The previous implementation of sweep mode had the M0 continuing to
receive and buffer samples during retuning. To avoid using data affected
by retuning, the code discarded two 16K blocks of samples after
retuning, before transferring one 16K block to the host.
However, retuning has to be done with the USB IRQ masked. The M4 byte
count cannot be advanced by the bulk transfer completion callback whilst
retuning is ongoing. This makes an RX buffer overrun likely, and
overruns now stall the M0, causing sweep timing to become inconsistent.
It makes much more sense to stop the M0 receiving data during retuning.
Using scheduled M0 mode changes between the RX and WAIT modes, it's now
possible to do this whilst retaining consistent sweep timing. The
comment block added to the start of the `sleep_mode()` function explains
the new implementation.
The new scheme substantially reduces the timing constraints on the host
retrieving the data. Previously, the host had to retrieve each sample
block before the M0 overwrote it, which would occur midway through
retuning for the next sweep, with samples that were going to be
discarded anyway.
With the new scheme, buffer space is used efficiently. No data is
written to the buffer which will be discarded. The host does not need to
finish retrieving each 16K block until its buffer space is due to be
reused, which is not until two sweep steps later. A great deal more
jitter in the bulk transfer timing can therefore now be tolerated,
without affecting sweep timing.
If the host does delay the retrieval of a block enough that its buffer
space is about to be reused, the M0 now stalls. This in turn will stall
the M4 sweep loop, causing the sweep to be paused until there is enough
buffer space to continue. Previously, sweeping continued regardless, and
the host received corrupted data if it did not keep up.
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