K22F + 32 Khz Xtal + FLL + USB

[chriskeeser]

Is it acceptable to run the USB module from the FLL if the source for the FLL is the external 32 Khz xtal?  I have had a difficult time finding concrete information that says this is ok or not.

I did stumble across this site µTasker MCG Configuration Support  that seemed to indicate the FLL was not suitable for USB operation.

"Note that the output from the FLL has higher jitter than that from the PLL and is not suitable as clock source for USB. For USB host mode, only the PLL should be used to derive its 48MHz clock from unless a dedicated USB clock signal is available. The same is true for USB device unless the Kinetis part includes IRC48M, which can be used to fulfil USB device specification requirements."

Any help here would be appreciated.

Note the FLL output is 96 Mhz.

[mjbcswitzerland · Accepted solution]

Chris

All user manuals have this, which warns about using the FLL output since it doesn't meet USB certification requirements.

For serious work the FLL should not be considered as a possible source.

Regards

Mark

[chriskeeser]

Thank you for your concise response.