DS31256 datasheet(107/183 Pages) MAXIM

107 / 183 page

DS31256 256-Channel, High-Throughput HDLC Controller

107 of 183

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 1, 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 as follows:

1) Priority packets are transmitted as soon as the current standard packet (not packet chain) finishes

transmission.

2) All priority packets are transmitted before any more standard packets are transmitted.

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

Figure 9-13 shows 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 began 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.

DS31256 Datasheet(PDF) 107 Page - Maxim Integrated Products