3.5.5.10 Dual-FET circuit
For low-speed signals, two transistors and two resistors can be used for blocking backfeeding. The
first transistor is inverting the signal, while the second one makes it an open-drain type. Make sure
the pull-up resistors are on the input IO rail. Instead of using two transistors, it might be possible
to invert the signal in software and use only a single FET.
Figure 83: Dual-FET circuit
3.5.5.11 Tristate Buffer
Tristate buffers that feature an output enable control signal can be a solution. For example, RS232
transceivers often feature an output enable signal. The biggest challenge with this solution is to
control the output enable signal. Often you cannot directly use the IO rail or the PWR_1V8_MOCI
as an output enable signal. If there is backfeeding to the power rail used as the output enable
signal, the buffers might never turn off, and therefore the backfeeding remains. Likely another
circuit is required for generating a proper output enable signal.
Figure 84: Tristate buffer
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