GPIO: signals you can touch
Build a low-current LED circuit and learn to check a pinout before connecting hardware.
Understand it
GPIO pins are electrical connections. A logic output represents a low or high voltage; it is not a general-purpose power supply. Follow the current Flipper pinout and use 3.3 V logic for these exercises. The separate 5 V supply pin does not mean an arbitrary signal pin is safe for 5 V.
An LED needs a series resistor. With a 3.3 V output, a red LED of approximately 2 V forward voltage, and a 1 kΩ resistor, the estimated current is (3.3 − 2) / 1000 ≈ 1.3 mA. Forward voltage varies; this is a conservative teaching circuit, not a device measurement.
UART, SPI and I²C
UART exchanges asynchronous serial data: endpoints must agree on baud rate and framing, and TX connects to the other end’s RX. SPI uses a clock and a chip-select signal. I²C shares clock and data lines with pull-ups and device addresses. All require compatible electrical levels and a common reference. A connector’s shape is not enough to identify its voltage.
Lab: build the evidence
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Disconnect USB and turn off the Flipper before wiring. Open the official pinout. Identify PA7 (pin 2) and GND (pin 8) by label and orientation, not by counting from memory.
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On the breadboard connect PA7 → 1 kΩ resistor → LED anode; connect LED cathode → GND. Check that no supply pins are shorted. If you cannot identify LED polarity, use its datasheet first.
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Power the Flipper. Open GPIO → GPIO Manual Control, select PA7, and use the on-screen control to change its output. Observe the LED. Do not connect this circuit to a mains-powered board.
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If it does not light, power down before checking polarity, resistor value, continuity, and the selected pin. Change one thing at a time.
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Set the output inactive, switch off, and disconnect. Draw the circuit with pin names, resistor value, and expected current.
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For a follow-up, read the USB–UART bridge documentation and draw the TX/RX/GND connections to a documented 3.3 V training board. Verify its pinout and power arrangement before a future connection.
Show that you can do it
- An annotated circuit diagram and estimated LED current.
- An observed high/low result with a controlled troubleshooting step.
- A clear explanation of why unknown voltages, RS-232 levels, and mains boards are excluded.
Record the result in your learning notebook. Repeat the explanation tomorrow without looking at this page.
Check yourself
Sources
Read the primary sources: Flipper: GPIO pinout and electrical limits. Course review: 10 September 2026.