topic Re: Kinetis KL26xx - USB clock (48 MHz) from 32.768 KHz crystal? in Kinetis Microcontrollers

Kinetis KL26xx - USB clock (48 MHz) from 32.768 KHz crystal?

merlin69 — Mon, 10 Mar 2014 09:16:05 GMT

Hi!

Is it possible to get fully working USB only with 32,768 KHz crystal?

I am able to use FLL to get 47.972352 MHz. USB works but only with some USB host controllers. With the others USB enumeration failed with result "Unknown device". If I use 8 MHz crystal with PLL to get exactly 48 MHz, USB works with all tested USB host controllers.

Tested program is the same, it only differs in clock settings.

Is there anyone who was able to operate USB with 32,768 KHz crystal?

Thanks,

Jan

Re: Kinetis KL26xx - USB clock (48 MHz) from 32.768 KHz crystal?

mjbcswitzerland — Tue, 11 Mar 2014 17:01:29 GMT

Hi Jan

32.768 kHz crystals are not very accurate so it would be best to use a good crystal for USB reliability..8MHz have much better accuracy.

For experimental purposes it is possible to control the 32kHz crystal loading a bit to tune it better but I wouldn't use such for a real product since it would probably cause more trouble that any cost savings are worth.

Regards

Mark