Re: RP2040 reset idea

On Tue, 17 Sep 2024 10:49:58 +0200, Arie de Muijnck <noreply@ademu.nl>
wrote:

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.

My circuit doesn't delay a normal powerup. That's dominated by the
roughly 200 msec reset generated by the MAX809 after the 3.3 supply
stabilizes.

>
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.

I don't think the 10u cap will weld the contacts of the pushbutton.

>
Arie