There are a number of features on the 0.54" Alphanumeric Backpack.

STEMMA QT Revision-Only Features

These features are only available on the STEMMA QT revision.

STEMMA QT Connectors

The default I2C address is 0x70.

The STEMMA QT connectors provide a solder-free way to connect this backpack to development boards with STEMMA QT connectors, or to other things, with various associated accessories.

On LED and LED Jumper

  • On LED - On the left side of the back of the board is a little green LED labeled On. This LED lights up when the board is successfully powered.
  • LED jumper - To the right of the On LED is a jumper labeled LED. If you wish to disable the On LED, you can cut the trace between the two pads. To enable it again, use solder to reconnect the two pads.

Original and STEMMA QT Version Features

These features are available on both versions. There is one header pin difference between the two, which is explained in the next section. Everything else is the same.

Header Pin Through-Hole Pads

If you prefer to use a breadboard, there are through-hole header pin pads along the top of the board in the middle.

The default I2C address is 0x70.

On both versions:

  • VIO/VCC - This is power for the backpack. It can be 3V-5V. To power the backpack, give it the same power as the logic level of your microcontroller - e.g. for a 5V microcontroller like Arduino, use 5V.
  • GND - This is common ground for power and logic.
  • SCL - This is the I2C clock pin. Connect it to your microcontroller I2C clock line. This pin is level shifted so you can use 3-5V logic, and there's a 10K pullup on this pin.
  • SDA - This is the I2C data pin. Connect it to your microcontroller I2C data line. This pin is level shifted so you can use 3-5V logic, and there's a 10K pullup on this pin.

On the STEMMA QT revision ONLY:

  • VHi - This pin allows you to provide 5V to only the 14-segment displays when using a 3V device to control the backpack. If you're using a 3V device and you want your displays to be brighter, you can maintain the 3V I2C power level, and connect 5V to the VHi pin to make the 14-segment displays have a brighter look.

On the original version ONLY:

  • Vi2c - This is the I2C voltage, which sets the logic level to I2C. Connect this pin to the voltage pin on your device that matches the device's logic level. For example, if you're using a 3.3V microcontroller, connect it to 3.3V.

Display Pin Through-Hole Pads

These two rows of through-hole pads are for soldering the alphanumeric LED displays onto the backpack. See the Assembly page for details on attaching the displays properly.

HT16K33 Matrix Driver

The chip located in the center of the back of the backpack is the HT16K33 matrix driver, which drives the 14-segment LED displays.

Address Jumper Pins

On the back of the board are three address jumpers, labeled A0, A1, and A2. These jumpers allow you to chain up to 8 of these boards on the same pair of I2C clock and data pins. To do so, you solder the jumpers "closed" in various combinations by connecting the two pads.

The default I2C address is 0x70. The other address options can be calculated by “adding” the A0/A1/A2 to the base of 0x70.

A0 sets the lowest bit with a value of 1, A1 sets the next bit with a value of 2 and A2 sets the next bit with a value of 4. The final address is 0x70 + A2 + A1 + A0 which would be 0x77.

So for example if A2 is soldered closed and A0 is soldered closed, the address is 0x70 + 4 + 1 = 0x75.


If only A0 is soldered closed, the address is 0x70 + 1 = 0x71


If only A1 is soldered closed, the address is 0x70 + 2 = 0x72


If only A2 is soldered closed, the address is 0x70 + 4 = 0x74

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

This guide was first published on Jul 29, 2012. It was last updated on Feb 26, 2024.

This page (Pinouts) was last updated on Sep 22, 2022.

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