Kinetis MicrocontrollersのトピックRe: USB_CLKIN Specificatin for K27

USB_CLKIN Specificatin for K27

masahirokiniwa — Thu, 09 Aug 2018 11:36:48 GMT

Hi,

I'm considering to use USB_CLKIN(USB0_CLKIN) for USB FS OTG of K27.

I think that its required accuracy of clock frequency is 500ppm same as USB2.0 specification.

But I couldn't found its required specification of jitter, duty, rise-time and fall-time.

Could you tell me them?

Best regards,

M.Kiniwa

Re: USB_CLKIN Specificatin for K27

miduo — Fri, 10 Aug 2018 05:01:02 GMT

Hi,

First please look at this document on the USB clock source.

Kinetis USB Clock Sources

So if you are using the USB_CLKIN pin to inject a 48MHz clock the USB will operate correctly if the clock source itself meets the usb specificacion. Just be note that FLL cannot be used to clock the USB module because it do not meet the USB specification.

Re: USB_CLKIN Specificatin for K27

masahirokiniwa — Fri, 10 Aug 2018 09:40:07 GMT

Hi Mr.Li,

Thank you for your reply.

I couldn't look you attached link. I'm unauthorized for its link.

If I check the usb specification, should I find the Table7-9 Full-speed Source Electrical Characteristics of USB specification Rev.2.0?

It's written based on 12MHz, but USB_CLKIN require higher frequency 48MHz.

Can I convert their specs for 48MHz?

ex.) Rise Time min 4ns / max 20ns based on 12MHz -> min 1ns / max 5ns base on 48MHz

Best regards,

M.Kiniwa

Re: USB_CLKIN Specificatin for K27

miduo — Tue, 14 Aug 2018 06:09:52 GMT

Hello,

Well, according to USB 2.0 specification, USB Full Speed communication rate accepts a maximum deviation of ±2500 ppm (±0.25%). This is what you should consider when selecting OSC. We don't have requirements for the rise/fall time because our oscillator circuitry is robust enough to support different rise times. Anyway, from the experience, it is as below:

---For high speed clocks we would say ~5ns for rise-fall times
-- For above 40MHz it would need to be ~3ns or faster
-- For frequencies below 1MHz it could be slower than 5ns