MK20DX128VLF5 does not boot without an external 16Mhz xtal

[chris_seto]

Hi,

I have an MK20DX128VLF5 on my board. It is connected to an external single ended oscillator. This chip will seemingly not exit POR. In debugging, I was trying to determine the clock configuration on a fresh reset. It seems like the chip should boot from its internal osc and run off of that until switched to the external clock by user code. I have a known good board with the same chip and an xtal, so I pulled the xtal to see if the chip still booted without it. It did not. Jtag works on both chips, but it looks like the reset handler is never running. Additionally, it seems like the reset line is oscillating up and down. The resrt line is not connected to anything else outide of jtag, and this behavior persists if htag is disconnected. 

The K20 should be able to boot by itself without any external clock, correct?

Thanks,

Chris

[mjbcswitzerland]

Hi

If your reference board/SW is using the crystal it will not start if you remove it (it will try to start it and wait for it to lock, which will never happen). A saw-tooth on the reset line means that the board is continuously resetting (usually due to watchdog).

Typical mistakes on new boards are:

- when using crystal: the oscillator gain is not set to match the crystal circuit (high gain or low gain) if it doesn't match the crystal doesn't start and the effect is the same
- EZPORT input held low - then nothing happens at all

Regards

Mark

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[Robin_Shen]

Hi Chris Seto,

The K20 should be able to boot by itself without any external clock, correct?

Yes. The MCG comes out of reset configured for FEI mode. In FEI mode, the slow Internal Reference Clock (IRC) can be used as the FLL source.

About the "the reset line is oscillating up and down", please refer: How To Sucessfully Program a Custom made Kinetis Microcontroller Board.

Best Regards,

Robin

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