ISO1540 and ISO1541 isolate the logic high and low input voltage levels of I2C devices

Table 1 lists the recommended logic input voltage levels for the 1 and 2 side inputs of ISO 1540 and ISO 1541 devices.

Table 1: ISO154x Input Voltage Levels

These input voltage levels apply to both I2C data and clock signals.

In order to achieve the bidirectional function of the isolated I2C device, the device needs to design a unidirectional channel with two back-to-back connections to implement a single bidirectional channel. Direct connection of two unidirectional channels results in a lock condition, and both channels are low. To avoid this, the diode on the 1-side output makes the low-level output of the 1-side output channel look like the high level of the 1-side input channel.

Due to this diode, the voltage of VOL1 can be up to 0.8 V. When the 2 side detects that the 2 side is low, the 1 side will turn on the field effect transistor, turning on the diode and generating a non-zero forward voltage. The thresholds in ISO154x devices are carefully designed to ensure smooth operation of the bidirectional channel . This approach has become a common practice in the industry for implementing bidirectional I2C functionality. Non-zero voltages with low logic levels will still be compatible with the I2C specification.

Please note that this only applies to VOL1. Since diodes are not required on the 2 sides of the device, VOL2 has a maximum of 0.4 V, which is common in most digital isolators.