United States Patent
`
`[19]
`
`Chapman
`
`[11] Patent Number:
`
`4,903,251
`
`[45] Date of Patent:
`
`Feb. 20, 1990
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`
`[22]
`
`[51]
`[52]
`[58]
`
`[56]
`
`ACCURACY ADJUSTMENT FOR
`TIlVIE-OF-DAY CLOCK USING A
`MICROCONTROLLER
`
`4,513,259 4/1985 Frerking ............................. 331/176
`
`4,644,484 2/1987 Flynn an.
`368/200
`4,708,491 11/1987 Luitje .................................. 368/156
`
`Inventor:
`
`Assignee:
`
`Max C. Chapman, Plymouth, Mich.
`
`Ford Motor Company, Dearbom,
`Mich.
`
`Primary Examiner—Vit W. Miska
`Attorney. Agent, or Finn—Mark L. Mollon; Paul K.
`Godwin
`
`Appl. No.: 402,726
`
`Filed:
`
`Sep. 5, 1989
`
`Int. C1.4 ....................... G04C 13/08; G04B 17/12
`U.S. C1. ..................................... 368/156; 368/200
`Field of Search ................................ 368/155—157,
`368/200—202; 364/569; 331/47, 66, 176
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`........... 368/200
`8/1981 Lowdenslager et a1.
`4,282,595
`4,321,698 3/1982 Gomi et a1.
`.................
`368/202
`
`4,407,589 10/1983 Davidson et al.
`368/200
`
`5/1984 Vail .........................
`4,448,543
`368/202
`4,502,790 3/1985 Yokoyama .......................... 368/200
`
`[57]
`
`ABSTRACT
`
`A microcontroller, software-based time-of-day clock
`compensates for inaccurate oscillator frequency by
`periodically correcting the time-of-day value using a
`calibration offset measured by manufacturing test
`equipment and permanently stored in nonvolatile mem-
`ory. A high degree of time keeping accuracy is
`achieved without the use of trimming capacitors or
`presorted piezoelectric crystals in the oscillator. Fur-
`thermore, frequency error compensation is incorpo-
`rated into the microcontroller system without requiring
`a significant amount of processing time overhead.
`
`11 Claims, 3 Drawing Sheets
`
`FROM
`OSCILLATOR 11
`
`REGISTER
`
`COUNT DOWN
`REGISTER
`FOR
`ONE MINUTE
`INTERVAL
`
`TIME
`
`TIME-OF-DAY
`VALUES
`
`RELOAD
`VALUE
`
`6,144
`
`COUNT DOWN
`REGISTER
`FOR
`ONE MILLION
`INTERVAL
`
`1
`
`‘
`
`APPLE 1031
`
`1
`
`APPLE 1031
`
`

`

`US. Patent
`
`Feb. 20, 1990
`
`Sheet 1 of3
`
`4,903,251
`
`
`
`FROM
`
`OSCILLATOR 11
`
`IO
`
`
`
`
`COUNT DOWN
`RELOAD
`COUNT DOWN ‘
`
`
`
`REGISTER
`VALUE
`REGISTER
`
`
`
`FOR
`REGISTER
`FOR
`
`
`
`ONE MILLION
`- — — —
`ONE MINUTE
`
`
`
`6,144
`INTERVAL
`INTERVAL
`
`
`
`
`_
`TlME-OF-DAY
`
`VALUES
`
`
`
`_____ _I
`
`_
`
`
`
`2
`
`

