Design Considerations for Low-Voltage Crystal Oscillator Circuit in a 1.8-V Single Chip Microprocessor

Shigeo KUBOKI  Takehiro OHTA  Junichi KONO  Yoji NISHIO  

Publication
IEICE TRANSACTIONS on Electronics   Vol.E76-C   No.5   pp.701-707
Publication Date: 1993/05/25
Online ISSN: 
DOI: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Low-Power and Low-Voltage Integrated Circuits)
Category: 
Keyword: 
crystal oscillator,  CMOS oscillation circuit,  open loop-gain,  microprocessor,  low voltage oscillator,  

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Summary: 
A low-voltage, high-speed 4-bit CMOS single chip microprocessor, with instruction execution time of 1.0µs at a power supply voltage of 1.8V, has been developed. A single chip processor generally includes crystal oscillation circuits to generate a system clock or a time-base clock. But when the operating voltage is lowered, it becomes difficult to get oscillations to start reliably and to continue stably. This paper describes a low voltage circuit design method for built-in crystal oscillators. Simple design equations for oscillation starting voltage and oscillation starting time are introduced. Then effects of the circuit device parameters, such as power supply voltage, loop gain values, and subthreshold swing S, on the low voltage performance of the crystal oscillators are considered. It is shown that the crystal oscillators operate in a tailing (subthreshold) region at voltages lower than about 1.8 V. Subthreshold swing, threshold voltage, and open loop gain have a significant influence on low voltage oscillation capability. This design method can be applied to crystal oscillators for a wide range of operating voltages.