A microprocessor-based analog wristwatch chip with 3 seconds/year accuracy

Previous developments on a 2.1MHz quartz have demonstrated that thermal, ageing and shock performances are improved by at least one order of magnitude compared to a 32kHz quartz. The temperature behavior is a flat cubic function, and therefore positive and negative deviations with respect to the reference temperature are compensated. A field test on 20 watch samples containing an oscillator built around this quartz showed that over a 3-year period, the maximum deviation is less than 3 seconds/year. Note that these results are obtained without any temperature compensation scheme. As the power consumption of a 2.1 MHz quartz oscillator (2.3/spl mu/A) is not compatible with the power consumption of standard watch oscillator circuits (<300nA), the 2.1MHz quartz oscillator is periodically powered for a short time and locks a continuously-running 32MHz, 200nA quartz oscillator. While short term precision is provided by the 32MHz oscillator, longer-term deviations are corrected by the periodically activated 2.1MHz oscillator. The chosen periodicity is 10 minutes, corresponding to the thermal constants in a watch.<>