`

`US. Patent
`
`Feb. 20, 1990
`
`Sheet 2 of 3
`
`4,903,251
`
`ENTER TIMER
`INTERRUPT SERVICE
`ROUTINE
`
`40
`
`
`
`4I
`
`
`INCREMENT THE
`
`DECREMENT THE
`
`
`TIME-OF-DAY
`
`
`ONE MINUTE
`
`MINUTES VALUE
`
`INTERVAL
`
`COUNTER
`
`
`
`
`
`
`
`
`
`RELOAD THE
`ONE MINUTE
`INTERVAL
`COUNTER WITH
`RELOAD VALUE
`
`RESET THE
`
`
`
`
`M'NUTT§§OX§LUE
`
`
`
`
`
`INCREMENT THE
`
`SET THE ONE
`
`
`TIME-OF-DAY
`MINUTE INTERVAL
`
`
`
`HOURS VALUE
`COUNTER RELOAD
`
`
`VALUE TO 6144
`
`
`
`TO "01"
`
`RESET TH E
`HOURS VALUE
`
`FIG.3
`
`DECREMENT THE
`ONE MILLION
`INTERRUPTS
`COUNTER
`
`
`0YES
`
`44
`
`RELOAD THE
`ONE MILLION
`INTERRUPTS
`COUNTER WITH
`1 000,000
`
`
`
`SET THE ONE
`MINUTE INTERVAL
`COUNTER RELOAD
`VALUE TO 6144
`+/- PPM ERROR
`
`-45
`
`3
`
`

`

`US. Patent
`
`Feb. 20, 1990
`
`Sheet 3 013
`
`4,903,251
`
`60
`
`IIEEHHHHII
`
`62
`
`MBROCONTROUER
`
`OUTPUT
`
`:
`
`é
`
`70
`
`é.
`
`TEST
`COMPUTER
`
`63
`
`7]
`
`.5
`
`FREQUENCY
`COUNTER
`
`FlG.4
`
`4
`
`

