samd21 blink LED in C, no SDK

blink_led_samd.c

// originally from: https://github.com/dwelch67/atsamd_samples/tree/master/ATSAMD21/sparkfun/blinker01

#include <stdint.h>  // for uint32_t

int main(void) {
    
    unsigned int ra;
    unsigned int rx;

    uint32_t* PORT_A_DIR = (uint32_t*)0x41004400;  // "PORT" from Table 12-1 & 23.7 of SAMD21 Datasheet 
    uint32_t* PORT_A_OUT = (uint32_t*)(0x41004400 + 0x10);  //  from 23.7 "Register Summary" of SAMD21 Datasheet

    *PORT_A_DIR |= ( 1 << 10 ); // PA10 is red LED on Trinket
    *PORT_A_OUT |= ( 1 << 10 );
    
    for(rx=0;;rx++) {
        
        *PORT_A_OUT |= ( 1 << 10 );
        for(ra=0;ra<80000;ra++) { asm("nop"); }
        
        *PORT_A_OUT &=~ ( 1 << 10 );
        for(ra=0;ra<20000;ra++) { asm("nop"); } 
    }
    
    return(0);
}