My old circuits use ATmega88 , 16MHz crystall, UART 250kbps, 8bit , 2 stop, no parity, both RX and TX
ATtiny816 has inetrnal 16Mhz and 20MHz RC factory calibrated with +-2% accuracy in 0-70°C range
attracts me quit from ATmega88 and get rid of crystall
my previous attempts to use internal 8Mhz RC of ATmega88 was unsuccessfull,
with 2% tolerance 250kbps will slow down to 245kbps and rise up to 255kbps ,cause bit spaces interfere eachother at last bits of the frame
according to datasheet and sampling 8,9,10 slices out of 16 slices of 1 bit, it seems possible,
I appreciate everyone share experineces