`

`1
`
`4,903,251
`
`ACCURACY ADJUSTMENT FOR TIME-OF-DAY
`CLOCK USING A MICROCONTROLLER
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates in general to a time-of-
`day clock implemented using a microcontroller, and
`more specifically to correcting for time keeping inaccu-
`racies resulting from frequency errors of a crystal oscil-
`later.
`Microprocessor and microcontroller systems operate
`using internal timing circuits for coordinating process-
`ing operations and establishing machine timing cycles
`for execution of instructions. A crystal oscillator is
`frequently employed to provide timing signals for the
`microprocessor due to the relatively high accuracy and
`stability of piezoelectric crystals.
`One of the functions that is often performed using a
`microprocessor or microcontroller is that of a time-of-
`day clock. Such a clock is implemented using time val-
`ues stored in memory which are updated periodically
`by the microprocessor which counts timing signals
`received from the crystal oscillator. Typically, the time-
`of-day function is incidental to some other primary
`control
`function being implemented using the mi-
`crocontroller, such as control of an automotive audio
`system. Proper operation of that primary control func-
`tion is not highly dependent upon the accuracy of the
`internal clock which is driven by the piezoelectric crys.
`tal oscillator. However, in implementing a time—of-day
`clock using the microcontroller, a crystal oscillator
`error amounting to even a few parts per million causes
`a significant timing error to eventually be accumulated
`in the time-of-day clock.
`‘
`.
`Typical manufacture of piezoelectric crystals results
`in a variability of the crystal’s resonant frequency of
`about 20 parts per million (ppm). In order microcon-
`troller time-of—day clocks, crystals can be presorted into
`various value ranges by the crystal manufacturer and
`subsequently matched with particular values of load
`capacitors for constructing each individual crystal os-
`cillator. Such sorting and matching of components in-
`creases the cost of the clock.
`Another method for obtaining accurate oscillator
`signals is to provide a variable load capacitor to allow
`adjustment (i.e., trimming) of the oscillator frequency.
`However, the variable capacitor and the need for man-
`ual calibration add expense to the oscillator.
`It is preferable to manufacture an oscillator using
`unmatched, stande tolerance components without
`special trimming capacitors. Therefore, there have been
`attempts to allow the oscillator to run at an incorrect
`frequency, measure the frequency error, and compen-
`sate for the error within the microcontroller as it keeps
`track of time-of-day. For example, Luitje (U.5. Pat. No.
`4,708,491), discloses an untrirnmed crystal circuit con-
`nected to the oscillator of a microcomputer. After fre-
`quency prescaling,
`the timing signal is measured to
`determine an error value. A correction term is stored in
`the microcomputer via entry from a keyboard. During
`clock operation, the value in a free-running counter is
`compared to a programmable register. Interrupts are
`generated when the number in the programmable regis-
`ter equals the number in the free-running counter. The
`interrupts provide a time base for keeping track of time-
`of-day. The correction term is used in loading the pro-
`grammable register once each second so that the time
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`2
`intervals of the interrupts themselves are error cor-
`rected.
`The ability to change the period between interrupt
`signals is essential to the technique disclosed in Luitje.
`However, not all microcontrollers provide user access
`to the interrupt interval. For example,
`the COP888
`family of microcontrollers manufactured by National
`Semiconductor Corporation use an idle timer for pro-
`viding a time base. The idle timer is a free-running 16-bit
`timer which is clocked from the crystal oscillator via a
`divide-by—lO circuit. Whenever the 12th bit toggles its
`value, an interrupt is generated by the idle timer which
`can be used to update various time-of-day counters. The
`period between these interrupts is not programmable
`and depends only upon oscillator frequency. Therefore,
`the technique described in Luitje cannot be used. Fur-
`thermore, the calculations required to apply error cor-
`rection to the programmable register value and the need
`to correct for inaccuracies each and every second in.
`creases the processor overhead associated with use of
`the Luitje technique to an undesirable level.
`SUMMARY OF THE INVENTION
`
`It is a principal object of the present invention to
`improve time-of-day clock accuracy for a microcon-
`troller software-based clock in an inexpensive manner.
`It is another object of the present invention to pro-
`vide a method and apparatus for correcting for time—of—
`day inaccuracies resulting from the use of unmatched,
`unsorted components in a crystal oscillator.
`It is a further object of the invention to reduce mi-
`crocomputer processing time overhead associated with
`correction of time-of—day measurement.
`These and other objects are achieved in an apparatus
`comprising idle timer means for generating interrupts
`separated by a fixed time period which differs from an
`ideal time period by an error. First counting means are ‘
`coupled to the idle timer means and have a first register
`for counting the interrupts and for generating an incre-
`ment signal when a number of interrupts have occurred
`equal to a load value which is loaded into the first regis-
`ter. Reload means are coupled to the first counting
`means for storing a reload value and for (1) transferring
`the reload value to the first register as the load value
`upon the occurrence of the increment signal and (2)
`then resetting the reload value to a predetermined
`value. Second counting means are coupled to the idle
`timer means and to the reload means for counting the
`interrupts and for transfering an adjusted reload value
`including an accumulated error value to the reload
`means as the reload value when a number of interrupts
`have occurred equal to a large count number relatively
`greater than the predetermined value. Time means are
`coupled to the first counting means and receives the
`increment signal for keeping track of time-of-day. Pref-
`erably,
`the apparatus further comprises nonvolatile
`storage means coupled to the reload means for storing
`the accumulated error value or the adjusted reload
`value. The accumulated error value is substantially
`equal to the algebraic sum of the predetermined value
`(i.e., normal reload value) and the quantity resulting
`from the time period error times the large count number
`divided by the ideal time period.
`The invention also provides a method for maintaining
`accurate time keeping with an interrupt-driven clock,
`the interrupts being separated by a fixed time period.
`The method includes the steps of maintaining a first
`count of interrupts corresponding to a first time interval
`
`5
`
`

