Serial Peripheral Interface (SPI)
Queuing Transmission Data
MC68HC08AZ32A — Rev 1.0
Technical Data
MOTOROLA
Serial Peripheral Interface (SPI)
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245
For a slave, the transmit data buffer allows back-to-back transmissions
to occur without the slave having to time the write of its data between the
transmissions. Also, if no new data is written to the data buffer, the last
value contained in the shift register will be the next data word
transmitted.
Figure 16-10. SPRF/SPTE CPU Interrupt Timing
BIT
3
MOSI
SCK (CPHA:CPOL =‘1’:0)
SPTE
WRITE TO SPDR
1
CPU WRITES BYTE 2 TO SPDR, QUEUEING
BYTE 2 AND CLEARING SPTE BIT.
CPU WRITES BYTE 1 TO SPDR, CLEARING
SPTE BIT.
BYTE 1 TRANSFERS FROM TRANSMIT DATA
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
3
1
2
2
3
5
SPRF
READ SPSCR
MSB BIT
6
BIT
5
BIT
4
BIT
2
BIT
1
LSB MSB BIT
6
BIT
5
BIT
4
BIT
3
BIT
2
BIT
1
LSB MSB BIT
6
BYTE 2 TRANSFERS FROM TRANSMIT DATA
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
CPU WRITES BYTE 3 TO SPDR, QUEUEING
BYTE 3 AND CLEARING SPTE BIT.
BYTE 3 TRANSFERS FROM TRANSMIT DATA
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
5
8
10
8
10
4
FIRST INCOMING BYTE TRANSFERS FROM SHIFT
REGISTER TO RECEIVE DATA REGISTER, SETTING
SPRF BIT.
6
CPU READS SPSCR WITH SPRF BIT SET.
4
6
9
SECOND INCOMING BYTE TRANSFERS FROM SHIFT
REGISTER TO RECEIVE DATA REGISTER, SETTING
SPRF BIT.
9
11
12 CPU READS SPDR, CLEARING SPRF BIT.
BIT
5
BIT
4
BYTE 1
BYTE 2
BYTE 3
7
12
READ SPDR
7
CPU READS SPDR, CLEARING SPRF BIT.
11 CPU READS SPSCR WITH SPRF BIT SET.
F
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