MC68HC908GZ8 Datasheet, PDF (291/344 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Serial Peripheral Interface (SPI) Module
I/O Signals
21.12.1 MISO (Master In/Slave Out)
MISO is one of the two SPI module pins that transmits serial data. In full duplex
operation, the MISO pin of the master SPI module is connected to the MISO pin of
the slave SPI module. The master SPI simultaneously receives data on its MISO
pin and transmits data from its MOSI pin.
Slave output data on the MISO pin is enabled only when the SPI is configured as
a slave. The SPI is configured as a slave when its SPMSTR bit is logic 0 and its SS
pin is at logic 0. To support a multiple-slave system, a logic 1 on the SS pin puts
the MISO pin in a high-impedance state.
When enabled, the SPI controls data direction of the MISO pin regardless of the
state of the data direction register of the shared I/O port.
21.12.2 MOSI (Master Out/Slave In)
MOSI is one of the two SPI module pins that transmits serial data. In full-duplex
operation, the MOSI pin of the master SPI module is connected to the MOSI pin of
the slave SPI module. The master SPI simultaneously transmits data from its MOSI
pin and receives data on its MISO pin.
When enabled, the SPI controls data direction of the MOSI pin regardless of the
state of the data direction register of the shared I/O port.
21.12.3 SPSCK (Serial Clock)
The serial clock synchronizes data transmission between master and slave
devices. In a master MCU, the SPSCK pin is the clock output. In a slave MCU, the
SPSCK pin is the clock input. In full-duplex operation, the master and slave MCUs
exchange a byte of data in eight serial clock cycles.
When enabled, the SPI controls data direction of the SPSCK pin regardless of the
state of the data direction register of the shared I/O port.
21.12.4 SS (Slave Select)
The SS pin has various functions depending on the current state of the SPI. For an
SPI configured as a slave, the SS is used to select a slave. For CPHA = 0, the SS
is used to define the start of a transmission. (See 21.5 Transmission Formats.)
Since it is used to indicate the start of a transmission, the SS must be toggled high
and low between each byte transmitted for the CPHA = 0 format. However, it can
remain low between transmissions for the CPHA = 1 format. See Figure 21-12.
MC68HC908GZ8
MOTOROLA
Serial Peripheral Interface (SPI) Module
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Data Sheet
291