Subj : Re: RP2040 reset idea
To   : john larkin
From : Arie de Muijnck
Date : Tue Sep 17 2024 10:49:58

XPost: sci.electronics.design

On 2024-09-17 04:58, john larkin wrote:
> The RP2040 has a cool mode where it can be made to boot up with the
> USB port looking like a memory stick. That's great for software
> installs or upgrades. The Pico board has a BOOT button on the board;
> if the pico is powered up with the button pushed, it goes into that
> boot mode.
>
> But if I have a product (actually a family of products) in a nice
> aluminum box, a user would have to remove the top cover, remove all
> power sources (there can be three), and hold the button down while
> reconnecting power.
>
> Some of my customers also want to lock a box such that it's impossible
> to write to any nonvolatile memory while it's in a secure area.
>
> So here's an idea: a small hole in the box allows a toothpick or a
> paper clip to push a button. A short push is a regular reset. A long
> push is a memory-stick mode boot. We can have a rotary switch LOCK
> hole too.
>
> https://www.dropbox.com/scl/fi/bbhdy053xsdzv77g0brc5/B-box_Reset_Button_3.jpg?rlkey=40ozzdvczf4z8625u8i081a9b&raw=1
>
> Does that look like it will work? The paranoid customer can pave over
> the two holes, and even the end-plate screws, with an official-looking
> sticker.
>
> Schmitt trigger U3 is unfortunate and likely unnecessary, but it
> insures against a low-probability hazard.
>

I used the same idea, but with less hardware. The switch triggered (RC differentiator) the reset pulse and the MCU senses the state of the switch after the reset pulse trailing edge. I used the bootloader code for that timing. The RP2040 could sense the 
switch state on the BOOT- pin after a long (5 second?) reset pulse, but that would mean a slow start on each power on.

I think your idea would work OK, just a bit complex. Just NEVER short a cap with a switch (or relay) without a series resistor.

Arie

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