GSM/GPRS Module Series
M85 Hardware Design
M85_Hardware_Design Confidential / Released 34 / 88
3.6. RTC Backup
The RTC (Real Time Clock) function is supported. The RTC is designed to work with an internal power
supply.
There are three kinds of designs for RTC backup power:
Use VBAT as the RTC power source
When the module is turned off and the main power supply (VBAT) is remained, the real time clock is still
active as the RTC core is supplied by VBAT. In this case, the VRTC pin can be kept floating.
Use VRTC as the RTC power source
If the main power supply (VBAT) is removed after the module is turned off, a backup supply such as a
coin-cell battery (rechargeable or non-chargeable) or a super-cap can be used to supply the VRTC pin to
keep the real time clock active.
Use VBAT and VRTC as the RTC power source
As only power the VRTC pin to keep the RTC will lead an error about 5 minutes a day, it is recommended
to power VBAT and VRTC pin at the same time when RTC function is needed. The recommended supply
for RTC core circuits are shown as below.
Normal Mode
AT+QPOWD
, use PWRKEY
pin, or use EMERG_OFF pin
Use AT command
AT+QSCLK=1
and pull up
DTR
SLEEP Mode
Use PWRKEY pin, or use
EMERG_OFF pin
Pull DTR down or
incoming voice call or
SMS or GPRS data
transmission
