Clock Calibration Calculator - STMicroelectronics

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.

If data retention lifetime is a critical parameter for the system, it is important to review the data retention current specifications for the particular SRAMs being evaluated. Most SRAMs specify data retention current (IccDR) at 3.0V. Manufacturers generally specify a typical condition for room temperature along with a worst case condition (generally at elevated temperatures.) The system level requirements will determine the choice of which value to use. The data retention current value of the SRAMs can then be added to the Ibat value of the Supervisor to determine the total current requirements for data retention. The available battery capacity for the SNAPHAT^® of your choice can then be divided by this current to determine the amount of data retention available (48mAh or 120mAh).

To calculate the typical and worse case data retention lifetimes, select the Supervisor Device and Battery Type in the form below. Enter the typical and worse case IccDR currents (nA) for the SRAM being evaluated and click on Calculate. The calculated typical and worse case Data Retention Lifetime in years will be then be displayed.

Calculating calibration value using the 512Hz. test mode signal
Calculating the calibration value for a measured time variation
Calculating the calibration value over a temperature range
Calculating the calibration value for multiple operating temperatures
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)

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%.

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

Enter the frequency at 25°C

Calculated frequency error in ppm

ppm

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