MC68HC08AZ32A Datasheet, PDF (250/456 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Freescale Semiconductor, Inc.
Serial Peripheral Interface (SPI)
16.12.4 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
Transmission Formats on page 234. 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 throughout the transmission for the CPHA = ‘1’ format. See Figure
16-11.
MISO/MOSI
MASTER SS
SLAVE SS
(CPHA =’0’)
SLAVE SS
(CPHA = ‘1’)
BYTE 1
BYTE 2
BYTE 3
Figure 16-11. CPHA/SS Timing
When an SPI is configured as a slave, the SS pin is always configured
as an input. It cannot be used as a general purpose I/O regardless of the
state of the MODFEN control bit. However, the MODFEN bit can still
prevent the state of the SS from creating a MODF error. See SPI Status
and Control Register (SPSCR) on page 254.
NOTE:
A logic ‘1’ on the SS pin of a slave SPI puts the MISO pin in a high-
impedance state. The slave SPI ignores all incoming SPSCK clocks,
even if transmission has already begun.
When an SPI is configured as a master, the SS input can be used in
conjunction with the MODF flag to prevent multiple masters from driving
MOSI and SPSCK. See Mode Fault Error on page 241. For the state of
the SS pin to set the MODF flag, the MODFEN bit in the SPSCK register
must be set. If the MODFEN bit is low for an SPI master, the SS pin can
be used as a general purpose I/O under the control of the data direction
register of the shared I/O port. With MODFEN high, it is an input-only pin
Technical Data
250
MC68HC08AZ32A — Rev 1.0
Serial Peripheral Interface (SPI)
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