Page Comparison

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Uses a 32kHz crystal oscillator for its RTC. Can save data in backup RAM if the main voltage supply fails and the RTC is connected to a backup battery.

RTC Backup Power Alternatives Comparison

Battery

Commonly, a lithium coin cell is used as the backup battery.

To calculate cell life:

cell life = cell capacity (Ah) / timekeeping current draw of RTC (A)

Pros

Cons

- High energy density: take up little room on the PCB

- At room temperature: < 1% self-discharge per year

- Cannot use IR reflow: must be soldered on after reflow or placed in a holder

- Above 60oC: rapid self-discharge

Supercapacitor

Large low-leakage capacitors.

To calculate backup time:

backup time = C * (VBACKUPSTART - VBACKUPMIN) / IBACKUPMAX

C = capacitance of capacitor

VBACKUPSTART = initial voltage

VBACKUPMIN = ending voltage

IBACKUPMAX = maximum VBACKUP current (obtained from data sheet)

Pros

Cons

- Allows for IR reflow

- Fewer regulations for shipping and disposal (environmentally friendly)

- Almost infinite cycle life

- Charges faster than a battery

- Require a charging circuit

- Much shorter backup time

- Capacity may decrease with use and at high temperatures

- Requires some initial time before being fully charged

Sources

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