DS3134 datasheet(121/203 Pages) DALLAS

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DS3134

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Transmit DMA Main Operational Areas Table 8.2.1A

Name

Section

Description

Packet

Descriptors

8.2.2

A dedicated area of memory that describes the location

and attributes of the packet data.

Pending

Queue

Descriptors

8.2.3

A dedicated area of memory that the Host will write to

inform the DMA that packet data is queued and ready

for transmission

Done Queue

Descriptors

8.2.4

A dedicated area of memory that the DMA will write to

inform the Host that the packet data has been

transmitted

Host Linking of Data Buffers

As mentioned earlier, the data buffers are limited to a length of 8191 bytes. If an outgoing packet requires

more memory space than the available data buffer contains, then the Host can link multiple data buffers

together to handle a packet length of any size. The Host does this via the End Of Frame (EOF) bit in the

Packet Descriptor. Each data buffer has a one-to-one association with a Packet Descriptor. If the Host

wishes to link multiple data buffers together, then the EOF bit will be set to zero in all but the last data

buffer. Figure 8.2.1A contains an example for HDLC channel number 5 where the Host has linked three

data buffers together. The transmit DMA knows where to find the next data buffer when the EOF bit is

set to zero via the Next Descriptor Pointer field.

Host Linking of Packets (Packet Chaining)

The Host also has the option to link multiple packets together in a chain. Via the Chain Valid (CV) bit in

the Packet Descriptor, the Host can inform the transmit DMA that the Next Descriptor Pointer field

contains the descriptor of another HDLC packet that is ready for transmission.

The transmit DMA

ignores the CV bit until it sees EOF = 1 which indicates the end of a packet. If CV = 1 when EOF = 1,

then this indicates to the transmit DMA that it should use the Next Descriptor Pointer field to find the

next packet in the chain. Figure 8.2.1C provides an example of packet chaining. Each column in Figure

8.2.1C represents a separate packet chain. In column 1, three data buffers have been linked together by

the Host for Packet #1 and then the Host has created a packet chain by setting CV = 1 in the last

descriptor of Packet #1.

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