3.3 volts versus SD pad for components...?

So I’m wondering what the difference is between using the 3.3 volt pad or the SD card pad for powering stuff like Bluetooths, OLEDs etc.

Both output 3.3 volts, and both stay powered after the LED Idle Timeout has kicked in. Each one appears in the manual for powering different items too.

So is there any benefit or disadvantage to using one over the other?

Obviously the 3.3 volt pad has the benefit of its proximity to the Ground pad for adding Bluetooth capacitors, but that’s about the only difference as far as I can tell.

Is there anything I’m missing?

Thoughts welcome. Thanks in advance.

Which version board?
A 2.2 board’s 3.3V pad has a direct line from the regulator, while the sdd power pad goes through some other components first.
SD power working correctly and uninfluenced is kind of vital IMO FWIW.

You mean any other current draws being applied to that pad might start messing with SD function?
Yeh, that makes sense.

I guess I was just curious as occasionally I get an install with lots of bells and whistles, and cramming all the extras’ positives into the one 3.3 volt pad can be a squeeze - especially if it’s partially blocked by a Bluetooth capacitor. Hence I was curious about the viability of spreading the load, and the wires. But your logic makes sense so I guess I’ll stick with the 3.3 volt pad for those installs.

Thanks as always.


The difference between SD_VDD and 3.3v is that SD_VDD can be turned off. There is no code that actually does that right now, but that is still the intention. In theory, we could also use this to cut the power to an OLED, bluetooth module and anything else that runs on 3.3v, but again, there is nothing that actually does that right now.

It turns out that cutting power to OLEDs is actually not required. Once you give the OLED a power down command, it uses so little power that it doesn’t really matter.

Bluetooth modules makes more sense though, however, since none of this is implemented, most people use a LED pad to power down their bluetooth modules, which also works.

For right now 3.3v and SD_VDD comes from the same source, so if you hook up too much to either one of them, everything that runs on 3.3v can fail, and this includes: The CPU, the sd card and the motion chip. The CPU will probably fail last, because it runs on 1.8v internally and has an internal down-converter.

Hopefully there will be an update in the future where SD_VDD turns on and off. There will also be some defines that lets you control how this works, like “don’t turn it off if the blutooth module is active” or “never turn it off”.


Thanks Prof. This is all good to know.
So up until ProffieOS 7.10 at least, the two pads are pretty much interchangeable. But I see where you’re coming from regarding possible future updates.
But at least now when someone wants a component added to an existing crowded install, I’ll know I have anther option rather than unpicking a crowded 3.3 volt pad. :slight_smile: Always good info to have in the toolbox.

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This will stay true for all 7.x releases, since new features always require a new major number.

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