XC908AS60ACFU Motorola Semiconductor Products, XC908AS60ACFU Datasheet - Page 308

• Manufacturer Part Number: XC908AS60ACFU
• Description: MC68HC908AZ60A, MC68HC908AS60A Hcmos Microcontroller Unit Technical Data
• Manufacturer: Motorola Semiconductor Products

Serial Peripheral Interface (SPI)
19.13.4 SS (Slave Select)
Technical Data
308
NOTE:
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
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
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
and Control Register
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
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.
MASTER SS
MISO/MOSI
SLAVE SS
SLAVE SS
CPHA = 0
CPHA = 1
Serial Peripheral Interface (SPI)
Formats.) Since it is used to indicate the start of a
Figure 19-10. CPHA/SS Timing
BYTE 1
on page 312).
Mode Fault Error
BYTE 2
on page 299). For the state
MC68HC908AZ60A — Rev 2.0
BYTE 3
Figure
SPI Status
MOTOROLA
19-10.