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Comment by neltnerb | Hacker News Reader

I don't personally usually bother, but an EE approach would be to put a diode between the 3.3V supply and the MCU/flash power supply pins with a large bulk capacitor, and use the voltage just before the diode as your brownout detection. It can't discharge back into the power supply or other parts on the board because of the diode so the capacitor just needs to store enough charge to run for a specified time after power loss.

Then you can configure how long the MCU runs after power shuts down. Put 470uF there and I imagine you'd have your milliseconds easily.

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Another reply mentioned that many MCUs don't like it when power is applied too slowly. Here is a version I just made as an example that uses two MOSFETs and a resistor divider to delay application of power to the MCU and flash memory. It relies on the N-channel MOSFET turn on voltage along with the resistor divider to set the turn on voltage, and when it reaches the threshold the P-channel MOSFET turns on.

Note the resistor R4 provides hysteresis -- after the P-channel MOSFET activates it acts to keep the N-channel MOSFET active unless the voltage drops below some level set by R2, R3, R4, and the turn on voltage of Q1. You just need to tune them appropriate for your components, and granted of course it adds some amount a bit to your BOM cost. But if you need it you need it.

So basically, R2/R3/Q1 set your turn on voltage (experimentally determined or maybe with Spice) and R4 sets your turn off voltage with R2/R3/Q1. The MCU probably uses under 500mA so these parts are pretty cheap... the capacitor is about 5 cents (https://www.digikey.com/product-detail/en/kemet/ESK477M6R3AE...) and the MOSFETs that looks appropriate are in the 6 cent each range.

https://i.postimg.cc/Df9N8fM5/delayed-turn-on-circuit.png

Most mcus are fine with power being applied slowly as long as they have a separate reset signal. It's good practice to have a separate reset signal anyway, to avoid unintended behaviour while the circuit is powering up.

You are still asking for trouble. Once something goes into latchup, reset is unlikely to recover. If not the MCU, then some peripheral on the same supply may express it's dislike for intermediate voltages.

A compact and reliable 470 uF capacitor could cost comparable to to a low-end uC.

The low end uC's use so little power that a 1uF capacitor would be plenty for a few milliseconds.

I mean, you get what you pay for. If your are cost-optimising that hard, you probably aren't controlling something with critical data.

If you aren't cost-optimizing that hard, you aren't shipping meaningful volumes yet. 470uF isn't cheap at 10k+ units.

My last project couldn't use a giant cap because of size, not cost constraints.

A lot of time is way easier and more reliable to have a secondary battery (Li coin cell) for this kind of purpose.

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