> I understand the meanings of the flags but my question
> still remains.  Can you give an example where
> testing TRMT allows you to do something that just
> testing TXIF exclusively couldn't do?

The two flags have different meanings and purposes.  Most of the time you
want to know when it's OK to write another byte to TXREG.  You check TXIF
for this, or use the TXIF interrupt.  See the HOS_UART.ASPIC module at
http://www.embedinc.com/pic/hos.htm for an example UART handler that uses
the TXIF interrupt.  The application calls UART_PUT, which writes the byte
to a FIFO.  It doesn't get written to the UART until the UART is ready for
it.  The TXIF interrupt is used to transfer one more byte from the FIFO to
the UART.

The TRMT flag tells you that the whole byte has actually been sent on the
wire.  Most of the time you don't care, but it can be important to know if
the wire is being shared.  I've only used the TRMT flag twice that I can
remember.  Once was when the UART was connected to a RS-485 bus.  The
firmware had to know when the last packet byte was actually sent before
reversing the bus driver hardware from transmit to receive.  The other
time was making sure the last byte was physically sent before powering
down.


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