For usb 1.1 low speed only 6mhz are need, but nowadays all is 2.0...curious to see why they don't offer low speed usb options....


On Mon, Jun 2, 2014 at 1:07 PM, Werner Almesberger <werner@almesberger.net> wrote:
Felix wrote:
> Wow, it's the first time I hear of FLL generators, but the idea seems good
> enough to use a low power crystal for main clk source :D

Even the Wikipedia article is rather terse :)
en.wikipedia.org/wiki/Frequency-locked_loop

> Btw, the jitter is too much for certification, but, how much is the
> tolerance of usb controllers chips? Will they be able to handle it?

According to the data sheet (dsv kl26, page 25), the FLL has a period
jitter of 180 ps at 48 MHz. (With the PLL, we need to run at 96 MHz
to obtain the 48 MHZ USB clock because there's a mandatory /2 divider
in the clock path. When using the FLL there's no such divider.)

The USB specification 2.0 (dsv usb, page 181) states that full-speed
sources shouldn't jitter by more than -2/+3.5 ns (Worst-case values,
Tfdeop(min) and Tdj1(max).) Receivers have even more tolerant timing.

I don't know what the certification requirements are. The checklists
on http://www.usb.org/developers/compliance/low_full/
don't even mention jitter. The test procedures in
http://www.usb.org/developers/compliance/electrical_tests/USB-IFTestProc1_3.pdf
do mention jitter but don't say what is acceptable.

- Werner

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--
Felix