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K22 FRDM CDC Demo with IRC48?

Question asked by Frank Roberts on Jul 20, 2016
Latest reply on Jul 22, 2016 by Jorge Antonio Alcala Vazquez


  We have been working on converting from using the PLL for USB/System/Core Clocks to just using a IRC48M.  In going back to a simple USB Example being the CDC example for the K22 FRDM board it is interesting that the XTal-Less capability of the K22 is a STRONG feature but the CDC example does not include a IRC48 Clock Example that works?  In inspecting the firmware briefly I see that the Vlpr clock config mode selects the IRC48 as the PLLFLLSEL but if one runs the CDC out of the box with this clock configuration the K22 USB port does not enumerate correctly.  I would have initially thought this was because the CORE_CLOCK_FREQ = 4MHz as defined in board.h when this mode is selected so this was quickly changed to 20MHz.  I did not think it would that simple and it wasn't.


That being said could someone provide some insight as to the remaining steps needed to get this example running using the IRC48 for the USB Clock Source?  I don't think it is far off...


Seems like having a couple of Clock Config examples using the IRC48 would be ideal to showcase this feature say one with the 4MHz feeding the PLL for the MCGOUT and one entirely driven by the IRC48 seems like it would be quite useful also.