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Smart Module Series
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VBAT
PWRKEY
Others
> 8 s
Power down
Figure 10: Timing of Turning off Module
3.4. VRTC Interface
The RTC (Real Time Clock) can be powered by an external power source through VRTC when the
module is powered down and there is no power supply for the VBAT. The external power source can be
a capacitor according to application demands. The following figure shows the reference circuit when an
external battery is utilized for powering RTC.
Coin Cell
Module
RTC
Core
VRTC
C
Figure 11: RTC Powered by Coin Cell
⚫
When VBAT is disconnected, the recommended input voltage range for VRTC is 2.0
–3.25 V and the
recommended typical value is 3.0 V.
⚫
When powered by VBAT, the RTC error is 50 ppm. When powered by VRTC, the RTC error is about
200 ppm.
⚫
If a rechargeable battery is used, ESR of the battery should be less than 2 k
Ω.
⚫
If RTC function is not needed, it is recommended to connect a 0.1
μF capacitor to VRTC.