ITP value non-periodically change. Is it normal? How actually it work?

[ChongHan]

Hi, Good day.

From USB 3.0 Spec, it stated "Isochronous Timestamp Packet, sent periodically by a host to inform devices on the USB of the current bus time."

I extract the ITP value in device and send it back to host application so that my software only analyzer can see what the ITP value. Here, I can see that the bus interval counter work fine that increase 1 by 1 unti 0x3FFF and roll back to zero, however, the Delta is zero at almost time, it will sometime have delta in some ITP packet.

As stated in Spec, ITP suppose transmitted in periodic time, so why it will suddenly change the delta value in certain ITP packet? Or it is normal? If it is normal, how actually ITP work?

Thanks in advance.

[Jan Axelson]

My interpretation is that the reason for the delta value is to indicate when the ITP's time has varied for any reason, and that is what you are seeing. I don't know why the time might vary but it seems that the spec recognized that this might occur and that a device might want to know this.