The two rows of pins along the sides of the PiCowbell Proto are connected to each other, to provide access to all of the pins, even when you solder headers onto the PiCowbell. Both rows of pins along the two sides of the PiCowbell match up exactly with Raspberry Pi Pico, Pico W, etc. The major difference from the Pico is the inclusion of the STEMMA QT connector which is connected to GPIO4 and GPIO5 (as well as power and ground). Let's take a look!

Power and Power-Related Pins

The following pins are either power or power related.

Power Pins

These are the available 3V power pins.

• 3V pins - The pair of pins highlighted in the image, along the top of the PiCowbell, are the standard Pico 3V out pins. These are used to power external devices, etc.
• 3V proto power rail - The 3V out Pico pin connects to a power rail that extends into the proto area. These pins all provide 3V power and are available to power anything you've connected to the proto area.
• 3V I2C pin - This pin next to the STEMMA QT connector is the 3V power pin for the broken out I2C pins. It can power a manually wired I2C sensor.

Ground Pins

These are the available GND pins. They are common ground for power and logic.

All GND pins are highlighted in white on the silk, except for the I2C GND pin. The GND pins that match the Pico exactly have square pads as well, as they do on the Pico.

• GND pins - There are pairs of GND pins along the top and bottom of the board, dispersed evenly throughout the other pairs of pins.
• GND proto rail - The right-most pairs of GND pins are connected to a GND rail that extends into the proto area. These pins are available to connect to anything you wire up in the proto area.
• GND I2C pin - This single G pin, at the upper right corner of the STEMMA QT connector is the GND pin for the broken out I2C pins. It can ground a manually wired I2C sensor.

Power-Related Pins

These pins are power-related.

• VB (VBUS) - This is the micro-USB input voltage, connected to the micro-USB port on Pico. It is nominally 5V.
• VS (VSYS) - This is the main system input voltage, can range from 1.8V to 5.5V, and is used to generate the 3.3V needed for the RP2040 and the GPIO pins.
• 3V3_EN - This connects to the enable pin, and is pulled high (to VSYS) via a 100kΩ resistor. To disable the 3.3V power, which also disables power to the Pico's RP2040, short this pin low (to GND).
• VR (ADC_VREF) - This is the ADC power supply and reference voltage. It is generated on the Pico by filtering the 3.3V supply. It can be used with an external reference when ADC performance is required.

I2C Logic Pins

• C (SCL) - I2C clock pin, labeled C on the PiCowbell. It is connected to your microcontroller I2C clock line, which is GPIO5 on the Pico.
• D (SDA) - I2C data pin, labeled D on the PiCowbell. It is connected to your microcontroller I2C data line, which is GPIO4 on the Pico.
• STEMMA QT - The PiCowbell Proto comes with a built in STEMMA QT connector! Located on the left end of the board, this connector means you can connect up all sorts of I2C sensors and breakout, with no soldering required!

The I2C logic pins and STEMMA QT logic pins correspond with GPIO4 for I2C data (SDA), and GPIO5 for I2C clock (SCL).

When connected to the Pico, in CircuitPython, you can talk to the STEMMA connector using board.STEMMA_I2C(). In Arduino it is Wire0.

Reset Button

• The PiCowbell Proto comes with a built in RESET button! Located on the right end of the board, it allows you to reset the Pico microcontroller and everything attached, without needing to unplug the USB connector. Use the reset button in conjunction with the BOOTSEL button on the Pico to enter the UF2 bootloader.
• The reset button is wired to the RUN pin, located towards the center of the top of the board. As with the other pins along the top and bottom, there are a connected pair of RUN pins available. This is the enable pin for the RP2040 on the Pico, and has an internal pullup to 3.3V. To reset the Pico, you can also short this pin low (to GND).

GPIO Pins

Along the top and bottom of the board are pairs of GPIO pins. The outside set matches the Pico pinout exactly, and is intended for soldering headers. The inside set is intended to make the pins available to connect up sensors and so on once you've attached the PiCowbell to a Pico.

GPIO Pins - Bottom Row

• GPIO0 through GPIO15 - Along the bottom of the board, from left to right, the pin pairs are GPIO0 through GPIO15 (with four pairs among them being GND pins). Note that all pin pairs are labeled on the silk, except for GPIO10, which is located to the left of the third GND pair, and to the right of GPIO11.

GPIO Pins - Top Row

• GPIO16 through GPIO22 - Along the top-right of the board, from right to left, the pin pairs are GPIO17 through GPIO22 (with two pairs among them being GND pins). Note that all pin pairs are labeled on the silk, except for GPIO21, which is located to the right of the second GND pair, and to the left of GPIO20.
• A0 (ADC0) through A2 (ADC2) - Along the top of the board, to the left of center, the pins are A0 through A2 (with one pair among them being GND pins). Note that all pin pairs are labeled on the silk, except for A1, which is located to the right of the GND pair, and to the left of A0. 

Proto Area

In the center of the board, you'll find the proto area.

• The single column on the far left is four separate pins. They are not connected in any way.
• In the highlighted block more towards the right, there are twelve columns that are connected vertically. You can identify these columns by the white silk between each of the pins in a given column. Similar to a breadboard, wherein the rows on either half are connected, this allows you to wire something to one pin in a column, and use another pin in the same column to wire something else to the same pin.