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Header Pins 1, 2, 4, 17 are labled 3V3 and 5V on the schematics but the connections to the BCM and/or regulators are not in their documents. It appears the 5V node is coming from the raw USB/external power, is this true? What about the 3v3 node?

Edit: Note the answer in the duplicate question is not quite correct. The 3v3 comes from the first portion of the PAM2306 (on RPi Zero W for example) which does have and EN1 and EN2 pin that could be switched. So a more accurate question is on the BCM can VDDIO2_1 and VDDIO2_2 be toggled without any ill effect?

Edit2: There are a lot of BCM connections not shown on the schematics. Are there any documents that describe the pin connections for this processor? https://elinux.org/RPi_BCM2835_Pinout shows the labels, but are the functional descriptions all proprietary? Some elude to core or sdram, but it's unclear how they are connected or which voltages are applied.

marked as duplicate by Ghanima Jun 14 at 20:08

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    The 3v3 rail is controlled by the MxL7704 PMIC (Power Management IC). The PIMC does have a I2C bus that is controlled by the firmware (which you might be able to use to control the output), but there is no way to access it according to this. – scitronboy Jun 14 at 20:18
  • w.r.t. the edit, the Zero W's PAM2306 enable pins are hardwired. So there is still no way to programmatically change its state, joan's answer holds true. With the Pi3B+ there is a PMIC connected via I2C as @scitronboy said. So this question would likely be better if specific to a certain model. Do you want to narrow it down? (Though according to previous comment and joan's answers the answer will still be "no, cannot toggle" either way.) – Ghanima Jun 14 at 20:45
  • @Ghanima I plan to modify the board if the 2 Vdd pins are just bias for the GPIO bank. However I can't find any reference in the BCM specs to the pin functions. The question is generic to the processor more than a specific version. I can disable the gpio and have an external timer that can wake it up again, for example. – user6972 Jun 14 at 20:49
  • Looks like your question is something completely different from what you've started with. Good luck with that endavour however. Note that the pins are labelled G4 and J5, so they're in the middle under the BCM... and hardwired to the 3V3 rail (or even power plane, who knows). – Ghanima Jun 14 at 21:03
  • @Ghanima it's still not a duplicate question either way. The enable pin on the regulator is quite accessible. Typically SOC like this will separate their output drivers and it appears 3v3 is primarily just that with the 1V8 is the core elinux.org/RPi_BCM2835_Pinout Are there no pin descriptions? – user6972 Jun 15 at 0:14
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No it is not possible to toggle the power rails in software.

They are both powered whilst the Pi is powered.

Look at the schematics for your model here.

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