`

`3
`being measured and displayed by the clock. Time values
`for display are maintained by the clock. A second count
`of interrupts is maintained corresponding to a second
`time interval equaling a large number of interrupts and
`containing many of the first time intervals. In response 5
`to the first count being equal to the predetermined num-
`ber, the steps are performed of (A) restarting the first
`count to count to the predetermined number; (B) updat-
`ing the time values; and (C) setting the predetermined
`number to a first constant equaling the number of inter- 10
`rupts that would occur during the first time interval if
`the time between interrupts equaled an ideal time per-
`iod. In response to the second count being equal to the
`large number, the steps are performed of (A) restarting
`the second count; and (B) setting the predetermined 15
`number to a second constant related to the difference
`between the fixed time period and the ideal time period
`multiplied by the large number and divided by the ideal
`time period.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The novel features of the invention are set forth with
`particularity in the appended claims. The invention
`itself, however, both as to its organization and method
`of operation, together with further objects and advan- 25
`tages thereof, may best be understood by reference to
`the following description taken in conjunction with the
`accompanying drawings, in which:
`FIG. 1 is a schematic, block diagram illustrating the
`overall system used in the present invention.
`FIG. 2 is a block diagram illustrating apparatus ac-
`cording to the present invention.
`FIG. 3 is a. flowchart illustrating a preferred method
`according to the present invention.
`FIG. 4 is a schematic, block diagram illustrating ap- 35
`paratus for measuring an oscillator frequency error and
`for storing a correction value for accurate time keeping
`by a microcontroller.
`
`20
`
`30
`
`DETAILED DESCRIPTION OF PREFERRED 40
`EMBODIMENTS
`
`Referring to FIG. 1, a microcontroller 10 is con-
`nected to a crystal oscillator 11. In the illustrated em-
`bodiment, microcontroller
`10
`is comprised of a
`COP888CG microcontroller manufactured by National 45
`Semiconductor Corporation.
`Oscillator 11 includes a piezoelectric crystal 12 con-
`nected to terminals CKI and CKO of microcontroller
`10. A resistor 13 is connected across crystal 12. A fixed
`capacitor 14 couples one end of piezoelectric crystal 12 50
`to ground 16. A fixed capacitor 15 couples the other end
`of crystal 12 to ground 16.
`On microcontroller 10, a terminal 17 is connected to
`a supply voltage V“, preferably in the range of from
`4—6 volts DC. The time values maintained by microcon- 55
`troller 10 are provided via I/O lines 18 to a display
`driver 20 which controls a display 21.
`A set of input switches 22 are coupled to microcon-
`troller 10 via l/O lines 19 for allowing the setting of the
`time values by selective activation of the switches. For 60
`example, input switches 22 may be comprised of a key-
`pad matrix for activating separate interrupt signals to
`microcontroller 10.
`Microcontroller 10 includes an idle timer which re-
`ceives timing signals from oscillator 11 via a divide-by- 65
`10 circuit (not shown). The idle timer counts the signals
`received from the divide‘by- 10 circuit and generates an
`interrupt upon each count of 212 or 4096. A numerical
`
`\
`
`4,903,251
`
`4
`example will be presented throughout this description
`in which crystal 12 resonates at a frequency of 4.194304
