Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
268 Serial Peripheral Interface Module (SPI) MOTOROLA
Serial Peripheral Interface Module (SPI)
13.12.4 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 sele ct a slave .
For CPHA = 0, the SS is used to define the start of a transmission. S ee
13.6 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 13-12.
Figure 13-12. 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 r egardless 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 13.13.2 SPI
Status and Control Register.
NOTE: A logic 1 voltage 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 it was already in the middle of a transmission.
When an SPI is configured as a master, the SS input can be used in
conjunction with the MODF flag to prevent multip le masters from driving
MOSI and SPSCK. (See 13.7.2 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, th e SS pin can
be used as a general-purpose I/O under the control of the d ata direction
register of the shared I/O port. With MODFEN high, it is an input-only pin
to the SPI regardless of the state of the data direction register of the
shared I/O port.
BYTE 1 BYTE 3
MISO/MOSI BYTE 2
MASTER SS
SLAVE SS
CPHA = 0
SLAVE SS
CPHA = 1