CortexM0 GPO LED

#include <MKL25Z4.h>

//red -> PTB18

#define MASK(x) (1UL << (x))

int main(){
    SIM->SCGC5|=SIM_SCGC5_PORTB_MASK; //enable clock
    PORTB->PCR[18]&=~PORT_PCR_MUX_MASK; // clear PCR
    PORTB->PCR[18]|=PORT_PCR_MUX(1); //set PB19 as GPIO (MUX1)
    PTB->PDDR|=MASK(18); // set PDDR[18]=1 -> output
    PTB->PSOR=~MASK(18); // set PSOR[18]=0 -> set GPO PB19 LOW to turn on the active LOW LED
    while (1)
        ;
    return 0;
}