Data Sheet

Smart Machine Smart Decision
SIM800F_Hardware Design_V1.00 28 2015-07-31
4.5 RTC Backup
VRTC is the power supply for RTC circuit, and its typical value is 2.8V. To allow the RTC to operate even when
VBAT is turned off, it is recommended to connect the VRTC pin to a backup battery or an external
large-capacitance capacitor (100uF is recommended).
When VBAT is turned on, RTC circuit is supplied from an internal LDO, and the backup battery or the external
capacitor will be charged through VRTC pin.
When VBAT is turned off, RTC circuit will be supplied from the backup battery or the external capacitor.
The following figures show various references for RTC backup circuit:
 External capacitor backup
Figure 17: RTC supply from capacitor
 Non-chargeable battery backup
Figure 18: RTC supply from non-chargeable battery
 Rechargeable battery backup
Figure 19: RTC supply from rechargeable battery
Note:
1. For electrical characteristics, please refer to Table 44: VRTC characteristics.
2. The clock error becomes larger when VBAT is turned off and RTC circuit was supplied from the backup battery or the
external capacitor.