I have not used the emulated UART routine with the 219x. I talked
with another engineer who is versed on the 218x and he told me pretty
much that it's a bit banged UART setup.
The RX'ED start bit is picked up with a input flag pin and timer is
setup up to interrupt the processor at 3 times the clock rate. On
each timer interrupt the dsp samples the UART input. After 3 samples
are collect, the dsp takes a best 2 out of 3 approach to determine the
correct bit value. The TX'ED end uses a timer that interrupts at the
bit rate. So each time the transmit timer interupt gets issued it
sets a UART output signal to the next bit to transmit.