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.
If no battery, both RM0091 and AN4080 recommend to connect it to VDD.
Well, yes. You're right. But I was hoping I would not need extra components when I don't mount the battery. Or what if I do mount the battery-holder, but the user forgets to replace the battery when it's empty? That creates a very similar situation as what I have now: Simply a floating VBAT.
I just checked the "decoupling your STM32F0xx" section in the datasheet. No capacitor on VBAT. And then RM0091 says to install a capacitor when connecting to VDD...
I guess you can leave it unconnected as long as you don't use the RTC and other VBAT-related features.
Note that I am not ST so do this on your own risk.
Capacitor between VBAT pin and ground is not mandatory. If you don't plan to use backup domain, please simply connect VBAT to VDD.
A capacitor to ground on the VBAT pin is usually detrimental to the battery life
Not recommended unless you use a high quality poly cap.
Why does a capacitor drain the battery? Why would a better cap not do this? You're thinking about leakage current? I don't think of ceramic caps as "leaky". I'll see if I can measure the leakage of the 100nF 50V ceramic 0805 capacitors that I have.
If you are using a normal Coin Type cell,
I found the basic 0.1uF bypass cap,
reduced the voltage on the battery below 3V within a month.
I measured the leakage of my standard 100nF 50V capacitor. It is about 0.1 nA at 10V and about 1nA at 50V.
A 200mAh coin cell would be depleted by such a current after about 63 years.....
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 ?
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"...
use a jumper or solder blob under the battery itself.
if the battery is not installed, you must solder the blob , shorting out the Vbat to VDD
Retrieving data ...