DS31256 datasheet(85/183 Pages) MAXIM

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DS31256 256-Channel, High-Throughput HDLC Controller

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9.2 Receive Side

9.2.1 Overview

The receive DMA uses a scatter-gather technique to write packet data into main memory. The host keeps

track of and decides where the DMA should place the incoming packet data. There are a set of

descriptors that get handed back and forth between the DMA and the host. Through these descriptors, the

host can inform the DMA where to place the packet data and the DMA can tell the host when the data is

ready to be processed.

The operation of the receive DMA has three main areas, as shown in Figure 9-1, Figure 9-2, and

Table 9-B. The host writes to the free-queue descriptors informing the DMA where it can place the

incoming packet data. Associated with each free data buffer location is a free packet descriptor where the

DMA can write information to inform the host about the attributes of the packet data (i.e., status

information, number of bytes, etc.) that it outputs. To accommodate the various needs of packet data, the

host can quantize the free data buffer space into two different buffer sizes. The host sets the size of the

buffers through the receive large buffer size (RLBS) and the receive small buffer size (RSBS) registers.

RLBS

Receive Large Buffer Size Select

0790h

Bit #

Name

LBS7

LBS6

LBS5

LBS4

LBS3

LBS2

LBS1

LBS0

Default

Bit #

Name

n/a

LBS12

LBS11

LBS10

LBS9

LBS8

Default

Note: Bits that are underlined are read-only; all other bits are read-write.

Bits 0 to 12/Large Buffer Select Bit (LBS0 to LBS12)

0000000000000 (0000h) = buffer size is 0 Bytes

1111111111100 (1FFCh) = buffer size is 8188 Bytes

RSBS

Receive Small Buffer Size Select

0794h

Bit #

Name

SBS7

SBS6

SBS5

SBS4

SBS3

SBS2

SBS1

SBS0

Default

Bit #

Name

n/a

SBS12

SBS11

SBS10

SBS9

SBS8

Default

Note: Bits that are underlined are read-only; all other bits are read-write.

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

0000000000000 (0000h) = buffer size is 0 Bytes

1111111111100 (1FFCh) = buffer size is 8188 Bytes

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