Registers – Systems controlled by data bearing records – Credit or identification card systems
Patent
1988-08-19
1990-05-22
Pitts, Harold I.
Registers
Systems controlled by data bearing records
Credit or identification card systems
364270, 364271, 3642418, 364934, 3649408, 3649503, 235375, 235492, G06F 1520
Patent
active
049280007
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to oscillators, and is particularly but not exclusively applicable to oscillators used for generating a clock frequency to control the frequency of operation of a processor, in particular a microprocessor.
Generally, a microprocessor requires a high frequency clock signal applied to a clock input in order to determine the rate of operation of the microprocessor. It is desirable that the clock frequency be of close tolerance with respect to a desired frequency and of high stability, especially in the case where the microprocessor has to perform input and/or output functions which require accurate timing, as this timing will be dependent upon the accuracy of the clock frequency. Generally, in order to achieve accurate and highly stable clock frequencies, crystals are used.
However, the use of crystals results in disadvantages when attempting to produce miniaturized and inexpensive products. Crystals tend to be relatively expensive, mechanically fragile and bulky. They cannot normally be formed as part of an integrated circuit. Ceramic resonators could alternatively be used, but these are also costly and bulky.
It would be possible instead to use an LC or RC oscillator for generating a clock frequency. However, even if several discrete, bulky components were used in order to improve accuracy compared with using merely components formed on integrated circuits, the accuracy and stability of the oscillator would still not be sufficient for many purposes.
DESCRIPTION OF THE RELATED ART
One particular application in which such problems are serious is that of data-storing electronic tokens which are generally coin-sized and shaped, such as those described in EP-A-147099. In one arrangement described therein, a small electronic circuit is provided within a coin-shaped casing, and communication between an interrogator and the circuit is achieved using electromagnetic transmissions from the interrogator. A carrier wave having a predetermined frequency is 100% amplitude modulated by the interrogator in order to transmit data to the token. The token selectively absorbs the carrier wave in a manner dependent upon data to be transmitted to the interrogator by the token. This is detected by the interrogator.
In such an arrangement it is clearly important for the operations of the interrogator and the token to be synchronized. It is fairly easy to arrange for the interrogator's operations to take place at a closely-controlled frequency, so that the frequencies of the carrier wave and the modulation thereof are accurate and very stable, by providing within the interrogator a crystal-controlled oscillator. However, providing the tokens with such oscillators would be extremely inconvenient, not only because of the small size of the tokens but particularly because their widespread use requires that they be very inexpensive and this is not possible when each is provided with a crystal to control the oscillator frequency.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention there is provided an oscillator which generates a clock for a processor, the oscillator being controllable by the processor in order to adjust the frequency of the clock.
In accordance with another aspect of the invention, a processor coupled to an oscillator is operable to correct the frequency of the oscillator in dependence upon an externally generated frequency, for example the carrier frequency generated by an interrogator in the application described above.
In accordance with a still further aspect of the invention there is provided a circuit comprising a processor, an input for receiving an input signal, and a clock generator for generating a clock for the processor, the clock generator being operable to alter the frequency of the clock in response to a frequency-adjustment signal from the processor, wherein the processor is operable to compare the frequency of its own operation with the frequency of the input signal in order to produce for the generator said frequency-adjustment si
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Patent Abstracts of Japan, vol. 9, No. 224, (P-387), (1947), Sep. 10, 1985, & JP, A, 6081627, (Matsushita Denki Sangyo K.K.), May 9, 1985.
Synchronizing the Operation of a Computing Means with a Reference Frequency Signal (Publication #0161883).
Eglise David
Ruddell Alan J.
Pitts Harold I.
Ray Gopal C.
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