What is the mDot's Vbat pin connected to?

[Heath Raftery]

I gather there are no schematics available for the mDot. Bit of a pain trying to reverse engineer everything.

I haven’t been able to get confirmation on what the Vbat pin of the STM32F411 on the mDot is connected to.

According to this thread, it’s connected to a regulated supply.

Why then do I get 3.85V when running powered from the MDK and using the ADC to measure that pin? Measuring Vdda with the same method gives 3.00V, which is what I expect from the information buried in some thread somewhere. So does that mean Vbat’s regulator is 3.85V? Why such an odd voltage, and is anything else supplied by this rail?

Ultimately I’d like to measure the input voltage for battery operation. I suspect I’ll need to wire in an external voltage divider, but just trying to understand what’s already on the board.

[Leon Lindenfelser]

The power supplied to the mdot is connected to the input of a 3v regulator. VBAT and VDDA are both connected to the output of the regulator.

[Heath Raftery]

The power supplied to the mdot is connected to the input of a 3v regulator. VBAT and VDDA are both connected to the output of the regulator.

This doesn’t add up.

AnalogIn   adcVref(ADC_VREF);
float vdda = 1.21f / adcVref.read(); //Internal voltage reference is 1.21V 

gives vdda = 3.00V. Makes sense.

But

AnalogIn   adcBatt(ADC_VBAT);
float vbatt = 4.0f * adcBatt.read() * vdda; //VBAT pin is div by 4

gives vbatt = 3.85V. That doesn’t make sense if, as you say, Vbat and Vdda are connected to the same rail.

[Author]

Posts