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From: john larkin <JL@gct.com>
Newsgroups: comp.sys.raspberry-pi,sci.electronics.design
Subject: Re: RP2040 reset idea
Date: Tue, 17 Sep 2024 07:20:54 -0700
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On Tue, 17 Sep 2024 11:07:17 +0100, The Natural Philosopher
<tnp@invalid.invalid> wrote:

>On 17/09/2024 03:58, 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.
>> 
>Thats' what my PICO powered thermostats have. For the onboard button though
>Resetting is done by pulling the power.
>
>The point is that if you do power up with the button pressed, you wipe 
>the entire FLASH RAM I think.

If BOOT- is low at the end of reset, it powers up in USB memory stick
mode. I don't think that wipes the files stored in the big serial
flash.


>
>
>> 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.
>At a brief glance, yes, but I donbt see why a simmle disconnect power 
>switch in addition to the onboard switch wouldn't be as easy.

There are three potential power sources to disconnect, and I don't
really want a power switch.

>> 
>> Schmitt trigger U3 is unfortunate and likely unnecessary, but it
>> insures against a low-probability hazard.
>> 
>Once you start laying out a board, put everything on it you may need, 
>Can always leave it out later

The schmitt adds way under a tenth of a per cent to the board area and
the cost, so may as well leave it in.