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CHAPTER 15 SERIAL INTERFACE SIO3
(2) Communication operations
In the three-wire serial I/O mode, data is transmitted and received in 8-bit units. Each bit of data is transmitted
or received in synchronized with the serial clock.
The serial I/O shift register (SIO) is shifted in synchronized with the falling edge of the serial clock. Transmission
data is held in the SO latch and is output from the SO pin. Data that is received via the SI pin in synchronized
with the rising edge of the serial clock is latched to SIO.
Completion of an 8-bit transfer automatically stops operation of SIO3 and sets a serial transfer completion flag.
Figure 15-5. Three-Wire Serial I/O Mode Timing
SI
DI7
DI6
DI5
DI4
DI3
DI2
DI1
DI0
Serial transfer
completion flag
Serial clock
1
SO
DO7
DO6
DO5
DO4
DO3
DO2
DO1
DO0
2
3
4
5
6
7
8
Transfer completion
Transfer starts in synchronized with the serial clock’s falling edge
(3) Transfer start
A serial transfer starts when the following two conditions have been satisfied and transfer data has been set to
serial I/O shift register (SIO).
• SIO3 operation control bit (CSIE) = 1
• After an 8-bit serial transfer, the internal serial clock is either stopped or is set to high level.
• Transmit or transmit/receive mode
When CSIE = 1 and MODE = 0, transfer starts when writing to SIO.
• Receive-only mode
When CSIE = 1 and MODE = 1, transfer starts when reading from SIO.
Caution
After data has been written to SIO, transfer will not start even if the CSIE bit value is set
to “1”.
Completion of an 8-bit transfer automatically stops the serial transfer operation and sets a serial transfer
completion flag.
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