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'[PIC]: [EE]: Microcontroller with 4 USARTs'
2000\05\28@070501 by Alan B Pearce

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>Unfortunately, to receive you need to run your interrupt handler somewhat
>faster than the bit time.  I have successfully built a software UART using a
>interrupt 6x of the bit rate. I expect 5x would work pretty well.

You get away with 4 samples per bit time if you are not doing it synchronous to
the bit edges. Many years ago before high speed async modems were commonly
available we used to send async data through synchronous modems, and could do it
with no errors if the async baud rate was a quarter of the sync modem rate.

2000\05\28@082711 by Bob Ammerman

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Of course this depends on the quality of your signal.

This thread has got me to thinking, and coding, and I am pretty sure I've
come up with a way to get 4 in and 4 out at 31250buad on a PIC18CXX2 at 10
MIPS. The trick involves accumulating input samples on each channel and
processing them 5 at a time instead of one-by-one. This idea was inspired by
byte-at-a-time CRC algorithms. Basically you end up with a monster state
machine, about 70 states, with 32 (ie: 2^5) possible inputs for each state.
The state machine is then stored as one whopping big (about 8K) table. But
hey, what's 8K when you've got 32K on chip? :-)

Bob Ammerman
RAm Systems
(high function, high performance, low level software).

{Original Message removed}

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