Electronic – Tri State/Floating Signal Detection

It sounds like the internal pull-up resistor isn't working. It's possible the code has accumulated a bug, or maybe the pull-up has got damaged.

Try an external pull-up resistor.

The Discovery board button schematic shows:

• R22 is 0Ω, and so isn't doing anything
• R21 is a pull-down, instead of a pull-up, so it is necessary
• C22 capacitor as "N/A", which on ST's schematics often means the part isn't fitted.
• R20 - if C22 isn't fitted, then it does nothing.

So, one resistor + switch is equivalent.

Edit:
Noise, triggering the other, unused pins which have no defined state might be an explanation for relatively frequent and random triggering.

Looking at your picture of the project at github, I see that there is main power nearby. Noise may come from that path.

In general it is a good practice to ensure all unused pins are in a known state anyway. Pins might otherwise trigger, or even go into a 'mid-state' where one, or even both of the complementary MOSFET transistors conduct in the 'analogue' region, causing excessive heat.

You don't say what the main-loads are, but they may make ensuring pin stability, by excluding noise, even harder.

Set unused pins, which are not already wired to ground or power, to a known state using the internal pull-down or pull-up resistors. Leave them as inputs. Be careful to not do this for pins already tied to signals, ground or power.

You might start with any unused pins which trigger the same interrupt as you are using.

(You might also consider handling different pin-interrupts just to see if it is affecting more of the MCU. For example you might set an LED if any of the unused pins trigger. This isn't a fix, but is only to help an investigation or analysis.)

Try to ensure this deliberate pin-setting is highlighted/documented so that future changes to the system will not cause any pin to get into conflict with these pull-up/down settings.

Also ...
It might also be worth a test where the solenoid-board, and mains power, is completely removed from the vicinity of the Discovery board, on the small chance that noise is coming via that board. For example, there may be a ground loop.

You have many options.

1. If you need to connect very few optocouplers, you can connect them directly to the GPIO of your microcontroller (through a resistor), provided that:

   • You do not exceed the GPIO output current.
   • You do not exceed the total port current.
   • You do not exceed the total gnd/vdd current.

2. If you need to connect more optocouplers, you can try to use low-current, high current transfer ratio optocouplers such as SFH618 (https://www.vishay.com/docs/83673/sfh618a.pdf), and connect them directly to your GPIOs (through a resistor).

3. Or, you can use a BJT or MOSFET (see schematics below). Some notes:

   • Remember to put the pull-down/pull-up resistor, which ensures that the MOSFET/BJT are OFF when the GPIO is not initialized yet (e.g. during reset).
   • Pull-up or down resistor might be omitted if your MCU has GPIO pin with pull-up/down always enabled during reset.
   • If using MOSFETs, remember to use logic level MOSFETs (e.g. BSS138).
   • If you use the active-low solution, make sure that the high level voltage of the GPIO is VDD. I.e. do not use a 3.3V-GPIO and VDD = 5V in the active low solution!.

4. Still, if you need to drive many optocouplers (e.g. 6) you can use the 74LS07 you mentioned, as it allows a 40mA per pin, and you'll have to mount only one component (instead of 6 BJTs/MOSFETs). Remember that, unlike CMOS, TTL ICs are instrinsically pulled-up! However, you might still want the pull-up resistor (the datasheet also recommends not to leave inputs floating). And, since '07 is not inverting, this solution will be active LOW. The 74ABT126 is CMOS so you MUST use anyway the pull-up resistor!

^{simulate this circuit – Schematic created using CircuitLab}