STM32F101 unused VBat.

[antonio]

Posted on June 26, 2013 at 15:37

Dear Sirs,

I need to know when the VBat pin has only a 100nF capacitor to ground, isn't connected to Vdd and no battery is present.

Thanks for advising.

Antonio 

#vbat-battery-backup

[Tesla DeLorean]

Posted on June 26, 2013 at 16:32

What's the question?

I would generally recommend having a supply on VBAT, and an LSE crystal. This would permit the use of low power modes, running the RTC, when the internal regulator turns off.

[antonio]

Posted on June 26, 2013 at 16:49

Dear clive1.

Thanks for your comment.

Anyway, the point is that in my application I neither use the RTC nor the low power modes and the LSE pins are pulled down: in such case may the VBat pin left unconnected with a 100nF cap to ground?

Thanks for your time.

Antonio  

[John F.]

Posted on June 26, 2013 at 17:19

The Reference Manual for your part says, ''If no external battery is used in the application, it is recommended to connect VBAT externally to VDD with a 100 nF external ceramic decoupling capacitor (for more details refer to AN2586).''

[antonio]

Posted on June 26, 2013 at 17:28

Dear John,

The document says it's highly recommended to do so (not mandatory); since I don't use the low power modes, the RTC and the LFE, can this be left unconnected? This is the question.

Thanks for looking into.

Antonio

[Tesla DeLorean]

Posted on June 26, 2013 at 17:38

I'm sure you can leave it unconnected, but seeing as the cost to connect it on a PCB approaches zero that seems like a pretty poor choice. Connect it to a supply, with the option to decouple it.

Make sure you connect the analogue supplies, these aren't optional, even if you don't use the ADC/DAC.

[antonio]

Posted on June 26, 2013 at 19:59

Dear clive1,

The question is not so trivial.

Of course, all the mandatory voltage nodes will be present. Since the battery isn't needed in every application and the provision is made for a lithium-thionyl chloride non-rechargeable chemistry, (which doesn't admit trikle nor leakage recharge), it would be preferable avoid any path towards Vdd, mainly because the manual warns that in some circumstances a reverse current is injected towards the battery for a short time. 

I'm wondering what could happen when the battery is not populated or removed. That's all.

Thanks for your time.

Antonio

[Tesla DeLorean]

Posted on June 26, 2013 at 21:46

The question is not so trivial.

It was however poorly framed when first asked.

[antonio]

Posted on June 26, 2013 at 22:08

Dear clive1,

''It was however poorly framed when first asked.''

Now that you make me notice it, yes, you're right. That's because I use to write on a separate text editor and then paste the text onto the forum. Perhaps some special character or else trimmed some sentences.

Anyway, thanks for your opinions.

Antonio 

[John F.]

Posted on June 27, 2013 at 09:14

You may insert a diode in series between the lithium thionyl chloride cell and VBat. This will prevent any significant current from being forced into the cell. Since the terminal voltage is normally 3.6V and very flat over discharge and considering the very small current the diode will not materially affect the life obtained. Note that the use of lithium thionyl chloride chemistry may bring with it other regulatory restrictions which will vary in different countries and may depend on the electrical capacity (size / weight) of the cell.