NEC mPD789426 Series User Manual page 236

(3) Cautions related to UART mode
(a) When bit 7 (TXE20) of asynchronous serial interface mode register 20 (ASIM20) is cleared during
transmission, be sure to set transmission shift register 20 (TXS20) to FFH, then set TXE20 to 1 before
executing the next transmission.
(b) When bit 6 (RXE20) of asynchronous serial interface mode register 20 (ASIM20) is cleared during
reception, reception buffer register 20 (RXB20) and the receive completion interrupt (INTSR20) are as
follows.
RxD20 pin
INTSR20
When RXE20 is set to 0 at a time indicated by <1>, RXB20 holds the previous data and INTSR20 is not
generated.
When RXE20 is set to 0 at a time indicated by <2>, RXB20 renews the data and INTSR20 is not generated.
When RXE20 is set to 0 at a time indicated by <3>, RXB20 renews the data and INTSR20 is generated.
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CHAPTER 12 SERIAL INTERFACE 20
RXB20
User's Manual U15075EJ1V0UM00
Parity
<1> <3>
<2>