Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
266 Serial Peripheral Interface Module (SPI) MOTOROLA
Serial Peripheral Interface Module (SPI)
13.11 Low-Power Mode
The WAIT instruction puts the MCU in a low power-consu mption standby
mode.
The SPI module remains active after the ex ecution of a WAIT instructio n.
In wait mode the SPI module registers are not accessible by the CPU.
Any enabled CPU interrupt request from the S PI module can bring the
MCU out of wait mode.
If SPI module functions are not required during wait mode , reduce power
consumption by disabling the SPI module before executing the WAIT
instruction.
To exit wait mode when an overflow condition occurs, enable the OVRF
bit to generate CPU interrupt requests by setting the error interrupt
enable bit (ERRIE). See 13.8 Interrupts.
Since the SPTE bit cannot be cleared during a break with the BCFE bit
cleared, a write to the transmit data register in break mode does not
initiate a transmission nor is this data transferred into the shift register.
Therefore, a write to the SPDR in break mode with the BCFE bit cleared
has no effect.
13.12 I/O Signals
The SPI module has five I/O pins and shares four of them with a pa rallel
I/O port. The pins are:
MISO — Data received
MOSI — Data transmitted
SPSCK — Serial clock
•SS — Slave select
The SPI has limited inter-integrated circuit (I2C) capability (requiring
software support) as a master in a single-master environment. To
communicate with I2C peripherals, MOSI becomes an open-drain output
when the SPWOM bit in the SPI control register is set. In I2C
communication, the MOSI and MISO pins are connected to a
bidirectional pin from the I2C peripheral and through a pullup resistor
to VDD.