Re: Slightly unmatched UART frequencies

> When you say "your" UART, is this a design you did yourself in an FPGA?
Yes, you're right I have my design runs on CPLD. However, the qestion is
more in logic rather than implementation. The value 10% I have got from
www.8052.com's forum where "Software Uart" is a hot topic.


> If so you may not have designed the logic correctly.  In order for the
> receiver to synchronize to a continuous data stream, it has to sample
> the stop bit in what it thinks is the center and then *immediately*
> start looking for the next start bit.  This will allow a mismatch in
> speed of almost a half bit minus whatever slack there is for the sample
> clock rate.  BTW, you are sampling at at least 8x the bit rate, right?
I use 16x oversampling and check input values at middle of a bit (SampleCnt
= 7). You suggest exactly what I have done. I think receiver part will work
under any condition. But I need to know what should I do with transmitter
module. As I attempted to explain, this half-bit solution cannot be used to
synchronize transmitters. It is a bad idea to start transmitting next byte
at the middle of the stop bit. It may fail listening device with slow clock
as it reaches center of stop bit when start bit of next byte is being
transmitted. On the other hand, if data is coming slightly faster
transmitter should do something, otherwise I face buffer overrun condition.
I understand that I can ignore the problem with transmitter module, it is
receiver that should synchronize with transmitter. However, I had got buffer
overrun problem until I used a trick described in my message (early
transmit). It is defenetely not the problem with receiver because I have
solved it right before got problem with transmitter's buffer overrun. And I
want to know how should function correct logic; there should be a solution
as commertial UARTs work without any problems. My UART is the first one
where I've realized that it is at all possible to get a problem with slowly
transmitting uart.
Is now the problem become clearer?