Voltage Stacking with energy harvesting STM32WB55

[methene]

Hello, I am attempting in voltage stacking of 2 STM32WB55 board, but unlike arduino I could not connect Top board's GND to bottom board's 3.3 V pin. I would like to request for help or insights for alternative method I could make it work. I would really appreciate the help. 

Problem

1. When I connect the Top STM32 GND to the VDD of Btm STM32, it discharged the voltage in Btm capacitor.

2. On one attempt I rearranged the circuit in similar stacked configuration, it did not discharged the Btm capacitor voltage. Instead the system goes haywire.

Thank you.

[methene]

Using voltage regulators to regulate the harvested energy is it?

I am using a multimeter that could measure in micro ampere which I think can complete the job. 

[methene]

In this context it would meant the voltage in capacitor.

Once the voltage in capacitor reaches a certain threshold detected by the STM32 board, in this case is 3V, it consumes the voltage to carry out a task, I set it as sending bluetooth signal.

Because I am using 2 PV in a parallel arrangement, to ensure the testing environment is the same for both cases I am using 2 PVs. The arrangement I had could be incorrect, I am also unsure which is the configuration I need. I am still testing around and appreciate any info on I could make it work. 

As for the arrangement above, I remember trying it but I forgot the result I obtained. But it is one of the problems I mentioned in the post:

1. When I connect the Top STM32 GND to the VDD of Btm STM32, it discharged the voltage in Btm capacitor.

2. On one attempt I rearranged the circuit in similar stacked configuration, it did not discharged the Btm capacitor voltage. Instead the system goes haywire.

But I will still try it out again within this week. 

[Andrew Neil]

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@methene wrote:

Once the voltage in capacitor reaches a certain threshold detected by the STM32 board, in this case is 3V, it consumes the voltage to carry out a task. 

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As noted, that won't work - due to the STM32's minimum operating voltage.

You would need something else to monitor the voltage, and only allow the STM32 to start once there is sufficient energy stored.

[Andrew Neil]

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@methene wrote:

Using voltage regulators to regulate the harvested energy is it?

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Yes.

There are chips specifically designed to do this - including MPPT.

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@methene wrote:

I am using a multimeter that could measure in micro ampere which I think can complete the job. 

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No.

The microcontroller current is not a steady current - especially with a radio!

The current will vary between (many) tens of mA during transmit down to uA when idle.

The current consists of very rapid changes & narrow pulses - you won't be able to see this on a meter.

You will need something like this:

https://www.st.com/en/development-tools/stm32cubemonpwr.html

[Andrew Neil]

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@methene wrote:

I remember trying it but I forgot the result I obtained. . 

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key to doing this kind of experiments is to keep detailed & accurate notes.

Do schematics of the actual configurations you've tried, and record the results. Take photographs.

You didn't try it with the USB connected to the boards, did you?

[unsigned_char_array]

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@methene wrote:

Using voltage regulators to regulate the harvested energy is it?

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No. I meant to regulate the voltage that is used to supply the MCU. In your case if you have some circuit that will charge the capacitor and then when it reaches a certain level stop charging and connect the MCU you won't need a voltage regulator as stopping the charge is also a way to regulate. But you still need some type of circuit (there are energy harvesting chips that do this for you, they can switch on/off an MCU when charged and they can even to MPPT for you).

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@methene wrote:

I am using a multimeter that could measure in micro ampere which I think can complete the job. 

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No. That absolutely won't work. If you knew how a multi-meter worked you would know that it won't work.
Multimeters use fixed ranges of shunt resistors. Even when you use an autoranging multimeter there is a response time. So when the current spikes the voltage drop over the shunt is too great and the MCU voltage will drip too low. In other words the dynamic range is too low.
You need something like this: https://eevblog.store/products/ucurrent-gold-multimeter-adapter
Even if this worked a multimeter is for continuous DC voltage or continuous AC amplitude, not for short spikes. You would need something faster. Such as an oscilloscope.

[unsigned_char_array]

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@methene wrote:

, it consumes the voltage to carry out a task

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Nothing consumes a voltage. It consumes a current or a charge. In this case the charge of a capacitor. But not the full charge as it cannot use the voltage below a certain level. Unless you use a boost converter.

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@methene wrote:

Because I am using 2 PV in a parallel arrangement, to ensure the testing environment is the same for both cases I am using 2 PVs. The arrangement I had could be incorrect, I am also unsure which is the configuration I need. I am still testing around and appreciate any info on I could make it work. 

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Is this a first year project? You should know parallel vs series by now. That's high school level physics.

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@methene wrote:

As for the arrangement above, I remember trying it but I forgot the result I obtained. But it is one of the problems I mentioned in the post:

1. When I connect the Top STM32 GND to the VDD of Btm STM32, it discharged the voltage in Btm capacitor.

2. On one attempt I rearranged the circuit in similar stacked configuration, it did not discharged the Btm capacitor voltage. Instead the system goes haywire.

But I will still try it out again within this week. 

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Don't try to remember results. You write down the results or save the results from your oscilloscope or logic analyzer. Or you take a photo with your phone.

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@methene wrote:

1. When I connect the Top STM32 GND to the VDD of Btm STM32, it discharged the voltage in Btm capacitor.

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What does "it discharged the voltage in" mean? It's not correct use of terminology and probably not grammatically correct either. I have no idea what it means.

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@methene wrote:

2. On one attempt I rearranged the circuit in similar stacked configuration, it did not discharged the Btm capacitor voltage. Instead the system goes haywire.

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Define haywire. What happened? Or what didn't happen? I assume one MCU received all the voltage and was being fried while the other one shut down due to too low voltage.

[Andrew Neil]

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@unsigned_char_array wrote:

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Define haywire. What happened? Or what didn't happen? 

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As Bob Pease used to say,  "If you measure something 'funny', record the amount of 'funny'." 

PS:

Ah - that was actually Bob quoting Tom Milligan, one of his old production test managers:

https://www.electronicdesign.com/technologies/analog/article/21748808/whats-all-this-critical-thinking-stuff-anyhow#:~:text=So%2C%20when%20you,in%20his%20honor

#RecordAmountOfFunny

[methene]

I tried to keep it as detailed as possible, will be more careful

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@Andrew Neil wrote:

You didn't try it with the USB connected to the boards, did you?

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 No, I did not, since I am using PV generated voltage

[methene]

The operating voltage of STM32WB series is from 1.71V to 3.6V and the board I am using is configured to be operating within that range.

The STM32WB board monitors the voltage in capacitor and only start the task when it reaches 3V.