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CHAPTER 19 SERIAL INTERFACE CHANNEL 2
(d) Reception
When the RXE bit of the asynchronous serial interface mode register (ASIM) is set (to 1), a receive
operation is enabled and sampling of the RxD0 (RxD1) pin input is started.
RxD0 (RxD1) pin input sampling is performed using the serial clock specified by ASIM.
When the RxD0 (RxD1) pin input becomes low, the 5-bit counter of the baud rate generator (see
Figure
19-2
) starts counting, and at the time when the half time determined by specified baud rate has passed,
the data sampling start timing signal is output. If the RxD0 (RxD1) pin input sampled again as a result
of this start timing signal is low, it is identified as a start bit, the 5-bit counter is initialized and starts counting,
and data sampling is performed. When character data, a parity bit and one stop bit are detected after
the start bit, reception of one frame of data ends.
When one frame of data has been received, the receive data in the shift register is transferred to the receive
buffer register (RXB), and a reception completion interrupt request (INTSR) is generated.
If an error is generated, the receive data in which the error was generated is still transferred to RXB. If
bit 1 (ISRM) of ASIM is cleared (to 0) on occurrence of the error, INTSR is generated.
If the RXE bit is reset (to 0) during the receive operation, the receive operation is stopped immediately.
In this case, the contents of RXB and the asynchronous serial interface status register (ASIS) are not
changed, and INTSR and INTSER are not generated.
Figure 19-10. Asynchronous Serial Interface Reception Completion Interrupt Request Generation Timing
Caution
The receive buffer register (RXB) must be read even if a receive error is generated. If
RXB is not read, an overrun error will be generated when the next data is received, and
the receive error state will continue indefinitely.
D1
D2
D6
D7
Parity
D0
RxD0 (RxD1) (Input)
INTSR
STOP
START