In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Peter Van der Zee wrote:

Hi Michael;

Sorry, but I can't agree with your logic.

Let's say the UART receive clock and the re-transmit clock are identical, and also happen to be the same as the "foreign" data stream we are receiving. Also let's say that the continuous incoming stream and the receiving UART bit edges are aligned, and the re-transmitter is waiting for a byte in the buffer.

Now, as you say, let the receiving UART transfer the first character being received to the buffer at the instant the sample of the 8th data bit is taken, say in the middle of the bit. This leaves us with the balance of that bit plus the stop bit before the second character's start bit will fall.

So the re-rtansmitter senses the first character in the buffer, and will start its re-transmission of that first character at the same instant. That means we are starting the re-transmission of character one's start bit before we have received the "trailing end" of the first incoming character.

Now, if the baud clock of the re-transmitter were just a bit slow, then it would finish sending the first character a little later than the middle of the 8th data bit of the second character in the input stream, but still well before the start edge of the third character. So this has the EFFECT of making the re-transmit appear faster than the receive stream, and hence there is no "bunching up", or clock creep in the system. There is 1.5 bits of "slop" time available for the re-transmitter to "gow" into for every received charcter.

This should work perfectly for re-transmit clocks that are up to 1.5 bits "slow" over one character under ideal cases. In reality one needs to expect a little less because clock edges are not normally ideally aligned, and one cannot be sure exactly when receive samples are taken in any bit.

If you still feel my analysis is wrong, (and I feel I'm right) then I will spend the time to set up three independent UARTs to prove (or disprove) my position.

Cheers,

Peter (pjv)

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