Digi NS7520 manual Buffer descriptor field definitions, Field Description

Models: NS7520

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D M A b u f f e r d e s c r i p t o r

Buffer descriptor field definitions

Field

Description

 

 

Source buffer pointer/Buffer

The source buffer pointer field identifies the starting location of

pointer

the data buffer. The source buffer pointer can start on any byte

 

boundary for fly-by memory-to-peripheral operations. The

 

source buffer must be aligned on 32-bit boundaries to support

 

peripheral-to-memory operations.

 

 

Status

Peripherals can use the 16-bit status field to store transmit and

 

receive status words. The status field is updated when the

 

current descriptor is completed.

 

 

Buffer length: Peripheral-to-

In fly-by peripheral-to-memory operations, indicates the

memory

maximum number of bytes available in the receive buffer pointed

 

to by the source buffer pointer. After filling the receive buffer

 

with peripheral data, the DMA controller updates this field with

 

the actual receive data byte count.

 

 

Buffer length: Memory-to-

In fly-by memory-to-peripheral operations, indicates the number

peripheral

of bytes to move to the peripheral device, pointed to by the

 

source address pointer. After completing the transfer, the DMA

 

controller updates this field in a transmit buffer descriptor with

 

the actual data byte count (useful during error conditions).

 

 

Buffer length: Memory-to-

Indicates the number of bytes to move between the source and

memory

destination locations. After completing a memory-to-memory

 

transfer, the DMA controller updates this field with the

 

remaining data byte count (useful during error conditions).

 

 

Destination buffer pointer

Used only when the DMA channel is configured for memory-to-

 

memory operations. The destination address pointer must start

 

on the same byte boundary as the source address pointer. If the

 

source and destination byte boundaries are different, the data

 

operand size must be set for 8-bit operations (see the SIZE field

 

in "DMA Control register," beginning on page 136).

 

 

Table 48: Buffer descriptor field definitions

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Digi NS7520 manual Buffer descriptor field definitions, Field Description