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Rev. 5.00, 09/03, page 482 of 760
Bit 7 Bit 0 Bit 7 Bit 0 Bit 1 Bit 6
Serial
clock
Serial
data
Transfer direction
Bit 7
RXI interrupt
handler reads data
and clears RDRF
bit to 0
1 frame
RXI interrupt
request
generated
RXI interrupt
request
generated
ERI interrupt
request generated
by overrun error
RDRF
ORER
Figure 14.22 Example of SCI Receive Operation
Transmitting and Receiving Serial Data Simultaneously (Synchronous Mode): Figure 14.23
shows a sample flowchart for transmitting and receiving serial data simultaneously. The procedure
for setting the SCI to transmit and receive serial data simultaneously is:
1. SCI status check and transmit data write: Read the serial status register (SCSSR), check that
the TDRE bit is 1, then write transmit data in the transmit data register (SCTDR) and clear
TDRE to 0. The TXI interrupt can also be used to determine if the TDRE bit has changed from
0 to 1.
2. Receive error handling: If a receive error occurs, read the ORER bit in SCSSR to identify the
error. After executing the necessary error handling, clear ORER to 0. Transmitting/receiving
cannot resume if ORER remains set to 1.
3. SCI status check and receive data read: Read the serial status register (SCSSR), check that
RDRF is set to 1, then read receive data from the receive data register (SCRDR) and clear
RDRF to 0. The RXI interrupt can also be used to determine if the RDRF bit has changed from
0 to 1.
4. To continue transmitting and receiving serial data: Read the RDRF bit and SCRDR, and clear
RDRF to 0 before the MSB (bit 7) of the current frame is received. Also read the TDRE bit to
check whether it is safe to write (if it reads 1); if so, write data in SCTDR, then clear TDRE to
0 before the MSB (bit 7) of the current frame is transmitted.