I/O Signals
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 select a slave. For CPHA = 0, the SS is used to define the start of a transmission.
(See 13.5 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 13-12.
MISO/MOSI
MASTER SS
BYTE 1
BYTE 2
BYTE 3
SLAVE SS
CPHA = 0
SLAVE SS
CPHA = 1
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 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 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 multiple 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, 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 regardless
of the state of the data direction register of the shared I/O port.
The CPU can always read the state of the SS pin by configuring the appropriate pin as an input and
reading the port data register. (See Table 13-3.)
Table 13-3. SPI Configuration
SPE
SPMSTR
MODFEN
SPI Configuration
Not enabled
State of SS Logic
General-purpose I/O;
SS ignored by SPI
X(1)
0
0
1
1
1
X
X
0
Slave
Input-only to SPI
General-purpose I/O;
SS ignored by SPI
1
Master without MODF
Master with MODF
1
1
Input-only to SPI
Note 1. X = Don’t care
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
223
FREESCALE [ Freescale ]
