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.

Calculating the calibration value using the 512Hz test mode signal

 
 
 

Calculating the calibration value for a measured time variation

Slow Fast
 
Day Month
 
 

Calculating the calibration value over a temperature range

 
 
 
 
 for 
 
sec/month
 

Calculating the calibration value for multiple operating temperatures

 
 
°C 
 
% duty cycle
 
°C 
 
% duty cycle
 
°C 
 
% duty cycle
 
ppm
 
 for 
 
sec/month
 

Note: The total duty cycle of all temperatures must not be greater than 100%.

Calculating frequency error in ppm for a 32,768Hz crystal

 
Hz
 
ppm

Note 1: To avoid resetting these 2 bits, the user needs to mask them out before writing to the control register.

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