Motorola MPC823e - Page 896

Communication Processor Module
16-442 MPC823e REFERENCE MANUAL MOTOROLA
SPI
COMMUNICATION
16
PROCESSOR MODULE
16.12.3.4 SPI BUFFER DESCRIPTOR RING. The data associated with the serial
peripheral interface is stored in buffers, which are referenced by buffer descriptors
organized in a buffer descriptor ring located in the dual-port RAM. This ring has the same
basic configuration as the SCCx, SMCx, USB, and I
2
C controllers.
The buffer descriptor ring forms a circular queue that helps you organize the buffers you
want to transmit or receive. Using the buffer descriptors, the communication processor
module confirms reception and transmission or indicates error conditions so that the
processor knows the buffers have been serviced. The actual buffers can reside in either
external or internal memory and the data buffers can reside in the parameter area of another
unused controller if it is not enabled.
Figure 16-123. SPI Memory Format
FRAME STATUS
DATA LENGTH
DATA POINTER
POINTER TO SPI
TX RING
DUAL-PORT RAM
EXTERNAL MEMORY
TX BD RING
TX DATA BUFFER
POINTER TO SPI
RX RING
RX BD RING
RX DATA BUFFER
TX DATA BUFFER
FRAME STATUS
DATA LENGTH
DATA POINTER