VBAT circuit for STM32F429

[gregw]

Posted on May 21, 2015 at 22:20

We are using the STM32F429ZIT6 and we need to keep the RTC running when VDD is not present.  Can we hook up a coin-cell battery directly to VBAT, with no switching/monitoring circuit?  Any help would be MUCH appreciated!!

Greg

[waclawek.jan]

Posted on May 22, 2015 at 07:54

> We areusing the STM32F429ZIT6 and we need to keep the RTC running when VDD is not present.

> Can we hook up a coin-cell battery directly to VBAT, with no switching/monitoring circuit? Yes. See the respective EVAL board (STM32439I-EVAL perhaps) for inspiration/reference (but there's nothing more than the plain battery holder connected through a jumper there). The gotchas are, that you are supposed to

https://my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/Flat.aspx?RootFolder=https://my.st.com/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/STM32F407I%20with%20LQFP176%20%20RTC%20registers%20resets%20when%20Vbat%20below%1V&FolderCTID=0x01200200770978C69A1141439FE559EB459D7580009C4E1...

; and the warning in 5.1.2 Battery backup domain chapter of RM0090 rev.9 (applies for unusually low voltage batteries only). Another gotcha may be the VBAT measurement in ADC, the 50k divider may suck the juice of a smaller battery quite rapidly if left connected for too long or the measurement is repeated too often. JW

[gregw]

Posted on May 22, 2015 at 14:07

Thanks!  This is a big help.  We'll be sure to tie VDDA to VDD and we'll also watch for any current injection into VBAT and use a diode to VDD accordingly.

Take care...

[tm3341]

Posted on May 23, 2015 at 10:42

Actually, F4xx devices have built-in option to measure VBAT with internal ADC. You don't need to connect external resistors and everything.

Just enable VBAT measurements by software and on some ADC channel (maybe 18) you will read VBAT value divided for some factor.Read datasheet for more info, Reference manual for F4xx series.