Safe to connect I2C device with a 3.6V limit to powerdock2?

George Gerber

Hi,
Is it safe to connect an I2C sensor with a limit of 3.6V to the Onion Omega Power Dock2?

The sensor manual has the following warning below:
Electrical connections
The DPS 5000 series sensors employ a 4 wire I2C-bus user electrical interface:
Supply +
Serial data (SDA)
Serial clock (SCL)
Supply –
The sensor may be used standalone or as part of a network of compatible I2C-bus devices.
CAUTION
The DPS 5000 is intended for use within networks operated from a single supply at a voltage within the range 2.7 V to 3.6 V. Operation outside these limits is not guaranteed and may damage the sensor.
The sensor interface includes 2 reserved signals. These should be left open circuit as connecting to these signals may result in incorrect sensor operation.

Do I make the following connections between the sensor and the power dock2(?):
Sensor Supply + ===== PowerDock2 3.3V
Sensor Supply - ===== PowerDock2 GND
Sensor SDA ===== PowerDock2 SDA
Sensor SCL ===== PowerDock2 SCL

Does this mean that all I2C connected devices can only be driven by a maximum of 3.3V?

Casey Holmes

It seems you'll need to check the data sheet of the sensor for current draw and check the current output on the data sheet of the specific regulator on the doc, then subtract the current draw of the omega and see what you have left... if it's enough current to power the sensor.

But I could be wrong. Completely new to onion.
.. first post in this forum as well

austin Sandford

The i2c bus is active low. Data is transmitted by pulling the data line to ground. When the data line isn't in use it floats to the supply voltage of the bus master.

In this case, everything is OK as you're powering the sensor off the 3v3 Omeaga supply.

If you needed to use a sensor that has (say) a 5v supply, then you have several options.

use a bidirectional opto isolator on tha data line. Only the clock and gnd are connected together.
use a bidirectonal level shifter to conver between the 5v and 3v3 systems , again only the clock and gnd are connected together.
the bus is driven by the master, and its voltage levels. You don't have to do anything special as 5v should never get anywhere near the data lines, and the sensor will pull the data line to gnd when it transmits data (connect the data, clock and gnd together).

Option1is the safest, the Aruinodo dock v2 uses option2 , but I've used option 3 to hook RPIs to Arduinos over i2c, with different supply voltages without anything going bang. The RPi (like the Omega) is a bus master and controls the line levels.

One caveat is if the sensor has pullup resistors to the supply line, so check any circuit diagrams if they are available.