[gary_0]

I designed the electronics for a heavy-duty industrial 3D printer and used a 555 in the failsafe circuit (alongside the manual e-stop). If it didn't get reset by a heartbeat from the embedded computer/software, it would unpower the heaters and actuators.

[buescher]

That's the only use for one that isn't (always) a design smell - it makes a really nice missing pulse detector, better than you can easily do with comparators. But if you have the budget, a purpose-made watchdog chip or a tiny microcontroller really can make a better watchdog.

[gary_0]

I would absolutely not want to use a microcontroller or a complicated chip for something like that. Simplicity is the point.

[buescher]

Supervisor chips are not complicated. In some ways simpler than a homebrew analog watchdog, and the good ones will handle failure modes a simple watchdog won't, like those that result in an oscillating output.

[gary_0]

Yes, a simple purpose-made chip designed to be used in safety-critical situations, with high tolerances for voltage etc, would probably be better. Although one thing the 555 design has going for it is that a seasoned EE could take one look at the physical circuit and know exactly what it does.

But I would never trust anything that ran software for something like this.

[buescher]

It depends on the system's potential failure modes and what's required by your safety standard, not on one engineer's opinion of what's "best".

[15155]

Modern 32-bit microcontrollers are cheaper than 555s.

[fmlpp]

Yes, but uses software, so you have another level of added complexity that may be or may be not desirable.

[gary_0]

The chips themselves add a bunch of new failure states to consider beyond software bugs, too. Maybe a bad wire or component puts too much load on the microcontroller's wee internal pin drivers and they melt into a permanent "on" state. Or a voltage fluctuation browns out the chip on boot, partially randomizing its RAM or registers. Or the chip manufacturer fixes some errata or discontinues a particular part number and now a pin you've left floating has become a hardware heisenbug. Or the wrong bit flips in your EEPROM after being in a hot machine for a few years. Suddenly a boring 555 looks pretty good. (Keep in mind, we're talking about "turn off heater after pulses stop", not "abort launch sequence if tank 3 pressure low". The latter is way above my pay grade.)

[15155]

For every task you could also use a 555 timer for (with dedicated analog support complexity,) you are talking about tens of lines of user code at most.

Even if you had to do everything directly with registers, the amount of C or Rust here is minuscule.

[kjs3]

There's the guy who's never shipped and supported a product.

[buescher]

Not if you go to the cheap "Asian brands" like you're thinking with micros, plus with your cheap micro you'll need a reset controller. And budget isn't all BOM cost.

[15155]

Who provisions dedicated reset monitors on $0.06 MCUs?