MC908GZ60CFUE Datasheet, PDF (247/352 Pages) Freescale Semiconductor, Inc – Standard features, Features of the CPU08

Queuing Transmission Data
16.5 Queuing Transmission Data
The double-buffered transmit data register allows a data byte to be queued and transmitted. For an SPI
configured as a master, a queued data byte is transmitted immediately after the previous transmission
has 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 SPTE is high. Figure 16-9 shows the
timing associated with doing back-to-back transmissions with the SPI (SPSCK has CPHA: CPOL = 1:0).
WRITE TO SPDR ¹
3
8
SPTE
SPSCK
CPHA:CPOL = 1:0
MOSI
SPRF
2
5
10
MSB BIT BIT BIT BIT BIT BIT LSB MSB BIT BIT BIT BIT BIT BIT LSB MSB BIT BIT BIT
654321
654321
654
BYTE 1
BYTE 2
BYTE 3
4
9
READ SPSCR
6
11
READ SPDR
7
12
1 CPU WRITES BYTE 1 TO SPDR, CLEARING SPTE BIT.
2 BYTE 1 TRANSFERS FROM TRANSMIT DATA
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
3 CPU WRITES BYTE 2 TO SPDR, QUEUEING BYTE 2
AND CLEARING SPTE BIT.
4 FIRST INCOMING BYTE TRANSFERS FROM SHIFT
REGISTER TO RECEIVE DATA REGISTER, SETTING
SPRF BIT.
5 BYTE 2 TRANSFERS FROM TRANSMIT DATA
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
6 CPU READS SPSCR WITH SPRF BIT SET.
7 CPU READS SPDR, CLEARING SPRF BIT.
8 CPU WRITES BYTE 3 TO SPDR, QUEUEING BYTE
3 AND CLEARING SPTE BIT.
9 SECOND INCOMING BYTE TRANSFERS FROM SHIFT
REGISTER TO RECEIVE DATA REGISTER, SETTING
SPRF BIT.
10 BYTE 3 TRANSFERS FROM TRANSMIT DATA
REGISTER TO SHIFT REGISTER, SETTING SPTE BIT.
11 CPU READS SPSCR WITH SPRF BIT SET.
12 CPU READS SPDR, CLEARING SPRF BIT.
Figure 16-9. 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 implies that a back-to-back write to the transmit data register is
not possible. SPTE indicates when the next write can occur.
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
Freescale Semiconductor
247