VDD - This is the positive power and logic level pin. It can be 2.7-5.5VDC, so fine for use with 3 or 5V logic. Power VDD with whatever logic level you plan to use on the i2c lines.
- GND - this is the ground power and logic reference pin.
- SCL - this is the I2C clock pin. There's a 10K pull-up already on the board, so connect this directly to the i2c master clock pin on your microcontroller
- SDA - this is the I2C data pin. There's a 10K pull-up already on the board, so connect this directly to the i2c master data pin on your microcontroller
These are pins you don't need to connect to unless you want to!
- Alert - This is the interrupt/alert pin from the MCP9808. The chip has some capability to 'alert' you if the chip temperature goes above or below a set amount. This output can trigger to let you know. It is open collector so you need to use a pull-up resistor if you want to read signal from this pin.
- A0 A1 A2 - These are the address select pins. Since you can only have one device with a given address on an i2c bus, there must be a way to adjust the address if you want to put more than one MCP9808 on a shared i2c bus. The A0/A1/A2 pins set the bottom three bits of the i2c address. There are pull-down resistors on the board so connect them to VDD to set the bits to '1'. They are read on power up, so de-power and re-power to reset the address
The default address is 0x18 and the address can be calculated by 'adding' the A0/A1/A2 to the base of 0x18
A0 sets the lowest bit with a value of 1, A1 sets the middle bit with a value of 2 and A2 sets the high bit with a value of 4. The final address is 0x18 + A2 + A1 + A0.
So for example if A2 is tied to VDD and A0 is tied to VDD, the address is 0x18 + 4 + 1 = 0x1D.
If only A0 is tied to VDD, the address is 0x18 + 1 = 0x19
If only A1 is tied to VDD, the address is 0x18 + 2 = 0x1A
If only A2 is tied to VDD, the address is 0x18 + 4 = 0x1C