VBAT Connection?

redgod9 · ‎2011-05-21

Posted on May 22, 2011 at 03:37

Hi.

The VL Manual (UM0919) says that by default solder bridge (SB1 VBAT) is factory set to be off. This should allow you to connect a battery to the board and run the real time clock when then board is powered off. There should be no connection to the MCU VDD supply from the battery.

When I connect a coin cell to the VBAT connector and GND and then disconnect USB power the board continues to run. If I look at the board carefully there is a component bridging the SB1 pads. Its too small to see what it is though.

Is this behaviour correct?

Do I have to detect that I'm on battery power and select a low power mode?

I'm a bit confused... (Nothing new).....

donald2 · ‎2011-05-22

Posted on May 22, 2011 at 09:27

The datasheet recommends powering Vbat with Vdd when there is no battery backup.

The schematic shows the solder bridge on Vbat, along with other locations, but it isn't always clear which are connected. On my board the bridges are connected with zero ohm resistors rather than solder. Probably because they cost almost nothing and are more predictable than expecting all of the solder bridges to close during reflow.

You can usually just cut/crush them with diagonal wire cutters to break the jumper if you don't have a soldering iron handy.

redgod9 · ‎2011-05-22

Posted on May 22, 2011 at 16:36

I can confirm that. I removed the bridge with a small knife and the RTC now behaves itself as i thought it should. The documentation is definitely incorrect.

If you want to remove the battery and reconnect VBAT to VDD you can just install a jumper between the adjacent VBAT and 3V3 pins, which Is why i suspect they are placed as they are.

In summary...

Minus : You have to chop the resistor out to battery back the clock.

Plus: The jumper is very conveniently located to effectively reconnect the link if you have to.

Remember not to keep either link in if you use the battery or you'll have current flowing into the battery.

Thanks for that Donald...

donald2 · ‎2011-05-23

Posted on May 23, 2011 at 17:06

I was curious where you got the idea that Vbat wasn't connected to Vdd.

I found that on page 12 of UM0919, the VLDiscovery User Manual, it says that ''Factory state is in bold type''. The Vbat solder bridge is indeed documented as ''Off'' / not connected.

SB1 is the only ''solder bridge'' on the component side of the board, located very close to pin 1 of the 32F100.