I am working with a mk22fn512cap , and wnated to run it at 120 MHz without using an external oscillator.
1) Since PLL are not connected to the internal sources I dont think I can use the internal ref clocks to get it upto 120?
2) So the option is to use IRC48M. in some posts I read that this is not stable and has some known errata..? Can someone tell me if using IRC48M is the correct/best way to get to run the controller to run at 120 Mhz.
There is no problem using the 48MHz IRC for 120MHz PLL operation. Just check the errata for your device to see whether it is affected by the one that requires the USB controller to be enabled to use it - this was only some of the first devices with IRC48M and has been corrected already in newer silicon.
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