Micropower Clock

Although ceramic resonators are a good choice for low-power, low-frequency clock sources (if you can stand their 30-ppm te
Micropower Clock - schematic

mperature coefficient), they have troublesome, spurious-resonance modes. This circuit rejects all but the resonator`s fundamental mode. This clock circuit works from -40 to +80°C and consumes only 2.8 mW. The rising edge of resonator Y1 toggles IC1A low. ac-coupled positive feedback from IC1D via CI and RI immediately confirms this state change at IC1B so that Miller loading, harmonic components, or below-minimum rise times at IC1A cannot force IC1C to relapse to its previous state. This tactic also applies to resonator Yl`s falling edge because IC1C, via C2 and R2, holds IC1B high. Choose time constants iiCi, and R2C2 to be equal and ranging from 60 to 75% of one-half of the clock`s period. Ceramic capacitors (10% tolerance) with X7R dielectric work well. With these time constants, the logic will be locked and unavailable to the ceramic resonator until just before it executes a legitimate transition. IC1D and IC1C are in parallel to isolate the resonator from external loads and, more importantly, from C2.

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