OSTCCR_PRESCALE1_MASK
prescale = FIELD_GET(OSTCCR_PRESCALE1_MASK, prescale);
val &= ~OSTCCR_PRESCALE1_MASK;
val |= FIELD_PREP(OSTCCR_PRESCALE1_MASK, prescale);
val &= ~(OSTCCR_PRESCALE1_MASK | OSTCCR_PRESCALE2_MASK);