Serial Peripheral Interface (SPI)
I/O Signals
MC68HC08AS32A — Rev. 1
Data Sheet
MOTOROLA
Serial Peripheral Interface (SPI)
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233
Figure 15-13. 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
15.13.2 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
a transmission already has 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
15.6.2 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 data register. See
Table 15-4
.
15.12.5 V
SS
(Clock Ground)
V
SS
is the ground return for the serial clock pin, SPSCK, and the ground for the port
output buffers. To reduce the ground return path loop and minimize radio frequency
(RF) emissions, connect the ground pin of the slave to the V
SS
pin.
BYTE 1
BYTE 3
MISO/MOSI
BYTE 2
MASTER SS
SLAVE SS
CPHA = 0
SLAVE SS
CPHA = 1
Table 15-4. SPI Configuration
SPE
SPMSTR
MODFEN
SPI Configuration
State of SS Logic
0
X
X
Not enabled
General-purpose I/O;
SS ignored by SPI
1
0
X
Slave
Input-only to SPI
1
1
0
Master without MODF
General-purpose I/O;
SS ignored by SPI
1
1
1
Master with MODF
Input-only to SPI
X = don’t care
F
Freescale Semiconductor, Inc.
n
.