`MHz. Thus, the period of the oscillator signals equals
`about 0.2384186 microseconds, and the output of the
`divide-by-IO circuit has a period of about 2.384186 mi-
`croseconds. The idle timer counts 4096 of these signals,
`which ideally yields 9765.625 microseconds between
`interrupts. Counting 6144 of the idle timer interrupts
`yields a measurement period of exactly one minute if the
`crystal resonant frequency is completely accurate. By
`keeping track of the idle timer interrupts, calculating
`real time minutes and hours is a straightforward matter
`for the microcontroller software. However, due to the
`nonideal resonant characteristic of piezoelectric crystal
`12, the actual real time between counts of 6144 inter-
`rupts differs from one minute by some error. According
`to the present invention, the error is allowed to accumu-
`late for some period of time, followed by a correction
`based upon a measurement of the error.
`FIG. 2 shows one preferred implementation of the
`present invention as embodied in microcontroller 10. A
`system clock 25 receives an oscillator signal from crys-
`tal oscillator 11. System clock 25 performs the divide-
`by-lO function and provides a system clock signal to
`idle timer 26. Idle timer 26 in the preferred embodiment
`is comprised of a free-running 16-bit timer which is
`tapped at its 12th bit to generate an interrupt when the
`12th bit changes its value, i.e., each 4096th system clock
`pulse. The interrupt output from idle timer 26 is con-
`nected to a countdown register 27 and a countdown
`register 28. Countdown register 27 has an output cou-
`pled to a reload means 30 and to time-of-day values
`store 33. Countdown register 28 has an output con-
`nected to an add and save means 35. Add and save
`means 35 has an output connected to reload means 30
`and has an input connected to an electrically erasable
`programmable read only memory (EEPROM) 34.
`Countdown registers 27 and 28 each has a reset line for
`reinitiating a count once zero has been reached in the
`previous countdown sequence.
`Reload means 30 includes a reload value register 31
`and a first constant storage portion 32. Storage portion
`32 stores the number 6144 which is the count number
`corresponding to the ideal performance of crystal oscil-
`lator 11. EEPROM 34 contains an error value which
`corresponds to the parts per million error in the crystal
`oscillator frequency. This error is algebraically added
`to 6144 and the result is stored in add and save means 35.
`Alternatively, the result of the addition could itself be
`stored in EEPROM 34, but this would require two
`bytes of EEPROM storage.
`In operation, countdown register 27 counts idle timer
`interrupts corresponding to a one minute interval.
`When countdown register 27 reaches zero, an incre-
`ment signal is sent to time-of-day values 33 in order to
`update the values to reflect that a minute has expired. A
`reload value in reload value register 31 is loaded into
`countdown register 27 which is then reset to initiate a
`countdown from the loaded value. The increment sig-
`nal from countdown register 27 is also connected to
`reload means 30 in order to cause the transfer of the
`predetermined constant 6144 from storage portion 32 to
`reload value register 31. Thus, reload value register 31
`most often contains 6144 and the value loaded into
`countdown register 27 is usually the number corre-
`sponding to ideal operation of the crystal oscillator.
`As a means for correcting the accumulated error in
`the interrupt signal from idle timer 26, the interrupts are
`
`6
`
`

