uPD789074 Subseries 8-Bit Single-Chip Microcontrollers UD - Home

CHAPTER 9 SERIAL INTERFACE 20(d) ReceptionWhen bit 6 (RXE20) of asynchronous serial interface mode register 20 (ASIM20) is set to 1, a receiveoperation is enabled and sampling of the RxD20 pin input is performed.RxD20 pin input sampling is performed using the serial clock specified by BRGC20.When the RxD20 pin input becomes low, the 3-bit counter starts counting, and at the time when half thetime determined by the specified baud rate has passed, the data sampling start timing signal is output. Ifthe RxD20 pin input sampled again as a result of this start timing signal is low, it is identified as a startbit, the 3-bit counter is initialized and starts counting, and data sampling is performed. When characterdata, 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 receivebuffer register 20 (RXB20), and a reception completion interrupt (INTSR20) is generated.If an error occurs, the receive data in which the error occurred is still transferred to RXB20, andINTSR20 is generated.If the RXE20 bit is reset to 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) arenot changed, and INTSR20 is not generated.Figure 9-9. Asynchronous Serial Interface Reception Completion Interrupt TimingRxD20 (Input)D0D1D2D6D7ParitySTOPSTARTINTSR20Caution Be sure to read receive buffer register 20 (RXB20) even if a receive error occurs. IfRXB20 is not read, an overrun error will occur when the next data is received, and thereceive error state will continue indefinitely.136User’s Manual U14801EJ3V1UD