Raspberry Pi pinout: the 40-pin GPIO header

Physical number or BCM number?

The physical number follows the position on the header: odd pins on the inner row, even pins on the board-edge row. Code uses the processor GPIO number instead (BCM numbering): pin 11 is GPIO17, pin 3 is GPIO2. On a Raspberry Pi, the pinout command prints the mapping.

The first 26 pins repeat the P1 header of revision 2 Pi 1 A and B boards; revision 1 differs on pins 3, 5 and 13 (see the table above). On Pi 5, the RP1 I/O controller drives the GPIO instead of the processor, but its functions were placed to match the Pi 4 header: BCM numbers and the main buses do not change.

Logic levels and current

3.3 V, not 5 V. The GPIOs are not 5 V tolerant; a 5 V signal applied directly can destroy the processor. A 5 V sensor or module needs a level shifter. Only pins 2 and 4 carry 5 V.

Current. The Raspberry Pi documentation gives up to 16 mA per pin and 50 mA in total for all GPIOs together. The drive-strength setting is not a current limit: it is the current up to which the output levels stay guaranteed. On Pi 4 (BCM2711) the default drive is 4 mA and the maximum 8 mA; on Pi 5 the RP1 offers 2, 4, 8 or 12 mA. An LED needs a series resistor, a motor or relay needs a driver stage.

3.3 V rail. Raspberry Pi states that the 3.3 V supply was designed for about 3 mA per GPIO, and does not publish a current rating for pins 1 and 17 in these documents: keep them for logic and sensors, and power anything else from 5 V through your own regulator. Since Pi 4, the STANDBY state keeps 5 V on and turns 3.3 V off: an add-on board must cope with it. Never connect a power source to the 3.3 V pins.

Pull-ups and power-on state

GPIO2 and GPIO3 (pins 3 and 5) have fixed 1.8 kOhm pull-ups to 3.3 V on the board (the HAT+ specification says about 2 kOhm). No need to add more on the I2C module; they also prevent using these pins as pulled-down inputs.

At power-on, every pin is an input with a weak pull. On BCM2835 and BCM2711, GPIO0 to GPIO8 start with a pull-up and GPIO9 to GPIO27 with a pull-down. The HAT+ specification asks boards to tolerate either: if a signal on your board must hold a defined state at power-on (power-stage enable, reset), fit an external resistor.

Pins 27 and 28: the ID EEPROM

ID_SD (GPIO0) and ID_SC (GPIO1) are reserved for a HAT ID EEPROM. The only allowed connections are the EEPROM and two 3.9 kOhm pull-ups to 3.3 V, on the add-on board. At boot, the firmware reads the EEPROM and loads the Device Tree overlay it names.

The HAT+ specification requires a 3.3 V 24Cxx I2C EEPROM with 16-bit addressing, no clock stretching and no address-bit paging, and a WP pin that protects the whole array; 100 kHz is enough. It recommends the CAT24C32, with WP pulled to 3.3 V through 1 kOhm and routed to a test point for production programming.

Powering the Pi through the 5 V pins

Pins 2 and 4 can power the Pi (PoE HAT, power board). The original HAT guide states that these pins join the 5 V net after the polyfuse and input diode: this path bypasses the input protection. The HAT+ specification requires a power board to supply at least 3 A at 5.1 V (5 A recommended), to only source current and to stay safe when USB-C power is plugged in at the same time.

The original HAT design guide recommended an ideal diode on the add-on board side, especially since Pi 3B+, 4B and Zero have no ideal diode on their 5 V input. When in doubt, we fit one.

Which UART is on pins 8 and 10?

On models with Bluetooth (Pi 3, Pi 4, Zero W, Zero 2 W), the full PL011 UART is taken by Bluetooth and GPIO14/GPIO15 carry the mini UART by default, which is less robust (no parity, baud rate tied to the VPU clock). On Pi 5, the console uses the dedicated 3-pin debug header (UART10); UART0 on GPIO14/GPIO15 is disabled by default and is enabled in config.txt. Every UART runs at 3.3 V.

What changes on Pi 5

Same pins, same BCM numbering, same main buses (I2C1, SPI0, SPI1, UART0, PWM, GPCLK), but everything goes through RP1. Differences that matter for design: input thresholds of 0.8 V and 2.0 V, internal pull-ups of 37 to 86 kOhm and pull-downs of 35 to 98 kOhm, drive selectable at 2, 4, 8 or 12 mA, four PWM channels (PWM0[2] and PWM0[3] also on GPIO14/15 and GPIO18/19), audio named I2S instead of PCM, and no mini UART.