`

`4,903,251
`
`10
`
`15
`
`25
`
`30
`
`35
`
`20
`
`5
`counted down in'countdown register 28 from a large
`number such as 1,000,000. The premeasured error of the
`crystal oscillator stored in EEPROM is in units of parts
`per million. This number is added to 6144 in block 5.
`When the countdown from the one million interval
`expires, the modified one minute interval count derived
`from adding the parts per million error to 6144 is loaded
`into reload value register 31. The modified value is
`guaranteed of being used one time since the usual value
`of 6144 will only replace the modified value after the
`next reload of countdown register 27 occurs.
`The present invention performs correction for time
`keeping inaccuracy only after some amount of error has
`been allowed to accumulate. Therefore, processing time
`overhead associated with the time correction is mini-
`mized. For example, if a crystal oscillator had a :100
`ppm error (which is five times greater than standard
`crystal
`tolerance), a 1.6% slower or faster minute
`would occur every 2.7 hours upon the counting down
`of the one million interval. Such an altered minute is not
`perceptible to the user of the clock, but overall accu-
`racy of the clock is maintained.
`EEPROM 34 is selected for storing the parts per
`million accumulated error value in order to provide
`nonvolatile storage of the correction factor so that it is
`not lost when power is removed from the microcon-
`troller system. In determining an accumulated error
`value A, measurements are made to determine the dif-
`ference between the actual fixed time period between
`interrupts from idle timer 26 and the ideal time period
`which would result from a crystal oscillator operating
`exactly at the designed frequency. In the present exam-
`ple, the ideal time period is 2.384186 microseconds. The
`time difference error (i.e., actual minus ideal) is multi—
`plied by 1,000,000 and divided by the ideal time period
`to derive the accumulated error value A in parts per
`million. Some number other than 1,000,000 could be
`used as long as it were larger than the one minute inter-
`val and preferably contained many one minute intervals
`to obtain the benefits of reduced processor loading and
`greater resolution of adjustment accuracy.
`'
`FIG. 3 shows a flowchart for a preferred software
`embodiment according to the present invention. Upon
`receiving an interrupt from the idle timer, a timer inter-
`rupt service routine is entered in step 40. In step 41, the
`one minute interval counter is decremented by one. In
`step 42, a test is made to determine whether the decre-
`mented one minute interval counter contains a value
`greater than zero. If the value is greater than zero, step
`43 is executed, otherwise step 46 is executed.
`In step 46, the one minute interval counter is reloaded
`with the reload value. The one minute interval counter
`reload value is set to 6144 in step 47. In step 48, the
`time-ofday minutes value is incremented. If the minutes '
`55
`value is less than 60 after incrementing, step 49 proceeds
`to step 43, otherwise step 50 is executed. In step 50, the
`minutes value is reset to “00”. In step 51, the time-of
`day hours value is incremented by one. In step 52, the
`hours value is tested to determine whether it is less than
`13 and, if so, step 43 is executed, otherwise the hours
`value is reset to “01” at step 53 before proceeding to step
`43.
`In step 43, the one million interrupts counter is decre-
`mented by one. In step 44, the one million interrupts
`counter value is checked for a value greater than zero.
`If the number is greater than zero, a return from the
`timer interrupt service routine is made in step 45. Other-
`wise, the one million interrupts counter is reloaded with
`
`4s
`
`50
`
`65
`
`6
`the number 1,000,000 at step 55. In step 56, the one
`minute interval counter reload value is set to the value
`of 6144 plus the parts per million accumulated error,
`then a return is made from the timer interrupt service
`routine in step 45.
`Turning to FIG. 4, an electronic system 60, such as a
`microprocessor-controlled audio system,
`including a
`time—of-day clock,
`is connected to a production test
`device 61. Microcontroller 10 includes crystal oscillator
`11 and EEPROM 34 as previously described. Mi-
`crocontroller 10 further includes a universal asynchro-
`nous receiver/transmitter (UART) 62 and a data output
`terminal which provides a test point 63. Test device 61
`includes a test computer 65 and a high accuracy fre-
`quency counter 67. Test computer 65 is coupled to
`frequency counter 67 and further includes a UART 66
`coupled to UART 62 via communication link 70. Fre-
`quency counter 67 includes an input line 72 connected
`to a test probe 71 which in turn is connected to test
`point 63 for purposes of testing the operation of elec-
`tronic system 60.
`In operation, during production testing of electronic
`system 60, test computer 65 instructs microcontroller 10
`through link 70 to output a test signal at its data terminal
`connected to test point 63. The test signal is propor—
`tional to the interrupt period of the microcontroller idle
`timer. The test signal is measured by frequency counter
`67 via test probe 71. Test computer 65 receives the
`actual frequency measurement from frequency counter
`67 and calculates the oscillator frequency error in parts
`per million. The parts per million error value can be
`either positive or negative and is transmitted via the
`UART interface to microcontroller 10 which stores the
`error value in EEPROM 34. For example, the error
`value may be an 8-bit number stored in two’s comple-
`ment format when negative. The 8-bit error value can
`represent correction factors within a range of :127
`ppm which is adequate for standard oscillator circuit
`components. Since the calibration or test signal gener-
`ated by microcontroller 10 is generated only during
`production testing, interference of the test signal with
`audio signals or other signals within electronic system
`60 during normal operation is avoided. Furthermore,
`the correction factor determination is fully automated
`and can be integrated with the normal production test-
`ing performed on electronic system 60. Therefore, no
`significant added expense is realized in providing accu-
`rate time keeping.
`While preferred embodiments of the invention have
`been shown and described herein, it will be understood
`that such embodiments are provided by way of example
`only. Numerous variations, changes, and substitutions
`will occur to those skilled in the art without departing
`from the spirit of the invention. Accordingly, it is in-
`tended that the appended claims cover all' such varia-
`tions as fall within the spirit and scope of the invention.
`What is claimed is:
`-
`1. Apparatus comprising:
`idle timer means for generating interrupts separated
`by a fixed time period which differs from an ideal
`time period by an error;
`first counting means having a first register and cou-
`pled to said idle timer means for counting said
`interrupts and for generating an increment signal
`when a number of interrupts have occurred equal
`to a load value loaded into said first register;
`reload means for storing a reload value and for (1)
`transferring said reload value to said first register as
`
`7
`
`

