Motorola MPC8260 - Manual (Page 937)

MOTOROLA Chapter 33. Serial Peripheral Interface (SPI) 33-13
Part IV. Communications Processor Module
33.7 The SPI Buffer Descriptor (BD) Table
As shown in Figure 33-10, BDs are organized into separate RxBD and TxBD tables in dual-
port RAM. The tables have the same basic conÞguration as for the SCCs and SMCs and
form circular queues that determine the order buffers are transferred. The CP uses BDs to
conÞrm 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.
Figure 33-10. SPI Memory Structure
33.7.1 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 33-11 and
Figure 33-12, has the following structure:
¥ The half word at offset + 0 contains status and control bits. The CP updates the status
bits after the buffer is sent or received.
¥ The half word at offset + 2 contains the data length (in bytes) that is sent or received.
Ñ For an RxBD, this is the number of octets the CP writes into this RxBDÕs buffer
once the BD closes. The CP updates this Þeld after the received data is placed
into the buffer. Memory allocated for this buffer should be no smaller than
MRBLR.
Ñ For a TxBD, this is the number of octets the CP should transmit from its buffer.
Normally, this value should be greater than zero. If the character length is more
INIT RX
PARAMETERS
Initializes all receive parameters in the parameter RAM to their reset state. Should be issued only when
the receiver is disabled. The
INIT TX AND RX PARAMETERS command can also be used to reset both the Tx
and Rx parameters.
Table 33-7. SPI Commands (Continued)
Command Description
Frame Status
Data Length
Buffer Pointer
Frame Status
Data Length
Buffer Pointer
Tx Buffer
Pointer to SPI
RxBD Table
Rx Buffer
Dual-Port RAM External Memory
TxBD Table
RxBD Table
Tx Buffer
Pointer to SPI
TxBD Table