The default I2C address is 0x20.
- 3-5V (VIN) - You can power the I2C/SPI LCD Backpack with 3 to 5V DC. There is a 3-5V boost circuit onboard so you can use this backpack to control 5V LCDs even with 3V power and logic.
- GND- Common ground for data and power.
Located along the top of the board, above the board label on the silk, are the input and output pins. These pins are for attaching an LCD screen to the backpack and are numbered 1-16. Pin 1 is located furthest to the left when looking at the front of the board.
Advanced users can repurpose these pins for general purpose I/O expansion. The MCP23008 has 8 I/O pins (7 are connected, backpack pins 4, 6, 11, 12, 13, 14 and 15) with optional pull-ups and the SPI 74HC595 has 7 connected outputs (backpack pins 4, 6, 11, 12, 13, 14 and 16).
- CLK (SCL) - I2C clock pin, connect 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.
- DAT (SDA) - I2C data pin, connect 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.
- STEMMA QT - These connectors allow you to connectors to dev boards with STEMMA QT connectors or to other things with various associated accessories.
- SPI Enable - The SPI Enable Jumper is located on the back of the board, between the lower STEMMA QT connector and the terminal block pins. You can solder this jumper closed to control the backpack over SPI.
The I2C/SPI LCD Backpack can be controlled over SPI after soldering the SPI Enable jumper closed.
- CLK (SCK) - SPI clock input pin
- DAT (MOSI) - This is the SPI MOSI (Microcontroller Out / Serial In) pin. It is used to send data from the microcontroller to the backpack.
- LAT - This is the latch input, which transfers the received data to the output lines on the shift register for the LCD.
On the back of the board are three address jumpers, labeled A0, A1, and A2, between the Adafruit logo on the board silk and the lower STEMMA QT port. 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" by connecting the two pads.
The default I2C address is 0x20. The other address options can be calculated by adding the A0/A1/A2 to the base of 0x20.
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 0x20 + A2 + A1 + A0 which would be 0x27.
So for example if A2 is soldered closed and A0 is soldered closed, the address is 0x20 + 4 + 1 = 0x25.
If only A0 is soldered closed, the address is 0x20 + 1 = 0x21
If only A1 is soldered closed, the address is 0x20 + 2 = 0x22
If only A2 is soldered closed, the address is 0x20 + 4 = 0x24
The table below shows all possible addresses, and whether the pin(s) should be high (closed) or low (open).