Vss (Clock Ground); I/O Registers; Spi Control Register - Freescale Semiconductor MC68HC908MR16 Datasheet

Serial Peripheral Interface Module (SPI)
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
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
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
SPE SPMSTR
(1)
0
X
1 0
1 1
1 1
1. X = don't care
15.11.5 V (Clock Ground)
SS
V is the ground return for the serial clock pin, SPSCK, and the ground for the port output buffers. To
SS
reduce the ground return path loop and minimize radio frequency (RF) emissions, connect the ground pin
of the slave to the V
SS

15.12 I/O Registers

Three registers control and monitor SPI operation:
• SPI control register, SPCR
• SPI status and control register, SPSCR
• SPI data register, SPDR

15.12.1 SPI Control Register

The SPI control register (SPCR):
• Enables SPI module interrupt requests
• Selects CPU interrupt requests or DMA service requests
• Configures the SPI module as master or slave
• Selects serial clock polarity and phase
• Configures the SPSCK, MOSI, and MISO pins as open-drain outputs
• Enables the SPI module
210
Table 15-3.
Table 15-3. SPI Configuration
MODFEN SPI Configuration
X Not enabled
X
0 Master without MODF
1 Master with MODF
pin of the master.
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
15.12.2 SPI Status and Control
NOTE
15.6.2 Mode Fault
General-purpose I/O; SS ignored by SPI
Slave
General-purpose I/O; SS ignored by SPI
Error.) For the state of
State of SS Logic
Input-only to SPI
Input-only to SPI
Freescale Semiconductor
Register.