DS3134 datasheet(122/203 Pages) DALLAS

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DS3134

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DMA Linking of Packets (Horizontal Link Listing)

The transmit DMA also has the ability to link packets together. Internally, the transmit DMA can store up

to two packets chains but if the Host places more packet chains into the Pending Queue, then the transmit

DMA must begin linking these chains together externally. The transmit DMA does this by writing to

Packet Descriptors. As an example, see Figure 8.2.1C. If columns 1 and 2 were the only two packet

chains queued for transmission, then the transmit DMA would not need to begin linking packet chains

together but as soon as column 3 was queued for transmission, the transmit DMA had to store the third

chain externally because it had no more room internally. The transmit DMA links the packet chain in the

third column to the one in the second column by writing the 1st descriptor of the third chain in the Next

Pending Descriptor Pointer field of the 1st descriptor of the second column (it also sets the PV bit to one).

As shown in Figure 8.2.1C, this chaining was carried one step farther to link the forth column to the third.

Priority Packets

The Host has the option to change the order in which packets are transmitted by the DMA. If the Host

sets the Priority Packet (PRI) bit in the Pending Queue Descriptor to a one, then the transmit DMA knows

that this packet is a priority packet and should be transmitted ahead of all standard packets. The rules for

packet transmission are:

1. Priority packets will be transmitted as soon as the current standard packet (not packet chain) finishes

transmission.

2. All priority packets will be transmitted before any more standard packets are transmitted.

3. Priority packets are ordered on a first come, first served basis.

Figure 8.2.1D provides an example of a set of priority packets interrupting a set of standard packets. In

the example, the first priority packet chain (shown in column 2) was read by the transmit DMA from the

Pending Queue while it was transmitting standard packet #1. It waited until standard packet #1 was

complete and then begins sending the priority packets. While column 2 was being sent, the priority

packet chains of columns 3 and 4 arrived in the Pending Queue so the transmit DMA linked column four

to column three and then waited until all of the priority packets were transmitted before returning to the

standard packet chain in column 1. Note that the packet chain in column 1 was interrupted to transmit the

priority packets.

In other words, the transmit DMA did not wait for the complete packet to finish

transmitting, only the current packet.

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