Working With 3.3V I2C FAQ (Frequently Asked Questions)

Working With 3.3V I2C.

3.3V Signals

The I2C Bus is an open-collector bus. Some place on the bus a pull-up on the clock (SCL) and data (SDA) lines hold the lines high when no device is actively pulling a line low. A pull-up to 3.3V is high enough for 5V devices, like our iPorts, to see the signal as a logical one (1). We have tested iPorts down to about 3.1V, and they seem to work OK. This is also true for our I2C Bus Monitor (#MIIC-101).


The only thing you should be careful of is the pull-ups inside the iPorts. These pull-ups are to our internal 5V, so if the iPort pull-up switch on the side of the case is ON, an external circuit will see the 5V. Some 3.3V devices may not tolerate the 5V on a pin, and this could damage the 3.3V device.

 iPort Power:

Our iPort (#MIIC-201), iPort/AI (#MIIC-202), and iPort/AFM (#MIIC-203) are 5V devices. They need about 75ma of regulated 5V power to operate. When attached to a 3.3V system, the iPort power should come from our wall power supply, and the power wire in the I2C Bus cable should NOT be attached to the 3.3V system.

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