Responder Packet Timing issue

[ericthegeek]

Originally Posted by berntd

Interpreting that, any controller must thus wait 2.8ms to recieve a response. If our responder thus uses a timeout of 2.1ms, it will work with all controllers.

Unfortunately, no. The 2.8ms time includes time for transmission delay. Transparent Inline Devices (hubs, splitters, etc.) are allowed to delay the signal by up to 88us each way. The timing values given in the standard allow for up to 4 transparent inline devices, which gives a round-trip delay of 704us. See E1.20-2010 Section 4.2.2.

The other problem you're going to run into is that if you wait until the end of the response time to get the DMA interrupt and start processing the packet, you'll have no time left to process the request and send the response before the controller gives up and assumes the response was lost.