MC68HC08AZ60A Datasheet, PDF (272/480 Pages) Motorola, Inc – Microcontrollers

Serial Peripheral Interface (SPI)
17.12.4 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. 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 17-
11.
MISO/MOSI
MASTER SS
SLAVE SS
(CPHA =’0’)
SLAVE SS
(CPHA = ‘1’)
BYTE 1
BYTE 2
BYTE 3
Figure 17-11. 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).
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. 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 to the SPI
Advance Information
272
Serial Peripheral Interface (SPI)
MC68HC08AZ60A — Rev 0.0
MOTOROLA