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Low Voltage I2C Interface to the Aardvark Adapter
Low Voltage I2C Interface to the Aardvark Adapter

Aardvark I2C/SPI Host Adapter Voltage Rating

The Aardvark I2C/SPI Host Adapter operates at 3.3V (+/- 10%). The adapter device is compatible with both 3.3V and 5V signal levels out of the box. The I2C bus is open-drain, and the device contains pull-up resistors for the SCL and SDA lines. As mentioned in the user manual, these lines are effectively pulled up to 3.3V.

For more information about the voltage rating, please consult the user manual.

Low Voltage Interface

Since the bus is open-drain and the Aardvark device already contains pull-up resistors, you could employ the following simple method to adapt the voltages to your target.

Sample I2C Circuit

Figure 1: Sample I2C Low Voltage Circuit.

In this example, the Aardvark I2C/SPI Host Adapter is interfaced with a 2.5V device. The Enable line is active high and should not exceed the lower of the two supply voltages. In this case, it should not go higher than 2.5V.

When the SCL line is high, both transistors are off. When the emitter of either transistor goes low, base current is drawn through the high Enable line, causing the transistor to saturate. Therefore, when either side of the SCL line goes low, the other side is pulled low, as well. The SDA line behaves in the same manner.

Essentially, this circuit acts as a bi-directional dual-voltage open collector buffer. The entire circuit can easily be created on a small prototyping board.

The total parts count would be:

4 NPN transistors
1 10K resistor
2 2.2K resistors

It is highly recommended that the circuit is tested to ensure that it does not apply more than 2.5V to your target before using it. Also, the transistors should be selected to provide sufficient switching performance for the desired bitrate. Total Phase has not tested this circuit and cannot guarantee its performance or reliability.