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.