3.5.5.5 Open Drain Signals
When using open-drain signals, it is crucial to use the right voltage domain for the pull-up resistor.
In Figure 67, the peripheral rail is used, and therefore backfeeding occurs. By using the same
voltage domain for the pull-up resistor as the input side, backfeeding is eliminated. When using a
computer module, using the IO rail might not be feasible. However, the Verdin module features
the PWR_1V8_MOCI voltage output, which is in the same power domain as the IO rail. Therefore,
it is recommended to use this PWR_1V8_MOCI for open-drain pull-up resistors.
Figure 77: Correct pull-up rail for open-drain signals
3.5.5.6 Simple FET Circuit for Open-Drain Signals
Sometimes, it is impossible to move the pull-up resistor to the IO rail domain since there is already
a pull-up resistor on the peripheral rail. Maybe this pull-up resistor is inside the peripheral device
and cannot be switched off. The simple FET circuit in Figure 78 can offer a solution for blocking
backfeeding. This circuit also works for bidirectional open-drain signals such as I
2
C and can be
used as a low-cost level shifter.
Figure 78: Simple FET circuit for open-drain signals
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