
CHAPTER 14 SERIAL INTERFACE 20
User’s Manual U14800EJ3V0UD
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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 at the time when half the
time determined by 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 14-9. Asynchronous Serial Interface Reception Completion Interrupt Timing
Stop
Parity
D7
D6
D2
D1
D0
Start
RxD20 (Input)
INTSR20
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 occurred when the next data is received,
and the receive error state will continue indefinitely.