Spi Commands; The Spi Buffer Descriptor (Bd) Table; Spi Buffer Descriptors (Bds) - Freescale Semiconductor MPC850 User Manual

31.6 SPI Commands

Table 31-7 lists transmit/receive commands sent to the CPM command register (CPCR).
Command
Initializes all transmit parameters in the parameter RAM to their reset state and should be issued only
INIT TX
when the transmitter is disabled. The
PARAMETERS
both the Tx and Rx parameters.
Forces the SPI controller to close the current RxBD and use the next BD for subsequently received
CLOSE RXBD
data. If the controller is not receiving data, no action is taken. Use this command to extract data from a
partially full buffer.
Initializes all receive parameters in the parameter RAM to their reset state. Should be issued only when
INIT RX
the receiver is disabled. The
PARAMETERS
Tx and Rx parameters.

31.7 The SPI Buffer Descriptor (BD) Table

As shown in Figure 31-10, buffer descriptors (BDs) are organized into separate Rx and Tx
BD tables in dual-port RAM. The tables have the same basic configuration as the SCCs and
SMCs and form circular queues that determine the order buffers are transferred. The CPM
uses BDs to confirm reception and transmission or to indicate error conditions so that the
core knows buffers have been serviced. The buffers themselves can be placed in external
memory or in any unused parameter area of the dual-port RAM.
Dual-Port RAM
Tx Buffer
Pointer to SPI
TxBD Table
Pointer to SPI
RxBD Table

31.7.1 SPI Buffer Descriptors (BDs)

Receive and transmit BDs report information about each buffer transferred and whether a
maskable interrupt should be generated. Each 64-bit BD, shown in Figure 31-11 and
Figure 31-12, has the following structure:
Table 31-7. SPI Commands
INIT TX AND RX PARAMETERS
INIT TX AND RX PARAMETERS
TxBD Table
Frame Status
Data Length
Buffer Pointer
RxBD Table
Frame Status
Data Length
Buffer Pointer
Figure 31-10. SPI Memory Structure
Chapter 31. Serial Peripheral Interface (SPI)
Description
command can also be used to reset
command can also be used to reset both the
SPI Commands
External Memory
Tx Buffer
Rx Buffer