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CMOS OSCILLATOR CIRCUIT
This note describes several square wave oscillators that can be built using
CMOS logic elements. These circuits offer the following advantages:
• Guaranteed startability
• Relatively good stability with respect to power supply
variations
• Operation over a wide supply voltage range (3V to 15V)
• Operation over a wide frequency range from less than 1
Hz to about 15 MHz
• Low power consumption (see AN-90)
• Easy interface to other logic families and elements including
TTL
Several RC oscillators and two crystal controlled oscillators are described.
The stability of the RC oscillator will be sufficient for the bulk of
applications; however, some applications will probably require the stability
of a crystal. Some applications that require a lot of stability are:
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Timekeeping over a long interval. A good deal of
stability is required to duplicate the performance of an ordinary wrist
watch (about 12 ppm). This is, of course, obtainable with a crystal.
However, if the time interval is short and/or the resolution of the
timekeeping device is relatively large, an RC oscillator may be adequate.
For example: if a stopwatch is built with a resolution of tenths of seconds
and the longest interval of interest is two minutes, then an accuracy of 1
part in 1200 (2 minutes x 60 seconds/minute x 10 tenth/second) may be
acceptable since any error is less than the resolution of the device.
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When logic elements are operated near their
specified limits. It may be necessary to maintain clock frequency accuracy
within very tight limits in order to avoid exceeding the limits of the logic
family being used, or in which the timing relationships of clock signals in
dynamic MOS memory or shift register systems must be preserved.
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Baud rate generators for communications
equipment.
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Any system that must interface with other
tightly specified systems. Particularly those that use a “handshake”
technique in which Request or Acknowledge pulses must be of specific widths.
Figure 1 illustrates a useful oscillator made
with three inverters. Actually, any inverting CMOS gate or combination of
gates could be used. This means left over portions of gate packages can be
often used. The duty cycle will be close to 50% and will oscillate at a
frequency that is given by the following expression.
The
Figure 1. Simple Cmos Oscillator Circuit
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