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TIMEKEEPER® Calibration Calculator
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Application Note AN934, TIMEKEEPER Calibration

The digital calibration feature of STMicroelectronics TIMEKEEPER products enables frequency calibration of the device to be software controlled. TIMEKEEPER products are driven by a quartz crystal controlled oscillator with a nominal frequency of 32,768Hz. Without calibration a typical TIMEKEEPER device is accurate to ± 1.53 minutes (± 35 ppm) per month at 25°C. TIMEKEEPER digital calibration (with the exception of the M48T86 device which does not have a calibration register) employs periodic counter correction to add or subtract counts from the oscillator divider circuit and hence compensate for temperature or crystal variations.

Information on TIMEKEEPER calibration is given in the Application Note AN934, TIMEKEEPER Calibration (62KB PDF). The TIMEKEEPER Calibration Calculator allows the Calibration Register value to be calculated for a measured value of the 512Hz test mode signal or for a measured time variation (fast or slow) per day or month. The calculator will also calculate the Calibration Register value for operation at a specified temperature given the frequency error in ppm at 25°C. Operation at different temperatures, each at a given duty cycle can also be specified. Finally the frequency error in ppm for a 32,768Hz crystal can be calculated.
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Calculating calibration value using the 512Hz. test mode signal
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Calculating the calibration value for a measured time variation
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Calculating the calibration value over a temperature range

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Calculating the calibration value for multiple operating temperatures
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Calculating frequency error in ppm for a 32,768Hz. crystal
Calculating the calibration value using the 512Hz test mode signal:

Enter the measured 512Hz output frequency:
(511.93- 512.043Hz, Frequency Test bit enabled)

Hz

Calculated result

ppm

Calibration Register value

 
'
Calculating the calibration value for a measured time variation:

The TIMEKEEPER device is

Slow Fast

for

seconds

per

Day Month

Calculated result

ppm

Calibration Register value

 
'
Calculating the calibration value over a temperature range:

Enter the frequency error in ppm at 25°C

ppm

Enter the temperature

°C

Calculated frequency

ppm

The time will be

 for  sec/month

Calibration Register value

 
'
Calculating the calibration value for multiple operating temperatures:

Enter the frequency error in ppm at 25°C

ppm

Enter temperature and duty cycle per day at temperature 1

°C  % duty cycle

Enter temperature and duty cycle per day at temperature 2

°C  % duty cycle

Enter temperature and duty cycle per day at temperature 3

°C  % duty cycle

Calculated frequency

ppm

The time will be

 for  sec/month

Calibration Register value

 

Note: The total duty cycle of all temperatures must not be greater than 100%.
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Calculating frequency error in ppm for a 32,768Hz crystal:

Enter the frequency at 25°C

Hz

Calculated frequency error in ppm

ppm
 

Note 1:

xx - To avoid resetting these 2 bits, the user needs to mask them out before writing to the control register.

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