Improves RX reliability

[ulde]

The RX GPIO was polled exactly at the transition time from one bit to the next. If the transmitting device presents some jitter or is simply a bit late driving its TX line, soft_uart could fail to detect the correct value on bit level transitions. This change simply starts the polling timer not immediately on the start bit interrupt, but a half bit period later, so that all RX GPIO level polling now occurs at the center of each bit, making the RX side of soft_uart more robust.

[adrianomarto]

Hi Ulde, Thanks for your contribution. Your modification makes sense to me. Have tested it enough to notice an actual improvement in the reliability? Cheers, Adriano.

[ulde]

Yes. Actually I made the chance because I was unable to receive with soft_uart from a serial line coming from a micro controller. The oscilloscope showed that the MCU occasionally had a tiny delay driving the line transition to the next bit. Even a few microseconds off, even at very low bps rates, are enough to corrupt the data. This change addresses this kind of situations.

Best, Ulde