MPC823 datasheet(8/74 Pages) MOTOROLA

8 / 74 page

Communication Processor Module

16-386

MPC823 REFERENCE MANUAL

MOTOROLA

AT—Address Type 1–3

These bits contain the function code value used during this SDMA channel memory

access. The AT0 pin is driven with a 1 to identify this SDMA channel access as a DMA

type.

• MRBLR—Each serial management controller has one maximum receive buffer length

number of bytes that the MPC823 writes to a receive buffer on a serial management

controller before it moves on to the next buffer. The MPC823 can write fewer bytes to

the buffer than MRBLR if a condition, such as an error or end-of-frame occurs, but it

never writes more bytes than the MRBLR value. It follows then, that the buffers you

supply must always be at least as long as the MRBLR. The transmit buffers for a serial

management controller are not affected in any way by the value programmed into the

MRBLR. Each transmit buffer can have a different length. You can choose the number

of bytes to be transmitted by programming the DATA LENGTH field in the TX buffer

descriptor.

• RBPTR—The receiver buffer descriptor pointer for each SMCx channel points to the

next buffer descriptor the receiver transfers data to when it is idle or to the current buffer

descriptor during frame processing. After a reset or when the end of the buffer

descriptor table is reached, the communication processor module initializes this pointer

to the value programmed in the RBASE entry. Although you will not usually need to

write the RBPTR in most applications, you can modify it when the receiver is disabled

or when you are sure no receive buffer is currently being used.

• TBPTR—The transmitter buffer descriptor pointer for each SMCx channel points to the

next buffer descriptor the transmitter transfers data from when it is idle or to the current

buffer descriptor during frame transmission. After a reset or when the end of the buffer

descriptor table is reached, the communication processor module initializes this pointer

to the value programmed in the TBASE register. Although you will not usually need to

write TBPTR in most applications, you can modify it when the receiver is disabled or

when you are sure no receive buffer is currently being used.

Note:

The MRBLR is not intended to be dynamically changed while a serial

management controller is operating. However, if it is modified in a single bus

cycle with one 16-bit move (not two 8-bit bus cycles back-to-back), then a

dynamic change in the receive buffer length can be successfully achieved. This

occurs when the communication processor module transfers control to the next

RX buffer descriptor in the table. Thus, a change to MRBLR does not have an

immediate effect. To guarantee that the change occurs on a particular RX buffer

descriptor, you must only change the MRBLR while the SMCx receiver is

disabled. The value of MRBLR must be greater than zero and it must be even if

the character length of the data is greater than 8 bits.

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