Peter, you are thinking along the same lines as I am, The software approach is plain vanilla if you have the resources (external interrupt, UART, 2 timers). The potential problem with the software interrupt is that character 0 - or is it ff - has a start bit only and if other (ongoing same level or higher priority) activities hold the serial off for half a bit time, you will lose then start bit interrupt on EXn). I was merely trying to bulletproof so I did not have to calculate/verify that this would never happen. Of course, if nothing else is going on (the external small processor) no such risk exist.

If anyone know of a UART with a "receive in progress" flag, I would still love to know about it.

Have fun,

Erik