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Filter and Temperature Compensation Method for Real Time Clock Calibration

IP.com Disclosure Number: IPCOM000122986D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 75K

Publishing Venue

IBM

Related People

Capps, LB: AUTHOR [+4]

Abstract

A method of increasing the accuracy of time of day clocks by periodically recalibrating the time base.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Filter and Temperature Compensation  Method for Real Time Clock Calibration

      A method of increasing the accuracy of time of day clocks by
periodically recalibrating the time base.

      Computer systems in many cases contain two clock sources to
keep the current time of day.  When the system is powered down,
typically a battery backed up Real Time Clock (RTC) module keeps
accurate time to the nearest second.  When the computer system is
powered up, the current time value in the battery backed RTC is
transferred to a high precision clock counter (decrementer) inside
the system microprocessor that keeps time to milliseconds or even
microseconds in some cases.  As long as the system is powered up, any
requests for the current time are answered with the value that is in
the microprocessor decrementer.

      The battery backed RTC is based on a dedicated very accurate
crystal circuit that is custom designed to keep accurate time.  The
microprocessor decrementer circuit, though, is based on an oscillator
that is not as accurate since the microprocessor oscillator also
controls many other devices in the system, and is chosen as the best
solution to fill requirements of all devices in the system.  Because
of these design trade-offs, the battery backed RTC is more accurate
and results in better time accuracy when the system is powered off,
than when the system is powered on.

      A method to increase the accuracy of the microprocessor
decrementer clock is to calibrate the microprocessor decrementer with
the battery backed RTC as the system boots.  This is done by reading
the RTC, reading the decrementer and determining their relative
rates.

      The problem with the calibration approach is that since the
microprocessor decrementer resolution is high and the battery backed
RTC resolution is low, this cali...