Your interrupt service routine is probably not using the
'swap' instruction to save/ restore W correctly.
The microchip ISR examples use swap to get status without changing
it's value, and a later swap to restore it. If done incorrectly
you'll get the nibble reversal you're seeing.
Turn off your timer interrupt to confirm this.
R

Roberts II, Charles K. wrote:

> I have my USART running in asynchronus mode with continuous receive
> enabled. I also have Timer1 running from the internal oscillator
> interrupting every 1ms. The timer1 interrupt increments a counter that
> is used to determine when to send an incrementing or decrementing value
> via I2C. 
> 
> What happens is on some of the characters sent out the USART the upper
> and lower nibbles are swapped, so that instead of sending out 0x0D it
> sends 0xD0. What could cause a "nibble" swap like this? It does not
> happen on every character.
> 
> Let me know what code you want to see and I will post it if it will help
> you come up with any ideas.
> 
> Charles K Roberts II 
> 
> 
> 
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