Abstract: A power saving sleep timer has a first clock and a second clock having greater frequency and temporal stability than the first clock. The second clock has an associated second clock period value which is accumulated once for each said second clock interval during one or more first clock periods, thereby forming a calibrated period value. During an operational interval, the calibrated period value is accumulated once per first clock interval until the accumulated value is equal or greater than a sleep time value, after which a power-up output is asserted.