Target devices will most likely have some type of protection on their input and output pins. This protection is in the form of a diode which ensures that the devices do not experience latch-up due to excessive input voltages.
Figure 1: A Protection Diode with VDD = 3.3V and input <= 3.3V
The cathode of a protection diode is connected to VDD while the anode of the diode is connected to the input line. In Figure 1, VDD is at 3.3V and the input signal is between 0V and 3.3V, so the diode is reverse-biased and no current flows.
Most diodes require at least a 0.5V threshold to activate. Therefore, as shown in Figure 2, an input signal of 3.8V would switch on the diode and allow current to flow from the input to VDD.
Impact of Power Cycling
The presence of protection diodes on target devices requires special consideration in the case of power cycling. Figure 3 illustrates the situation in which VDD goes to 0V.
When the power is cycled, VDD drops to 0V. This means that the cathode of the diode has been grounded to 0V and current is free to flow from the signal to ground.
If the Aardvark I2C/SPI Host Adapter is driving the signal pin, this scenario represents a short circuit condition. The Aardvark adapter and driver will become unstable, necessitating a reset or power-cycle of the Aardvark adapter.
Proper Power Cycling Procedure
The best way to ensure proper operation is to disconnect the Aardvark adapter from the target system before power cycling either the target device or the Aardvark adapter.