`

`7
`said load value upon the occurrence of said incre-
`ment signal and 2) then resetting said reload value
`to a predetermined value;
`second counting means coupled to said idle timer
`means and to said reload means for counting said
`interrupts and for
`transferring an accumulated
`error value to said reload means as said reload
`value when a number of interrupts have occurred
`equal to a large count number relatively greater
`than said predetermined value; and
`time means coupled to said first counting means and
`receiving said increment signal for keeping track of
`time-of-day.
`2., The apparatus of claim 1 further comprising:
`non-=volatile storage means coupled to said reload
`means for storing said accumulated error value,
`said accumulated error value being substantially
`equal to the sum of said predetermined value and
`the quantity of said error times said large count
`number divided by said ideal time period.
`3. The apparatus of claim 1 further comprising:
`non-volatile storage means for storing an offset value;
`and
`
`adding means coupled to said non-volatile storage
`means and to said reload means for adding said
`offset value and said predetermined value to obtain
`said accumulated error value, said offset value
`being substantially equal to said error times said
`large count number divided by said ideal time per-
`iod.
`4. The apparatus of claim 1 further comprising:
`test means coupled to said idle timer for causing said
`idle timer to produce a test signal for measuring
`said error.
`5. The apparatus of claim 1 wherein said first count-
`ing means, said second counting means, said idle timer
`means, said reload means, and said time means are im-
`plemented using a microcontroller, said apparatus fur-
`ther comprising a crystal oscillator coupled to said
`microcontroller,
`6. The apparatus of claim 5 further comprising:
`interface means coupled to said microcontroller for
`interfacing said microcontroller to test equipment
`for measuring a test signal having a frequency de-
`pendent on said oscillator; and
`
`8
`non-volatile storage means coupled to said microcon-
`troller for storing a representation of said accumu-
`lated error value.
`7. The apparatus of claim 6 wherein said non-volatile
`storage means is comprised of an EEPROM.
`8. The apparatus of claim 5 further comprising a
`display means coupled to said time means for displaying
`time-of-day information.
`9. A method for. maintaining accurate time keeping
`10 with an interrupt-driven clock, said interrupts being
`separated by a fixed time period, said method compris-
`ing the steps of:
`maintaining a first count of interrupts corresponding
`to a first time interval being measured and dis-
`played by said clock;
`maintaining time values for display by said clock;
`maintaining a second count of interrupts correspond-
`ing to a second time interval equalling a large num-
`ber of interrupts and containing many of said first
`time intervals;
`in response to said first count being equal to a prede-
`termined number, performing the steps of (A) re-
`starting said first count to count to said predeter-
`mined number;(B) updating said time values; and
`(C) setting said predetermined number to a first
`constant equalling the number of interrupts that
`would occur during said first time interval if the
`time between interrupts equalled an ideal time per-
`iod; and
`in response to said second count being equal to said
`large number, performing the steps of (A) restart—
`ing said second count; and (B) setting said predeter-
`mined number to a second constant related to the
`difference between said fixed time period and said
`ideal time period multiplied by said large number
`and divided by said ideal time period.
`10. The method of claim 9 wherein said first and
`second counts are maintained by counting down from
`said predetermined number and said large number, re—
`40 spectively, to zero.
`11. The method of claim 9 further comprising the
`steps of;
`determining the error between said fixed time period
`and said ideal time period using test equipment;
`calculating said second constant; and
`storing a representation of said second constant in
`non-volatile memory associated with said clock.
`I‘
`i
`It
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`
`4,903,251
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`25
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`30
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`35
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`50
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`55
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