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Serial Peripheral Interface Module (SPI)
General Release Specification
MC68HC(7)08LN56 — Rev. 2.0
176
Serial Peripheral Interface Module (SPI)
MOTOROLA
NONDISCLOSURE
AGREEMENT
REQUIRED
completed. The SPI transmitter empty flag (SPTE) indicates when the
transmit data buffer is ready to accept new data. Write to the transmit
data register only when the SPTE bit is high. Figure 11-8 shows the
timing associated with doing back-to-back transmissions with the SPI
(SPSCK has CPHA: CPOL = 1:0).
Figure 11-8. SPRF/SPTE CPU Interrupt Timing
The transmit data buffer allows back-to-back transmissions without the
slave precisely timing its writes between transmissions as in a system
with a single data buffer. Also, if no new data is written to the data buffer,
the last value contained in the shift register is the next data word to be
transmitted.
For an idle master or idle slave that has no data loaded into its transmit
buffer, the SPTE is set again no more than two bus cycles after the
transmit buffer empties into the shift register. This allows the user to
queue up a 16-bit value to send. For an already active slave, the load of
the shift register cannot occur until the transmission is completed. This
BIT
3
MOSI
SPSCK
SPTE
WRITE TO SPDR
1
CPU WRITES BYTE 2 TO SPDR, QUEUEING BYTE 2
CPU WRITES BYTE 1 TO SPDR, CLEARING SPTE BIT.
BYTE 1 TRANSFERS FROM TRANSMIT DATA
3
1
2
3
5
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
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
CPU WRITES BYTE 3 TO SPDR, QUEUEING BYTE
BYTE 3 TRANSFERS FROM TRANSMIT DATA
5
8
10
8
10
4
FIRST INCOMING BYTE TRANSFERS FROM SHIFT
6
CPU READS SPSCR WITH SPRF BIT SET.
4
6
9
SECOND INCOMING BYTE TRANSFERS FROM SHIFT
9
11
AND CLEARING SPTE BIT.
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
REGISTER TO RECEIVE DATA REGISTER, SETTING
SPRF BIT.
3 AND CLEARING SPTE BIT.
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
REGISTER TO RECEIVE DATA REGISTER, SETTING
SPRF BIT.
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.
(CPHA:CPOL = 1:0)