Method and apparatus for real time clock frequency error correction
AssigneeTelefonaktiebolaget LM Ericsson (publ)
InventorsRolf Erik Ledfelt, Anders Wilhelm Östsjö
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Abstract
A real time clock, suitable for use in mobile units in a radiocommunication system, is calibrated during production, and corrects for timing inaccuracies during normal operation. As a result, the accuracy of the real time clock's crystal oscillator is increased economically and efficiently. Frequency error correction is performed by periodically adjusting the frequency of the real time clock by a fixed amount once for every interval of time defined by an adjustment value. The adjustment value is determined by a calibration procedure performed during manufacture of the mobile unit.
Claims39
A method of generating a real time clock signal, comprising the steps of:
- generating a high frequency clock signal;
- generating a low frequency clock signal from the high frequency clock signal;
- generating a corrected low frequency clock signal by adjusting a frequency of the low frequency clock signal by a predefined amount, r, once out of every CountAdjust assertions of the low frequency clock signal, wherein CountAdjust is an integer greater than one that is calculated as a function of r;
- and
- supplying the corrected low frequency clock signal as the real time clock signal.
A method of determining an amount CountAdjust for use in an apparatus that generates a low frequency clock signal from a high frequency clock signal, and generates a corrected low frequency clock signal by adjusting a frequency of the low frequency clock signal once out of every CountAdjust assertions of the low frequency clock signal, wherein CountAdjust is an integer greater than one, the method comprising the steps of:
- counting assertions of a first oscillator signal from which the high frequency clock signal is derived;
- simultaneously counting assertions of a reference oscillator signal;
- determining a measured count value representing a number of assertions of the reference oscillator signal that occurred for a predetermined number of assertions of the first oscillator signal;
- generating an error count value by comparing the measured count value with an expected count value;
- and
- converting the error count value to the CountAdjust.
A real time clock generator, comprising:
- a frequency divider for generating a low frequency clock signal from a high frequency clock signal;
- and
- means for generating a corrected low frequency clock signal by adjusting a frequency of the low frequency clock signal by a predefined amount, r, once out of every CountAdjust assertions of the low frequency clock signal, wherein CountAdjust is an integer greater than one that is calculated as a function of r.
A calibration unit for determining an amount CountAdjust for use in an apparatus that generates a low frequency clock signal from a high frequency clock signal, and generates a corrected low frequency clock signal by adjusting a frequency of the low frequency clock signal once out of every CountAdjust assertions of the low frequency clock signal, wherein CountAdjust is an integer greater than one, the calibration unit comprising:
- a first counter for counting assertions of a first oscillator signal from which the high frequency clock signal is derived;
- a second counter for simultaneously counting assertions of a reference oscillator signal;
- means for determining a measured count value representing a number of assertions of the reference oscillator signal that occurred for a predetermined number of assertions of the first oscillator signal;
- a comparator for generating an error count value by comparing the measured count value with an expected count value;
- and
- means for converting the error count value to the CountAdjust.
A mobile unit for use in a radiocommunication system, the mobile unit comprising:
- a controller for controlling components in the mobile unit;
- and
- a real time clock generator, comprising:
- a frequency divider for generating a low frequency clock signal from a high frequency clock signal;
- and
- means for generating a corrected low frequency clock signal by adjusting a frequency of the low frequency clock signal once out of every CountAdjust assertions of the low frequency clock signal, wherein CountAdjust is an integer greater than one.
Dependencies are inferred from the claim text.