1 master mode, Master mode, Figure 15-3 – Freescale Semiconductor MC68HC908MR32 User Manual
Page 198
Serial Peripheral Interface Module (SPI)
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
198
Freescale Semiconductor
15.4.1 Master Mode
The SPI operates in master mode when the SPI master bit, SPMSTR, is set.
NOTE
Configure the SPI modules as master or slave before enabling them.
Enable the master SPI before enabling the slave SPI. Disable the slave SPI
before disabling the master SPI. See
Only a master SPI module can initiate transmissions. Software begins the transmission from a master SPI
module by writing to the SPI data register. If the shift register is empty, the byte immediately transfers to
the shift register, setting the SPI transmitter empty bit, SPTE. The byte begins shifting out on the MOSI
pin under the control of the serial clock. See
Figure 15-4. Full-Duplex Master-Slave Connections
The SPR1 and SPR0 bits control the baud rate generator and determine the speed of the shift register.
See
15.12.2 SPI Status and Control Register
. Through the SPSCK pin, the baud-rate generator of the
master also controls the shift register of the slave peripheral.
As the byte shifts out on the MOSI pin of the master, another byte shifts in from the slave on the master’s
MISO pin. The transmission ends when the receiver full bit, SPRF, becomes set. At the same time that
SPRF becomes set, the byte from the slave transfers to the receive data register. In normal operation,
Addr.
Register Name
Bit 7
6
5
4
3
2
1
Bit 0
$0044
SPI Control Register
(SPCR)
Read:
SPRIE
R
SPMSTR
CPOL
CPHA
SPWOM
SPE
SPTIE
Write:
Reset:
0
0
1
0
1
0
0
0
$0045
SPI Status and Control
Register (SPSCR)
Read:
SPRF
ERRIE
OVRF
MODF
SPTE
MODFEN
SPR1
SPR0
Write:
R
R
R
R
Reset:
0
0
0
0
1
0
0
0
$0046
SPI Data Register
(SPDR)
Read:
R7
R6
R5
R4
R3
R2
R1
R0
Write:
T7
T6
T5
T4
T3
T2
T1
T0
Reset:
Unaffected by reset
R
= Reserved
Figure 15-3. SPI I/O Register Summary
SHIFT REGISTER
SHIFT REGISTER
BAUD RATE
GENERATOR
MASTER MCU
SLAVE MCU
V
DD
MOSI
MOSI
MISO
MISO
SPSCK
SPSCK
SS
SS