// interval of calling do_stuff() in pru cycles
#define INTERVAL 20000000

// make sure do_stuff() never takes more than INTERVAL cycles!
void do_stuff() {
    //...
}

int main() {
    timestamp_initialize();

    uint32_t next = timestamp();
    for(;;) {
        next += INTERVAL;
        do_stuff();
        while( (int)( timestamp() - next ) < 0 ) {}
    }
}