tristate vs schottky

[electrolust]

This is a tangent on a different thread I started.  Sufficiently tangent and self contained as to warrant its own thread.

I'm switching an LED via a FET.  The FET can be turned on either by an MCU output or by an "external" signal.  I already put a schottky diode on the external signal line so that the MCU output doesn't do nasty things upstream to the external device.  But I didn't consider what happens to the MCU pin when the external signal is active.

The MCU I'm using is a TI MSP430.  The particular '430 doesn't have tristate output.  To remedy this, shouldn't I just put a schottky diode inline?

I used a schottky diode due to my thought that 3.3V - 0.7V = 2.6V wasn't quite where I wanted it to be for the Vgs of the FET.  But now seeing as I can choose a different FET, should I just use a regular diode instead of the schottky?

[Rerouter]

As your just switching a mosfet to actuate an led (assuming slow indicator), i would just fit a resistor that keeps the fault current from something external within what your device can handle without damage,

[bson]

You should be able to configure the MSP430 GPIO pin as an input to put it into high impedance.

[electrolust]

yeah but at some point I need to switch it to output mode, and at that time I won't know what's going on with the other signal

[tautech]

yeah but at some point I need to switch it to output mode, and at that time I won't know what's going on with the other signal

Your suggested method was widely used at logic level of yesteryear with 1N4148's to provide isolation between 2 signal sources but as the Vf for silicon diodes is ~0.7V a Schottky will be a better choice if there is little voltage headroom over a threshold you need to trigger like your FET gate. BTW Schottky Vf is ~0.3V.
I wouldn't drive a gate with a 3 state device unless the gate was being held off (high/low) with say a 1K resistor.

[IconicPCB]

A diode resistor OR gate?

[electrolust]

Ah, great. Thanks all.

[David Hess]

A diode can work fine assuming that the forward voltage drop is not a problem.  A bipolar transistor used in common base mode can be used as a "zero" voltage drop diode by driving the emitter but the reverse breakdown voltage is limited.

[bson]

yeah but at some point I need to switch it to output mode, and at that time I won't know what's going on with the other signal

But the same would happen even if the GPIO pin could be put into high-Z mode: at some point you need to drive it and to do that you need to make sure it's not fighting something else.

[Vtile]

That diode OR is interesting, especially the inverting AND gate, which could be modified to be zero drop diode with high reverse breakdown voltage. ?? Assuming that 3.3V is not only power rail.

Basicly a diode OR with diode protected transistor buffer? I need to add this to my "notebook".