Here are some technical specs if you'd like to make devices and sensors that work with STEMMA
STEMMA 4-Pin I2C (both standard & STEMMA QT)
This connector is for I2C devices and has a 4 pin connector
For JST PH (2mm pitch) the pin order is
- Green for SCL
- White for SDA
- Red for V+
- Black for GND
For the STEMMA QT cables, we follow the Qwiic convention:
- Black for GND
- Red for V+
- Blue for SDA
- Yellow for SCL
Note the colors are slightly different for SDA/SCL but the pin order is the same
Power Lines
Ground is easy, thats the power and data reference ground.
V+ can be anything from 3-5V DC. I2C devices must accept 3-5V DC and must use a regulator if the device requires 3V or less power voltage! I2C controllers may provide 3 or 5V.
The reason we don't force 3.3V is that there are some high-current controlled devices (servos, RGB LEDs) that may benefit from having a high voltage/current availability.
Data Lines
SDA and SCL are the classic I2C data lines. The I2C device is expected to have pullups from SDA & SCL to V+. For that reason, both sides (I2C controller and I2C device) must be OK with 3-5V logic level and must implement I2C level shifting to the desired final voltage of the controller/device
The reason we don't go with the simple 3.3V logic level is that there are still a lot of folks using 5V logic devices like the Arduino UNO, level shifting is incredibly inexpensive (4 x 10K resistor pack + dual BSS138 is very compact and 10 cents total), and level shifting provides a bit of line level protection from reverse polarity, or overvolting. It's a little more effort but we think it's essential for a good experience.
STEMMA 3-Pin Analog/Digital/PWM
This connector is for Analog/Digital/PWM devices and has a 3 pin 2.0mm connector
- Black for GND
- Red for V+
- White for Signal
Power Lines
Ground is easy, thats the power and data reference ground.
V+ can be anything from 3-5V DC. Devices must accept 3-5V DC and must use a regulator if the device requires 3V or less power voltage! STEMMA controllers may provide 3 or 5V.
The reason we don't force 3.3V is that there are some high-current controlled devices (servos, RGB LEDs) that may benefit from having a high voltage/current availability.
Data Line
There is a single data pin, that we try to make sure can be used for various purposes such as analog input, digital in/out, even PWM/servo control. For example, here's a NeoPixel strip that has a JST connector on it for easy attachment to add colorful lights! As you can see, it plugs in right into the HalloWing
The connected device is expected to be OK with either 3-5V power and 3-5V logic. Use level shifting/dividers and regulators if necessary to be compatible!
To make things a little safer for the controller when it has 3.3V logic level, we put on a 1K+3.6V Zener diode protection circuit. This will keep the incoming voltage from going above 3.3V or below 0V
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