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User’s Manual U11969EJ3V0UM00
CHAPTER 8 SERIAL INTERFACE FUNCTION
(3) Reception
When reception is enabled, sampling of the RXD pin is started, and reception of data begins when the start
bit is detected. Each time one frame of data or character has been received, the reception completion interrupt
(INTSR) occurs. Usually, the receive data is transferred from the receive buffer (RXB or RXBL) to memory
by this interrupt processing.
(a) Reception enabled status
Reception is enabled when the RXE bits of the ASIM registers are set to 1.
RXE = 1: Reception is enabled
RXE = 0: Reception is disabled
However, to set the reception enabled status, set both the CTXE and CRXE bits of the clocked serial
interface mode register (CSIM) to “0”.
When reception is disabled, the receive hardware stands by in the initial status.
At this time, the reception completion interrupt/receive error interrupt does not occur, and the contents
of the receive buffer are retained.
(b) Starting reception
Reception is started when the start bit is detected.
The RXD pin is sampled with the serial clock from baud rate generator. The RXD pin is sampled again
eight clocks after the falling edge of the RXD pin has been detected. If the RXD pin is low at this time,
it is recognized as the start bit, and reception is started. After that, the RXD pin is sampled in 16 clock
ticks.
If the RXD pin is high eight clocks after the falling edge of the RXD pin has been detected, this falling
edge is not recognized as the start bit. The serial clock counter is reinitialized, and the UART waits for
the input of the next falling edge or valid start bit.
(c) Reception completion interrupt request
When one frame of data has been received with RXE = 1, the receive data in the shift register is transferred
to RXB, and a reception completion interrupt request (INTSR) is generated.
If an error occurs, the receive data that contains an error is transferred to the receive buffer (RXB or RXBL),
and the transmission completion interrupt (INTSR) and receive error interrupt (INTSER) occur simultaneously.
If the RXE bit is reset (0) during receive operation, the receive operation stops immediately. In this case,
the contents of the receive buffer (RXB or RXBL) and the asynchronous serial interface status register
(ASIS) do not change, and neither reception completion interrupt (INTSR) nor reception error interrupt
(INTSER) is generated.
When RXE = 0 (reception disabled), no reception completion interrupt occurs.
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