NEC mPD780852 Series, Preliminary User'S Manual

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CHAPTER 13 SERIAL INTERFACE UART
Preliminary User’s Manual U14581EJ3V0UM00
(d) Reception
The receive operation is enabled when “1” is set to bit 6 (RXE) of the asynchronous serial interface mode
register (ASIM), and input via the RxD pin is sampled.
The serial clock specified by ASIM is used when sampling the RxD pin.
When the RxD pin goes low, the 5-bit counter of the baud rate generator begins counting and the start timing
signal for data sampling is output when half of the specified baud rate time has elapsed. If sampling the
RxD pin input with this start timing signal yields a low-level result, a start bit is recognized, after which the
5-bit counter is initialized and starts counting and data sampling begins. After the start bit is recognized,
the character data, parity bit, and one-bit stop bit are detected, at which point reception of one data frame
is completed.
Once reception of one data frame is completed, the receive data in the shift register is transferred to the
receive buffer register (RXB) and a receive completion interrupt (INTSR) occurs.
Even if an error has occurred, the receive data in which the error occurred is still transferred to RXB. INTSR
occurs if bit 1 (ISRM) of ASIM is cleared to 0 on occurrence of an error. If the ISRM bit is set to 1, INTSR
does not occur (see Figure 13-9).
If the RXE bit is reset (to “0”) during a receive operation, the receive operation is stopped immediately. At
this time, the contents of RXB and ASIS do not change, nor does INTSR or INTSER occur.
Figure 13-8 shows the timing of the asynchronous serial interface receive completion interrupt.
Figure 13-8. Asynchronous Serial Interface Receive Completion Interrupt Timing
RxD (input) D0 D1 D2 D6 D7 Parity STOP START
INTSR
Caution Be sure to read the contents of the receive buffer register (RXB) even when a receive
error has occurred. Overrun errors will occur during the next data receive operations
and the receive error status will remain until the contents of RXB are read.