Start-up circuit for low power MOS crystal oscillator

Abstract

A CMOS crystal oscillator circuit for producing a 32 KHz clock signal operates at low power over a range of power supply voltages without overdriving the crystal. To accomplish this, the complementary MOS transistors in the oscillator inverter are supplied with operating current from a constant low current source. The constant low current is at an optimum value for crystal stability and power consumption and is independent of supply voltage variations. To ensure quick initial start-up of the oscillator circuit, a second current source is connected in parallel with the constant current source; and the second source initially is switched on for a pre-established time interval to supply significantly greater operating current to the oscillator inverter only during initial start-up of the oscillator. After this pre-established time interval, the second current source is rendered inoperative or non-conductive for the duration of the operation of the oscillator circuit.

Dependencies are inferred from the claim text.