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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