On Mon, 10 Feb 2014, vichy wrote:
> >> 2. in tt_available, below is used to check whether tt time is bigger than 125us
> >> if (125 < usecs) {
> >> int ufs = (usecs / 125);
> >> int i;
> >> for (i = uframe; i < (uframe + ufs) && i < 8; i++)
> >> if (0 < tt_usecs[i]) {
> >> ehci_vdbg(ehci,
> >> "multi-uframe xfer can't fit "
> >> "in frame %d uframe %d\n",
> >> frame, i);
> >> return 0;
> >> }
> >> }
> >> is it possible tt time bigger than 1 uframe?
> >
> > Yes. Any isochronous transfer that is longer than 188 bytes will
> > require more than one uframe.
>
> from spec's explanation, the usecs in the above source is the budget
> calculated with Maxp instead of bandwidth indeed we need to transfer.
> Why it is still possible over 125us?
> for the case the ep max packet size over 188 bytes?
I don't understand your question.
At full speed (12 Mb/s), the time required to transfer 188 bytes is
(188 bytes) * (8 bits/byte) / (12 b/us) = 125.33 us,
which is greater than 125 us.
> > For each packet (token, data, and handshake), the overhead includes
> > inter-packet delay, cable delay, SYNC, bit-stuffing, and End-Of-Packet
> > -- all in addition to the PID, token, and CRC bytes.
> >
> > I don't know where you got that "13" from. If you assume the
> The 13 bytes I see is from usb spec section 5.8.4.
Then you must have noticed the sentence just above Table 5-9:
The table does not include the overhead associated with bit
stuffing.
The table also doesn't include the overhead associated with cable
delay and intermediate hubs.
> > inter-packet delays always require 1 byte time and count up the
> > inter-packet delays (3), SYNCs (3), token bytes (3), PID and CRC hytes
> > (3), and handshake (1), that adds up to 13 bytes of protocol overhead.
> > But it leaves out cable delay, bit-stuffing, and End-Of-Packet signals.
> >
> > You also have to remember that the spec allows the clock rate on
> > Full-Speed devices to vary by up to 0.25% of the nominal rate.
> so you mean FS speed will be 12M bits/s +- 30000 bits/s?(30000/12M = 0.25%)
That's right. Also, Low-Speed devices are allowed to vary by up to
1.5%, and High-Speed devices are allowed to vary by up to 0.05%.
Alan Stern
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