This invention relates to a novel integrated-circuit RC oscillator. More specifically, this invention relates to an RC oscillator which is entirely fabricated on an integrated-circuit chip, and has an output oscillation frequency which may be externally trimmed.
Oscillators are used in microprocessors to convert energy from a DC source into a cyclic energy which is usually supplied to the microprocessor clock. Crystal oscillators are sometimes employed to produce the desired oscillation frequency. While crystal oscillators produce good quality oscillating voltage signals, they are more expensive than most oscillators. Therefore, other types of less expensive oscillators have been used. In the past, RC oscillators were made having the passive resistive and capacitive elements positioned off the integrated-circuit chip because the elements occupied a considerable amount of semiconductor area. As semiconductor technology has advanced, it has been possible to position some of the resistive or capacitive elements on the IC chip.
It is oftentimes desirable for a user to be able to change the output voltage signal frequency of an oscillator after purchasing the oscillator. In order to do so, a user would purchase a new IC chip having an oscillator which produced the desired oscillation frequency. However, this method of modulation is expensive and inefficient. RC oscillators were then developed which included some resistive and capacitive elements off-chip so that the user could adjust the oscillating frequency. These oscillators were not entirely satisfactory, however, because the off-chip elements utilized too much space.
The difficulties in the preceding are not intended to be exhaustive but rather are many which may tend to reduce the usefulness of prior art RC oscillators. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that RC oscillators appearing in the past will admit to worthwhile improvement.
It is therefore a general object of the invention to provide a novel RC oscillator which will obviate or minimize difficulties of the type previously described.
It is a specific object of the invention to provide an RC oscillator which produces an output voltage signal at an oscillating frequency which may be easily adjusted.
It is another object of the invention to provide an RC oscillator which minimizes the number of components which are external to the semiconductor chip on which the oscillator is fabricated.
It is still another object of the invention to provide an RC oscillator which is inexpensive and which provides a good quality voltage signal which consistently oscillates at a desired frequency.