No VBAT pin on STM32L152 ?

[Tom]

Posted on April 14, 2014 at 18:48

Hi All,

Can anyone please advise why there is no VBAT pin on STM32L152 MCU?I read that RTC runs as long as the MCU is powered...but when its not, then what ?How to implement RTC backup supply on this MCU?Thanks!regardsTom #stm32l1-rtc

[waclawek.jan]

Posted on April 14, 2014 at 21:54

The 'L series is  different from the 'F series in that there is no dedicated VBAT pin. The backup parts (RTC, backup RAM) are connected to the same VDD than the rest of the circuit (although there is a regulator powering the core).

Review the datasheet pertaining to your model, chapters ''Power supply schemes'', ''Voltage regulator'', and the diagram under ''Power supply scheme'' chapter. In RM0038, focus to chapter 5, ''Power control''. JW

[Tom]

Posted on April 18, 2014 at 14:06

Hi,

Thank you for your response.What a sick idea with no separate Vbat pin!So how do I arrange the power supply in the system? I imagine  this:The MCU must be supplied from, say VBat and other ICs eg. LCD from, say Vcc.Now , as the Vcc turns off, VBat will take over the supply for MCU and the MCU must quickly tristate all its GPIOs so that the LCD won't be supplied via MCU's IO pins in the absence of  Vcc.Then the MCU has to go into sleep to spare the battery but run RTC, and then be able to wake up on external signal as Vcc returns.I don't know but I smells of trouble and constant RTC, and backup register reset or quick battery discharge.I would have to detect Vcc power off early enough to do the tristating and power down arrangements.How about taking voltage from the input of  LM7805 via resistor divider and connecting it to PVD_IN?  I haven't found any app note from ST on how to do the RTC backup in STM32L1 correctly. STM32L1 seemed almost perfect for my application until I came to this ridiculous solution of the backup domain in L1.Can you please advise how to solve this ?Best regardsTomPs: I fully agree with ''crap forum software''.  I cannot log in from Opera, (404: Forbidden) , in Firefox I can log in but cannot post a new topic (javascript crashes), Only Chrome works so far. I reported the problem many times to webmaster but they seem to sip drinks on Bora Bora as had no response at all.

[Tesla DeLorean]

Posted on April 18, 2014 at 16:09

The L series parts are designed to run off batteries, so you'd run it from the battery side of your charger circuit, and you'd have the chip monitor the presence of the charger supply, and you would SLEEP/STANDBY the part as you need too.

[Tesla DeLorean]

Posted on April 18, 2014 at 17:13

An LM7805, what on earth are you building, surely you can find a more effective low drop out regulator. There are devices out there with a POWER GOOD signal.

You could use a simple diode arrangement (supply with highest voltage wins, 2.4V battery 3.3V supply?).

http://www.nxp.com/documents/data_sheet/BAV70_SER.pdf

[Tom]

Posted on April 22, 2014 at 10:27

Hello Clive,

What do you think about the schematic I attach? I want to use the logic level and interrupt to detect the switch to battery power but I wonderif I can put the MCU to sleep quickly enough before the battery starts to feed current. I would like to avoid fast battery discharge due to frequent turning on/off of the device. Unfortunately the internal comparators are already in use . regards Tom ________________ Attachments : IMG_0131.jpeg : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006I13E&d=%2Fa%2F0X0000000bi8%2Fow5hOwUWjtuw1WT553n.W3GZxglXd_qJoh3WakK.VTo&asPdf=false