DS31256 datasheet(98/183 Pages) MAXIM

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DS31256 256-Channel, High-Throughput HDLC Controller

98 of 183

The host reads from the receive done queue to find which data buffers and their associated descriptors

are ready for processing.

The receive done queue is circular. A set of internal addresses within the device that are accessed by the

host and the DMA keep track of the queue’s addresses. On initialization, the host configures all the

registers, as shown in Table 9-F. After initialization, the DMA only writes to (changes) the write pointer

and the host only writes to the read pointer.

Empty Case

The receive done queue is considered empty when the read and write pointers are identical.

Receive Done-Queue Empty State

empty descriptor

read pointer >

empty descriptor

< write pointer

empty descriptor

Full Case

The receive done queue is considered full when the read pointer is ahead of the write pointer by one

descriptor. Therefore, one descriptor must always remain empty.

Receive Done-Queue Full State

valid descriptor

empty descriptor

< write pointer

read pointer >

valid descriptor

Table 9-F. Receive Done-Queue Internal Address Storage

NAME

ADDRESS

Receive Done-Queue Base Address 0 (lower word)

RDQBA0

0730h

Receive Done-Queue Base Address 1 (upper word)

RDQBA1

0734h

Receive Done-Queue DMA Write Pointer

RDQWP

0740h

Receive Done-Queue Host Read Pointer

RDQRP

073Ch

Receive Done-Queue End Address

RDQEA

0738h

Receive Done-Queue FIFO Flush Timer

RDQFFT

0744h

Note: Receive done-queue end address is not an absolute address. The absolute end address is “Base + RDQEA x 4.”

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