> I am messing around with a simple program that is supposed to simply echo
> characters received on the serial line.  When I run the program (in the
> YAP-II devel. board) and start hitting keys, I am not able to successfully
> receive a character from the PC.  From my testing so far, it appears that
> the program fails to receive a full start bit on the input line.  When I
> send ascii char 0, (i.e. binary 00000000) the interrupt routine isn't
> invoked at all.  It seeems like the start bit should trigger the interrupt
> routine -- so it seems like I'm not getting a start bit.  Hmm.

Hi, the start bit simply triggers the uart to shift in X bits (whatever
the wordlength is). Upon receipt of bit X+1 (stop bit) the shift register
is moved into the receive buffer, and RXD is set. Since RXD is NOR'd onto
the /INT output (I *think* it's a negated /INT), then THIS is when a
receive interrupt occurs. RXD is cleared when you read the receive buffer.

Note on the PC architecture, the uart's OUT2 output gates the interrupt
output with an (IIRC) NOR gate. So you need to set up the interrupt
controller for IRQ 3/4, set up the IER for RX interrupt, AND set the OUT2
bit.

But first you should hack the uart directly with debug to see if it's
actually receiving (i.e. RXD gets set and the receive buffer contains the
character transmitted). Once that's working, then figure out the
interrupts.

HTH / Rich

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