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3.3 V and 5 V — power rails, logic levels and the one diode rule

Question this page answers: “The board has a 5 V pin and a 3V3 pin — can I feed 5 V into a GPIO?”

Short answer: no. The GPIOs on all four boards are 3.3 V logic. 5 V belongs to power paths — the Espressif 5V/VBUS pin, the Pico’s VSYS (1.8–5.5 V input) and VBUS (USB 5 V) — never to signal pins.

Where the two rails live

Every MCUID board exposes both rails, but the official docs differ on how the power paths combine:

  • ESP32-DevKitC V4: three mutually exclusive supply paths — micro-USB (default), 5V+GND header pins, 3V3+GND header pins — with an explicit warning against combining them.
  • ESP32-S3-DevKitC-1: same three-path scheme; USB delivers 5 V to the board, the onboard regulator produces 3V3.
  • Pico: VSYS accepts 1.8–5.5 V input; the SMPS makes 3V3; VBUS carries the USB 5 V when present. Unlike the Espressif paths above, USB and an external VSYS supply can coexist — the board routes VBUS into VSYS through a diode. That diode describes the default power path, not a guarantee for every external supply: back-feed and isolation behaviour depend on your source, so keep one known power path unless you have checked the official power-supply notes for your setup.

On the Espressif boards the 5V/VBUS pin is an input for powering the board (or 5 V accessories); on the Pico the power-input pin is VSYS and VBUS is the USB 5 V at the connector. None of these pins is a logic level for the GPIOs.

GPIO logic is 3.3 V

The GPIOs on all four boards are 3.3 V logic. Interfacing 5 V sensors or modules is a real need, but the per-pin input limits are a datasheet matter — and MCUID does not publish a per-chip absolute-maximum field, so this page does not quote one. What is published, with sources, is the one explicit limit the Raspberry Pi datasheet states:

The ADC-capable pins (GP26-29) have an internal reverse diode to the 3V3 rail: input must stay below 3V3 + ~300 mV, and driving them while the board is unpowered leaks current through the diode into 3V3.

That is the Pico datasheet’s rule, quoted on the Pico and Pico 2 pages. For everything beyond it — 5 V tolerance of digital pins, series-resistor tricks, level-shifting part choices — check the chip datasheet for your exact board; do not infer from another board’s limits.

How 5 V reaches 3.3 V inputs safely on these boards

Two documented examples of the board doing the level-conversion for you:

  • Pico GP24 reads VBUS (5 V present) — on-board circuitry scales the rail down before it reaches the pin, so the GPIO sees a level inside its normal input range; the scaling is hardware the board provides, and firmware only reads the result.
  • Pico GP29 reads VSYS/3 (an ADC input behind a divider) — the divider is on-board hardware; the ADC and your code only ever see the divided value, never VSYS itself.

On the Espressif side the USB 5 V goes through the regulator and never appears on a GPIO unless you wire it there yourself.

Practical order of operations

  1. Power the board through its documented path. On the Espressif boards the three paths are mutually exclusive — pick exactly one. On the Pico, USB plus a VSYS supply can coexist per the board’s diode design, but that is not a blanket guarantee for every external supply — keep one known source unless you have checked the official power-supply notes.
  2. Treat every GPIO as 3.3 V logic until the specific datasheet says otherwise.
  3. Need to read a 5 V signal? Use a level shifter or divider sized from the datasheet limits — and label it as your circuit’s decision, not the board’s capability.

What this page cannot tell you

  • Per-chip input absolute maximums — not published as MCUID fields; the gap is recorded here rather than papered over with a generic “5 V tolerant / not tolerant” claim.
  • Level-shifter part recommendations — engineering choice outside the fixed-source scope of this site.

Sources