>
> > but testing stop bits seems less useful to me. Even if you
> > discover framing error what would you do ? Trash the byte ?
> > Ask for resend ?
>
> Hardware USARTs generally return a framing or overrun error
> and (I think) along with the data which was received.  Upper
> level software then chooses what do to with the octet (ask
> for resend, etc).

Different UART's behave differently things when they receive an
incorrectly-framed packet.  The approach I took in my interrupt-driven
software UART (which I personally would like to see done in hardware) is
to hold the last 32 bits that came in [sampled with a 3xbaud clock] and
grab a byte whenever a properly-aligned byte has been shifted in.  IMHO,
this should allow the UART to correctly-align itself after a framing error
much faster than the conventional techniques (other approaches include
delaying start-bit detect until the next *falling edge*, or regarding the
missing stop bit as the next byte's start bit).  Does anyone know if any
hardware designer has ever done a UART this way?