NEC Renesas V850/SC1 User Manual page 452

(c) Transmission
A transmit operation is started when transmit data is written to transmit shift register n (TXSn). A start bit,
parity bit, and stop bit(s) are automatically added to the data.
Starting a transmit operation shifts out the data in TXSn, thereby emptying TXSn, after which a transmission
completion interrupt (INTSTn) is issued.
The timing of the transmission completion interrupt is shown below.
Figure 11-50. Timing of Asynchronous Serial Interface Transmission Completion Interrupt
TxDn (output)
INTSTn
TxDn (output)
INTSTn
Caution Do not write to asynchronous serial interface mode register n (ASIMn) during a transmit
operation.
operations (in such cases, enter a RESET to restore normal operation).
Whether or not a transmit operation is in progress can be determined via software using
the transmission completion interrupt (INTSTn) or the interrupt request flag (STIFn) set by
INTSTn.
Remark n = 0 to 3
450
CHAPTER 11 SERIAL INTERFACE FUNCTION
(a) Stop bit length: 1
START D0 D1 D2
(b) Stop bit length: 2
START D0 D1 D2
Writing to ASIMn during a transmit operation may disable further transmit
User's Manual U15109EJ3V0UD
D6 D7 Parity STOP
D6 D7 Parity STOP