I've been getting some strange problems with a PIC16F877 relating to its
USART.  I'm running in synchronous mode, and That worked fine on the
breadboard.  Like a charm, even.  But, then when I ported it over to PCB, it
started misbehaving.  I'm confused as to what the problem actually is.

I'm running a bit-banged interface with a PS/2 device, and it appears to
work correctly.  and has worked correctly on a breadboard.  I'm outputting
synchronous serial data via the USART. I use code that ouputs a byte, waits
for completion, then outputs another.  4 bytes in total.  While the
synchronous data is being transmitted, the PS/2 is locked out.

The problem is that when I attach the receiver for the synchronous data, the
stream begins to transmit incessantly.  It is unstable up to roughly 5.1V.
Therefore, I'm led to wonder if the voltage applied affects the USART
at all.  The peripheral that attaches to the PIC USART draws
almost 500mA *from the power rails*.  I have tried attaching it to a
seperate power supply, and I get the same result, but if I increase the
voltage to the PIC slightly, it seems to stabilize a little, however, the
USART seems to affect the bitbanged interface, and is causing it to abort
transmissions part way through.

I've no idea what's happening.  Anyone else run into something like this
before?  (believe me, I'm rechecking all my communication code, even though
it worked before)

--Brendan

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