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In transmitting serial data, the SCI operates as follows.
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•
The SCI monitors the TDRE flag in SSR. When the TDRE flag is cleared to 0 the SCI
recognizes that TDR contains new data, and loads this data from TDR into TSR.
•
After loading the data from TDR into TSR, the SCI sets the TDRE flag to 1 and starts
transmitting. If the TIE bit in SCR is set to 1, the SCI requests a transmit-data-empty interrupt
(TXI) at this time.
Serial transmit data is transmitted in the following order from the TxD pin:
— Start bit:
— Transmit data:
— Multiprocessor bit: One multiprocessor bit (MPBT value) is output.
— Stop bit:
— Mark state:
•
The SCI checks the TDRE flag when it outputs the stop bit. If the TDRE flag is 0, the SCI
loads data from TDR into TSR, outputs the stop bit, then begins serial transmission of the
next frame. If the TDRE flag is 1, the SCI sets the TEND flag in SSR to 1, outputs the stop
bit, then continues output of 1 bits in the mark state. If the TEIE bit is set to 1 in SCR, a
transmit-end interrupt (TEI) is requested at this time.
Figure 13-11 shows an example of SCI transmit operation using a multiprocessor format.
Start
1
bit
0
D0
TDRE
TEND
TXI
TXI interrupt handler
request
writes data in TDR and
clears TDRE flag to 0
Figure 13-11 Example of SCI Transmit Operation (8-Bit Data with Multiprocessor Bit and
One 0 bit is output.
7 or 8 bits are output, LSB first.
One or two 1 bits (stop bits) are output.
Output of 1 bits continues until the start bit of the next transmit data.
Multi-
processor
bit
Stop
Start
Data
bit
bit
D1
D7
0/1
1
0
TXI
request
1 frame
One Stop Bit)
478
Multi-
processor
bit
Stop
Data
bit
D0
D1
D7
0/1
1
TEI request
1
Idle (mark)
state
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