PORTS Forum

Ports and Interfaces => USB => Topic started by: ChongHan on April 09, 2015, 05:49:10 am

Title: Different between SOF in USB 2.0 and ITP in USB 3.0
Post by: ChongHan on April 09, 2015, 05:49:10 am

Hi guys,

Good days.
I would like to know what is the differences between SOF and ITP?

1) Is it the clock used to produce SOF or ITP is same?
2) SOF is not guaranteed to be delivered in every frame, how about ITP? Will it guaranteed to be sent at each bus interval boundary within 8us windows?
3) For the clock for SOF generation, it has 500ppm from spec, how about ITP ppm?

I can't these answer in USB 3.0 spec. Hope someone here to help me. Thanks.

Best regards
Chong Han

Title: Re: Different between SOF in USB 2.0 and ITP in USB 3.0
Post by: Jan Axelson on April 09, 2015, 12:17:30 pm

8.7 in the USB 3.1 spec says:

The host shall transmit an ITP in every bus interval within tTimestampWindow from a bus interval boundary if the root port link is in U0.

4.3.1 says:

The USB host may send an Isochronous Timestamp Packet (ITP) within a relaxed timing window from a bus interval boundary. USB 3.0 added a mechanism for devices to send a Bus Interval Adjustment Message that is used by the host to adjust its 125 μs bus interval up to +/ -13.333 μs.

Title: Re: Different between SOF in USB 2.0 and ITP in USB 3.0
Post by: ChongHan on April 09, 2015, 11:19:54 pm

Hi Jan

Thanks for reply me. So host must transmit ITP at every boundary of bus interval when the link is U0, is this mean there will no miss of ITP transmission?

While, what is the clock and ppm for the clock to timestamp Bus interval and ITP?

Thanks in advance.

Best regards
Chong Han

Title: Re: Different between SOF in USB 2.0 and ITP in USB 3.0
Post by: Jan Axelson on April 10, 2015, 10:15:01 am

Yes, see 8.7 in the spec.

Search the spec for

ppm

and

PTM Clock

for more information on clocking.