Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 843

Powerquicc family

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The half word at offset + 2 contains the data length (in bytes) that is sent or received.

— For an RxBD, this is the number of octets the CPM writes into this RxBD's buffer once the BD

closes. The CPM updates this field after the received data is placed into the buffer. Memory

allocated for this buffer should be no smaller than MRBLR.

— For a TxBD, this is the number of octets the CPM should transmit from its buffer. Normally,

this value should be greater than zero. If the character length is more than 8 bits, the data length

should be even. For example, to send three characters of 8-bit data the data length field should

be initialized to 3. However, to send three characters of 9-bit data, the data length field should

be initialized to 6 since the three 9-bit data fields occupy three half-words in memory. The CPM

never modifies this field.

The word at offset + 4 points to the beginning of the buffer.

— For an RxBD, the pointer must be even and can point to internal or external memory.

— For a TxBD, the pointer can be even or odd, unless the character exceeds 8 bits, for which it

must be even. The buffer can be in internal or external memory.

SPI Receive BD (RxBD)

The CPM uses RxBDs to report on each received buffer. It closes the current buffer, generates a maskable

interrupt, and starts receiving data in the next buffer once the current buffer is full. The CPM also closes

the buffer when the SPI is configured as a slave and SPISEL is negated, indicating that reception stopped.

The core should write RxBD bits before the SPI is enabled. The format of an RxBD is shown in

describes the RxBD status and control fields.

Table 30-8. SPI RxBD Status and Control Field Descriptions

0 The buffer is full or stopped receiving because of an error. The core can examine or write to any

fields of this RxBD, but the CPM does not use this BD while E = 0.

1 The buffer is empty or reception is in progress. The CPM owns this RxBD and its buffer. Once E

is set, the core should not write any fields of this RxBD.

Reserved, should be cleared.

Freescale Semiconductor

Rx Buffer Pointer

Figure 30-11. SPI Receive BD (RxBD)

MPC885 PowerQUICC Family Reference Manual, Rev. 2

Serial Peripheral Interface (SPI)