The default I2C address is 0x44.

Power Pins

• VIN - this is the power pin. To power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V micro like Arduino, use 5V.
• GND - common ground for power and logic.

I2C Logic Pins

• SCL - I2C clock pin, connect to your microcontroller's I2C clock line. This pin can use 3-5V logic, and there's a 10K pullup on this pin.
• SDA - I2C data pin, connect to your microcontroller's I2C data line. This pin can use 3-5V logic, and there's a 10K pullup on this pin.
• STEMMA QT - These connectors allow you to connect to dev boards with STEMMA QT (Qwiic) connectors or to other things with various associated accessories.

Other Pins

• RST - This is the reset pin. If this pin is pulled low, it will reset the sensor.
• ALERT - This is the interrupt pin. It is a push-pull output, meaning that you can monitor the state of this pin for interrupts that can be configured over I2C.

I2C Address Jumpers

On the back of the board are two address jumpers, labeled A0 and A1, between the STEMMA QT logo and cutouts for the HDC3021 sensor. These jumpers allow you to chain up to 4 of these boards on the same pair of I2C clock and data pins. To do so, you solder the jumpers "closed" by connecting the two pads.

The default I2C address is 0x44. The other address options can be calculated by “adding” the A0/A1 to the base of 0x44.

A0 sets the lowest bit with a value of 1 and A1 sets the next bit with a value of 2. The final address is 0x44 + A1 + A0 which would be 0x47.

If only A0 is soldered closed, the address is 0x44 + 1 = 0x45

If only A1 is soldered closed, the address is 0x44 + 2 = 0x46

The table below shows all possible addresses, and whether the pin(s) should be high (closed) or low (open).

Power LED and Jumper

• Power LED - In the upper left corner, above the STEMMA connector, on the front of the board, is the power LED, labeled on. It is a green LED.
• LED jumper - This jumper is located on the back of the board and is labeled LED on the board silk. Cut the trace on this jumper to cut power to the "on" LED.