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CHAPTER 8 SERIAL INTERFACE FUNCTION
(3) Reception
When reception is enabled, sampling of the RXDn 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
(INTSRn) occurs. Usually, the receive data is transferred from the receive buffer (RXBn) to memory by this
interrupt processing.
(a) Reception enabled status
Reception is enabled when the RXEn bits of the ASIMn0 registers are set to 1.
RXEn = 1: Reception is enabled
RXEn = 0: Reception is disabled
However, to set the reception enabled status, set both the CTXEn and CRXEn bits of the clocked serial
interface mode register (CSIMn) 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 RXDn pin is sampled with the serial clock from baud rate generator n (BRGn). The RXDn pin is
sampled again eight clocks after the falling edge of the RXDn pin has been detected. If the RXDn pin
is low at this time, it is recognized as the start bit, and reception is started. After that, the RXDn pin is
sampled in 16 clock ticks.
If the RXDn pin is high eight clocks after the falling edge of the RXDn pin has been detected, this falling
edge is not recognized as the start bit. The serial clock counter is reinitialized, and the UARTn 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 RXEn = 1, the receive data in the shift register is
transferred to RXBn, and a reception completion interrupt request (INTSRn) is generated.
If an error occurs, the receive data that contains an error is transferred to the receive buffer (RXBn), and
the transmission completion interrupt (INTSRn) and receive error interrupt (INTSERn) occur simultaneously.
If the RXEn bit is reset (0) during receive operation, the receive operation stops immediately. In this case,
the contents of the receive buffer (RXBn) and the asynchronous serial interface status register (ASISn)
do not change, and neither reception completion interrupt (INTSRn) nor reception error interrupt
(INTSERn) is generated.
When RXEn = 0 (reception disabled), reception completion interrupt request does not occur.