MC68HC08AZ32A Datasheet, PDF (229/312 Pages) Motorola, Inc – HCMOS Microcontroller Unit

I/O Registers
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 16.12.2 SPI Status and Control Register
(SPSCR).
NOTE
A 1 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
transmission has already 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 16.5.7 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 16-3.
Table 16-3. SPI Configuration
SPE SPMSTR
0
X
1
0
1
1
1
1
X = don’t care
MODFEN
X
X
0
1
SPI CONFIGURATION
Not Enabled
Slave
Master without MODF
Master with MODF
STATE OF SS LOGIC
General-purpose I/O;
SS ignored by SPI
Input-only to SPI
General-purpose I/O;
SS ignored by SPI
Input-only to SPI
16.11.5 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.
16.12 I/O Registers
Three registers control and monitor SPI operation:
• SPI control register (SPCR)
• SPI status and control register (SPSCR)
• SPI data register (SPDR)
MC68HC08AZ32A Data Sheet, Rev. 2
Freescale Semiconductor
229