Serial Peripheral Interface Module (SPI)
I/O Registers
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Serial Peripheral Interface Module (SPI) 269
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 Table 13-3.
13.12.5 VSS (Clock Ground)
VSS is the ground return for the serial clock pin, SPSCK, and the grou nd
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 VSS pin of the master.
13.13 I/O Registers
Three registers control and monitor SPI operation:
SPI control register, SPCR
SPI status and control register, SPSCR
SPI data register, SPDR
Table 13-3. SPI Configuration
SPE SPMSTR MODFEN SPI Configuration State of SS Logic
0X(1)
1. X = don’t care
X Not enabled General-purpose I/O;
SS ignored by SPI
1 0 X Slave Input-only to SPI
11 0 Master
without MODF General-purpose I/O;
SS ignored by SPI
1 1 1 Master with MODF Input-only to SPI