledge `dout`. If asserted, the next entry will be | |||
| available on the next cycle (if `readable` is high then). | |||
| """ | |||
| def __init__(self, width, depth): | |||
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| def __init__(self, width, depth, fwft=False): | |||
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FIFOInterface does not use fwft and therefore should not have this parameter.
| _FIFOInterface.__init__(self, width, depth) | ||
| def __init__(self, width, depth, fwft=False): | ||
| assert not fwft, "fwft is not supported on the AsyncFIFO." | ||
| _FIFOInterface.__init__(self, width, depth, fwft=False) |
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This AsyncFIFO is always FWFT (in general there is no reason for an async FIFO not to be FWFT).
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With a sync FIFO it's either:
- non-FWFT
- asynchronous reads (lots of resources used, timing issues)
- 2 cycles of latency instead of 1
The async FIFO does not have to make this compromise (the latency is already high enough from the CDC and hides the memory latency).
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fwftto be part of _FIFOInterface.fwftstands for.fwftargument given to the FIFO.