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Rev. 1.00, 05/04, page 256 of 544
12.4.6 Serial Data Reception (Asynchronous Mode)
Figure 12.8 shows an example of the operation for reception in asynchronous mode. In serial
reception, the SCI operates as described below.
1. The SCI monitors the communication line, and if a start bit is detected, performs internal
synchronization, receives receive data in RSR, and checks the parity bit and stop bit.
2. If an overrun error (when reception of the next data is completed while the RDRF flag in SSR
is still set to 1) occurs, the ORER bit in SSR is set to 1. If the RIE bit in SCR is set to 1 at this
time, an ERI interrupt request is generated. Receive data is not transferred to RDR. The RDRF
flag remains to be set to 1.
3. If a parity error is detected, the PER bit in SSR is set to 1 and receive data is transferred to
RDR. If the RIE bit in SCR is set to 1 at this time, an ERI interrupt request is generated.
4. If a framing error (when the stop bit is 0) is detected, the FER bit in SSR is set to 1 and receive
data is transferred to RDR. If the RIE bit in SCR is set to 1 at this time, an ERI interrupt
request is generated.
5. If reception finishes successfully, the RDRF bit in SSR is set to 1, and receive data is
transferred to RDR. If the RIE bit in SCR is set to 1 at this time, an RXI interrupt request is
generated. Because the RXI interrupt routine reads the receive data transferred to RDR before
reception of the next receive data has finished, continuous reception can be enabled.
RDRF
FER
0
1 frame
D0 D1 D7 0/1 1 0 D0 D1 D7 0/1 0
1 1
DataStart
bit
Parity
bit
Stop
bit
Start
bit
Data Parity
bit
Stop
bit
ERI interrupt request
generated by framing
error
Idle state
(mark state)
RDR data read and RDRF
flag cleared to 0 in RXI
interrupt handling routine
RXI interrupt
request
generated
Figure 12.8 Example of SCI Receive Operation in Asynchronous Mode
(Example with 8-Bit Data, Parity, One Stop Bit)
Table 12.9 shows the states of the SSR status flags and receive data handling when a receive error
is detected. If a receive error is detected, the RDRF flag retains its state before receiving data.
Reception cannot be resumed while a receive error flag is set to 1. Accordingly, clear the ORER,
FER, PER, and RDRF bits to 0 before resuming reception. Figure 12.9 shows a sample flow chart
for serial data reception.