Can I leave VBAT unconnected?

re.wolff9 · ‎2018-04-11

Posted on April 11, 2018 at 14:03

Hi,

I Have a project that can be populated with or without RTC and a battery. Can I leave the VBAT pin unconnected when building PCBs without the battery?

I'm using an STM32F072.

#stm32f0 #stm32 #vbat

T J · ‎2018-04-21

Posted on April 22, 2018 at 02:00

You may well be correct, except to say it is very difficult to read 1nA.

I had real world experience on my board, but it was just a standard 0.1uF bypass cap.

Can you try a few different brands ?,

how did you measure it ?

re.wolff9 · ‎2018-04-22

Posted on April 22, 2018 at 07:46

Measuring 1nA is surprisingly simple. Put the multimeter in 200mV range and use it as a current measurement device: in series with the DUT. With the 10M input resistance, the full scale is now 20nA.

You really SHOULD verify the input resistance of your multimeter before you can do this. The DT9205 is good and consistenty 10Mohm across a bunch of ranges. While the Zotek VC17B+ does not give a consistent reading: I cannot get the input impedance more accurate than 'between 3.5M Ohm and more than 20M Ohm'.

We populated a bunch of boards yesterday and this was one of the 'standard bypass capacitors' that we use. Apparently while populating the boards, one of the caps escaped and we use the one 'lingering around'...