mc68hc908gp32 Freescale Semiconductor, Inc, mc68hc908gp32 Datasheet - Page 189

• Manufacturer Part Number: mc68hc908gp32
• Description: M68hc08 Microcontrollers Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

15.12.4 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
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
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
Register.)
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
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
15.12.5 CGND (Clock Ground)
CGND is the ground return for the serial clock pin, SPSCK, and the ground for the port output buffers. It
is internally connected to V
Freescale Semiconductor
15.5 Transmission
Note 1. X = Don’t care
SPE
0
1
1
1
MASTER SS
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.
MISO/MOSI
SPMSTR
SLAVE SS
SLAVE SS
CPHA = 0
CPHA = 1
X
0
1
1
(1)
Formats.) Since it is used to indicate the start of a transmission, the SS must be
MODFEN
SS
X
X
0
1
as shown in
MC68HC908GP32 Data Sheet, Rev. 10
Figure 15-12. CPHA/SS Timing
BYTE 1
Table
Table 15-3. SPI Configuration
Master without MODF
SPI Configuration
Master with MODF
15-3.)
Not enabled
Table
Slave
NOTE
15-1.
Figure
BYTE 2
15-12.
General-purpose I/O; SS ignored by SPI
General-purpose I/O; SS ignored by SPI
15.7.2 Mode Fault
15.13.2 SPI Status and Control
Function of SS Pin
Input-only to SPI
Input-only to SPI
BYTE 3
Error.) For the state of
I/O Signals
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