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Rev. 1.0, 07/01, page 204 of 372
14.5.3 Serial Data Transmission
Figure 14-10 shows an example of SCI operation for transmission in clocked synchronous mode.
In serial transmission, the SCI operates as described below.
1. The SCI monitors the TDRE flag in SSR, and if the flag is 0, the SCI recognizes that data has
been written to TDR, and transfers the data from TDR to TSR.
2. The SCI sets the TDRE flag to 1 and starts transmission. If the TIE bit in SCR3 is set to 1 at
this time, a transmit data empty interrupt (TXI) is generated.
3. 8-bit data is sent from the TxD pin synchronized with the output clock when output clock
mode has been specified, and synchronized with the input clock when use of an external clock
has been specified. Serial data is transmitted sequentially from the LSB (bit 0), from the TxD
pin.
4. The SCI checks the TDRE flag at the timing for sending the MSB (bit 7).
5. If the TDRE flag is cleared to 0, data is transferred from TDR to TSR, and serial transmission
of the next frame is started.
6.
If the TDRE flag is set to 1, the TEND flag in SSR is set to 1, and the TDRE flag maintains
the output state of the last bit. If the TEIE bit in SCR3 is set to 1 at this time, a TEI interrupt
request is generated.
7.
The SCK3 pin is fixed high at the end of transmission.
Figure 14-11 shows a sample flow chart for serial data transmission. Even if the TDRE flag is
cleared to 0, transmission will not start while a receive error flag (OER, FER, or PER) is set to 1.
Make sure that the receive error flags are cleared to 0 before starting transmission.
Synchronization
clock
Serial
data
Bit 1Bit 0 Bit 7 Bit 0
1 frame 1 frame
Bit 1 Bit 6 Bit 7
TDRE
TEND
LSI
operation
User
processing
TXI interrupt request generated
Data written
to TDR
TDRE flag
cleared
to 0
TXI interrupt
request
generated
TEI interrupt request
generated
Figure 14-10 Example of SCI Transmission in Clocked Synchronous Mode