UART Rx starts to slip when baud rate is above 700000 and no inter-frame gap

[ed-xmos]

This was found when using FTDI to test. It was not seen during loopback where we achieve almost 1.5Mbps. I strongly suspect that the turnaround time between sampling the last stop bit and waiting for the falling edge of the next start bit is the issue since we can receive a 1.5Mbps byte with stop->start gaps.

See https://xmosjira.atlassian.net/wiki/spaces/~870418189/pages/3572039681/RTOS+UART+interoperability+HW+test for evidence

[ed-xmos]

I have started a PR to look at optimisations here - #25

[ed-xmos]

The changes so far:

• inlined state machine
• Removed third start bit check
• Use get_ref_time always
• if buffer case simplified
• Retune latency hiding parameters

has resulted in the HIL Rx component now passing at 1152000 baud in the regression. It also passes the bare metal test at 1267200 baud. HOWEVER, the RTOS test only passes at 921600 baud for 128 bytes. It passes at 1000000baud for 4 bytes.

I have previously checked timing in the RTOS driver and am satisfied it is good (no back pressure on the HIL thread) so am unsure what is causing this.

[ed-xmos]

See also xmos/xcore_iot#491 which may answer the last question.

[ed-xmos]

Note that the work in #25 managed to yield more than 40% higher bit rate but xmos/xcore_iot#491 has since pegged that back. So we now verify at 700kbps Rx rate (assuming 75MHz threads) and there isn't much headroom. Tx is well in excess of 1Mbps. Leaving issue open for now since we still can't make 1Mbps Rx without using a 100MHz HIL Rx thread.