CHAPTER 13 SERIAL INTERFACE 20
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(d) Reception
When bit 6 (RXE20) of asynchronous serial interface mode register 20 (ASIM20) is set (1), a receive
operation is enabled and sampling of the RxD20 pin input is performed.
RxD20 pin input sampling is performed using the serial clock specified by ASIM20.
When the RxD20 pin input becomes low, the 3-bit counter starts counting, and when half the time
determined by the specified baud rate has passed, the data sampling start timing signal is output. If the
RxD20 pin input sampled again as a result of this start timing signal is low, it is identified as a start bit,
the 3-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 receive
buffer register 20 (RXB20), and a reception completion interrupt (INTSR20) is generated.
If an error is generated, the receive data in which the error was generated is still transferred to RXB20,
and INTSR20 is generated.
If the RXE20 bit is reset (0) during the receive operation, the receive operation is stopped immediately.
In this case, the contents of RXB20 and asynchronous serial interface status register 20 (ASIS20) are
not changed, and INTSR20 is not generated.
Figure 13-9. Asynchronous Serial Interface Reception Completion Interrupt Timing
Caution Be sure to read receive buffer register 20 (RXB20) even if a receive error occurs. If RXB20
is not read, an overrun error will be generated when the next data is received, and the
receive error state will continue indefinitely.
STOP
Parity
D7
D6
D2
D1
D0
START
RxD20 (Input)
INTSR20
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