The default I2C address is 0x70.
- V+ - this is the power pin. Since the multiplexer chip uses 3-5 VDC, 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.
- - - common ground for power and logic.
- 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. This connection is shared with the STEMMA QT port on the opposite side of the board.
- 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. This connection is shared with the STEMMA QT port on the opposite side of the board.
- STEMMA QT - These connectors allow you to connect to dev boards with STEMMA QT connectors or to other things with various associated accessories. There's one port at the end that connects to your microcontroller. The other four connectors in two rows of two are discussed below.
- STEMMA QT Ports 0 to 3 - There are four JST SH 1mm connectors in two rows of two, all with the power, ground, and SDA/SCL pins connected. There are 4 sets of SDA and SCL pins, from SD0/SC0 to SD3/SC3. These are the multiplexed pins. Each one is a completely separate I2C bus set. You can have 4 I2C devices with identical addresses, as long as they are on one I2C bus each. The power input for the connectors is the output from the Vout pin, and is either V+ or 3.3V 500mA.
On the back of the board are three address jumpers, labeled A0, A1, and A2, below the board description text on the silkscreen. 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 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).
- RST - Reset pin for resetting the multiplexer chip. Pulled high by default, connect to ground to reset.
These jumpers select between supplying the 4 STEMMA QT ports with either the incoming voltage on V+ (3-5 VDC) or the output from the 3V regulator at 3.3V 500mA (labeled 3V). For example, if you're using this with a 5V microcontroller like an Arduino 328-compatible, you can level shift all the QT ports to have 3V power and logic level by cutting/soldering these jumpers.
- V+ jumper (default) - This jumper is located on the back of the board, to the right of the address jumpers. It supplies the incoming voltage on V+ (3-5 VDC) to the attached multiplexed devices. Cut the trace to this jumper to disconnect V+ from the multiplexed devices and set the output voltage to 3.3V 500mA.
- 3V jumper - This jumper is located on the back of the board, to the right of the address jumpers. Solder this pad to the center Vout pad to set the output logic level to 3.3V 500mA from the onboard 3V regulator.