STM32F4 RTC VBat - Cap VS Battery

[MobileWill]

So I am working with the STM32F429ZI and was thinking I could use a super cap for the RTC/Backup SRAM instead of a coin cell. My question is it worth the extra design for the super cap or just stick with a battery? This project will mostly likely turn in to a product.

From my research the super cap will need 2 diodes, 1 resistor, and 1 cap close to Vbat input.

For the battery I would assume I just 1 diode to stop it from feeding the circuit when unpowered or should the battery only be connected to VBat and nothing else.

Thanks.

[matkar]

A CR2032 battery will last for more than 10 years. Super capacitor costs more. Any particular reason to consider using super cap?

A warning; I have experience with 32F407 and haven't checked if the same applies to your 429... Check if there is a PDR_ON on the pinout and tie it to Vss. With lower pin count devices (64, 100) this pin is tied to Vss internally, but on higher pin count devices (144, 176) you have to connect it to a positive or negative potential. If this pin is not tied to Vss you will drain the battery in a week. Check the datasheet for more.

Edit: PDR_ON should be tied to Vdd, not Vss. On the lower pin count devices the PDR_ON is not connected to Vss internally. Read further down.

[bktemp]

A warning; I have experience with 32F407 and haven't checked if the same applies to your 429... Check if there is a PDR_ON on the pinout and tie it to Vss. With lower pin count devices (64, 100) this pin is tied to Vss internally, but on higher pin count devices (144, 176) you have to connect it to a positive or negative potential. If this pin is not tied to Vss you will drain the battery in a week. Check the datasheet for more.

The STM32F427/429/469/476 datasheets say:

When PDR_ON pin is connected to VSS (Internal Reset OFF), the VBAT functionality is no more available and VBAT pin should be connected to VDD.

[MobileWill]

A CR2032 battery will last for more than 10 years. Super capacitor costs more. Any particular reason to consider using super cap?

Only reason I was thinking of super cap was for board space but mainly so users don't have to worry about a battery. Plus easier for assembly so a human doesn't have to place a battery.

[Jeroen3]

You can get small board mount (even smd) rechargeable lithium batteries that can be tied directly to 3v3 to charge them.
Works like a supercap, but longer and cheaper.

You have to be very picky about the diode though. The leakage is often more than the mcu will use on it's Vbat.

[Chriss]

From my side I advise to use a coin battery. Or that board for rechargeable batteries.

[MobileWill]

You can get small board mount (even smd) rechargeable lithium batteries that can be tied directly to 3v3 to charge them.
Works like a supercap, but longer and cheaper.

You have to be very picky about the diode though. The leakage is often more than the mcu will use on it's Vbat.

Sounds even better. I will see what I can find. The first prototype is an add-on to a discovery board so I have some time to figure out the details.

[exmadscientist]

You can get small board mount (even smd) rechargeable lithium batteries that can be tied directly to 3v3 to charge them.
Works like a supercap, but longer and cheaper.

You have to be very picky about the diode though. The leakage is often more than the mcu will use on it's Vbat.

Rechargeable coin cells are the way to go here. Supercap leakage current is usually enough to render them inapplicable for long-term hold-up applications. Many RTC modules even have a built-in trickle charger for rechargeable lithiums. I've used a combination of ML621 batteries with DS1374 RTCs in several designs now, and have been quite satisfied.

[matkar]

The STM32F427/429/469/476 datasheets say:

When PDR_ON pin is connected to VSS (Internal Reset OFF), the VBAT functionality is no more available and VBAT pin should be connected to VDD.

I verified and I was wrong. You should tie the PDR_ON to Vdd not Vss. I'll edit my first post as well.
Thank you bktemp!

[MobileWill]

Thanks for everyones help.

I found these

http://www.cymbet.com/products/enerchip-solid-state-batteries.php

But not something digikey seems to stock and probably would be pricey. Too bad they are hard to find.
Hmm. They also have a all in one RTC/Battery.

Would be such a nice solution too.

If I get a generic rechargeable coin cell, I would need a charge controller and probably can't reflow.