DS3134 datasheet(99/203 Pages) DALLAS

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DS3134

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Bits 0 to 12 / Small Buffer Select Bit (SBS0 to SBS12).

0000000000000 (0000h) = Buffer Size is 0 Bytes

1111111111111 (1FFFh) = Buffer Size is 8191 Bytes

On a HDLC channel basis in the Receive DMA Configuration RAM, the Host will instruct the DMA on

how to use the large and small buffers for the incoming packet data on that particular HDLC channel.

The Host has three options (1) only use Large Buffers, (2) only use Small Buffers, and (3) first fill a Small

Buffer then if the incoming packet requires more buffer space, use one or more Large Buffers for the

remainder of the packet. The Host selects which option via the Size field in the Receive Configuration

RAM (see Section 8.1.5). Large Buffers are best used for data intensive, time insensitive packets like

graphics files whereas small buffers are best used for time sensitive information like real-time voice.

Receive DMA Main Operational Areas Table 8.1.1A

Name

Section

Description

Packet

Descriptors

8.1.2

A dedicated area of memory that describes the location

and attributes of the packet data.

Free Queue

Descriptors

8.1.3

A dedicated area of memory that the Host will write to

inform the DMA where to store incoming packet data.

Done Queue

Descriptors

8.1.4

A dedicated area of memory that the DMA will write to

inform the Host that the packet data is ready for

processing.

The Done Queue Descriptors contain information that the DMA wishes to pass to the Host. Via the Done

Queue Descriptors the DMA informs the Host about the incoming packet data and where to find the

Packet Descriptors that it has written into main memory. Each completed Descriptor contains the starting

address of the data buffer where the packet data is stored.

If enabled, the DMA can burst read the Free Queue Descriptors and burst writes the Done Queue

Descriptors. This helps minimize PCI Bus accesses, freeing the PCI Bus up to do more time critical

functions. See Sections 8.1.3 and 8.1.4 for more details on this feature.

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