Datasheet

Functional Description
MC68HC908GZ16 • MC68HC908GZ8 Data Sheet, Rev. 4
Freescale Semiconductor 233
17.4 Functional Description
Figure 17-2 summarizes the SPI I/O registers and Figure 17-3 shows the structure of the SPI module.
The SPI module allows full-duplex, synchronous, serial communication between the MCU and peripheral
devices, including other MCUs. Software can poll the SPI status flags or SPI operation can be interrupt
driven.
If a port bit is configured for input, then an internal pullup device may be enabled for that port bit. See
13.5.3 Port C Input Pullup Enable Register.
The following paragraphs describe the operation of the SPI module.
17.4.1 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 17.13.1 SPI Control Register.
Only a master SPI module can initiate transmissions. Software begins the transmission from a master SPI
module by writing to the transmit 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 17-4.
The SPR1 and SPR0 bits control the baud rate generator and determine the speed of the shift register.
(See 17.13.2 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,
SPRF signals the end of a transmission. Software clears SPRF by reading the SPI status and control
register with SPRF set and then reading the SPI data register. Writing to the SPI data register clears the
SPTE bit.
Addr.Register Name Bit 7654321Bit 0
$0010
SPI Control Register
(SPCR)
See page 247.
Read:
SPRIE R SPMSTR CPOL CPHA SPWOM SPE SPTIE
Write:
Reset:00101000
$0011
SPI Status and Control
Register (SPSCR)
See page 249.
Read: SPRF
ERRIE
OVRF MODF SPTE
MODFEN SPR1 SPR0
Write:
Reset:00001000
$0012
SPI Data Register
(SPDR)
See page 250.
Read: R7 R6 R5 R4 R3 R2 R1 R0
Write: T7 T6 T5 T4 T3 T2 T1 T0
Reset: Unaffected by reset
R
= Reserved
= Unimplemented
Figure 17-2. SPI I/O Register Summary