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Use EXTAL0 and XTAL0 pins in UART mode

Question asked by Alexander Agudelo on Oct 29, 2015
Latest reply on Oct 31, 2015 by Alexander Agudelo

Hi, I have a custom board with a MKL25Z128 MCU. The MCG is configured in FEI mode (internal clock source) and I'm trying to repurpose the EXTAL0 and XTAL0 pins for serial communication (UART1).

 

(My project uses mbed libraries, but I'm comfortable manipulating the registers directly if required)

 

mbed has a DigitalOut library for generic I/O purposes that works fine over the PTA19 pin. The problem is that if I replace it with the serial instance (using the Serial library), the program stops working. My guess is that there is something in that library that conflicts with my configurations.

 

I tried to set the ALT mode 3 directly thinking that was the problem but no luck whatsoever:

 

PORTA->PCR[19] = PORT_PCR_MUX(3);

PORTA->PCR[18] = PORT_PCR_MUX(3);

 

I wonder if there is something else I'm supposed to do in order to free these pins and use the UART module available to them?

 

Any help would be greatly appreciated.

Regards,

Alex.

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