Broadcom BCM5722 Programmer's Manual page 80

BCM5722
Data SDI
The isochronous packet is sensitive to the latency delay from the ISO SDI to the TXMAC. This means that the flow control
of the normal data traffic must be carefully handled in order not to queue up too many normal data packets ahead of an ISO
packet in TXMBUF memory.
This flow control can be implemented in either of two methods:
Method 1
The throughput of the TXMAC is calculated such that by the time the ISO packet is written into TXMBUF, it is as close as
possible to the head of the send ring, or there is at most one normal data transmit packet in flight inside TXMBUF.
Method 2
The outgoing data packet is de-pipelined so that, at most, one best effort normal data packet is in-flight inside TXMBUF. In
other words, a request to RDMA is made only when TXMBUF is empty. This method is a simpler, more conservative
approach and is offered as an alternate algorithm when Method 1 is not preferred.
Method Selection
The method is selected by setting 0x0C20–0x0C23 bits [1:0] in the ISO Packet Transmit Support register. These settings
determine the flow control decision scheme used by normal data SDI packets for making an RDMA request:
00 Send Request unconditionally
01 Based on throughput calculation method (Method 1)
10 De-pipeline method (only 1 data packet in TXMBUF at a time, Method 2)
11 Undefined
Note: If the ISO SBD is empty, the DATA SDI makes requests to the RDMA without checking for any of the above
conditions.
RXMAC Time-sync
The Receive MAC compares the EtherType and Protocol ID from the incoming packets against a programmed value that
identifies an inbound time-sync packet. When a match is found, the receive MAC flags the ISO SDI to log the local network
clock into register 0x0C28.
Page 21 Transmit Data Path
Bro adco m C orp or atio n
Programmer's Guide
10/15/07
Document 5722-PG101-R