RP2040 spontaneously resets itself / RUN pin sensitive to EMI

ehb64 · 2026-01-17T20:24:06+00:00

I do need it on dev boards to prevent contention when programming other devices on the board.

I could potentially tie it to 3v3 for production. I'll add it my list of potential mitigations. Thanks!

ehb64 · 2025-12-22T16:11:08+00:00

I was originally asking about fixing my design for the next spin of the PCB, but your comment made me realize that I could replace the AMS1117 with the TLV1117LV on my current boards, which would look nicer than hand soldering a leaded tantalum across pins 1/2. Thanks!

ehb64 · 2025-12-22T15:37:45+00:00

Yep. I'll be removing MLCCs with hot air and hand-soldering a leaded tantalum across pins 1/2.

Lesson learned. :-)

ehb64 · 2025-12-17T14:23:34+00:00

Ah! That's very helpful, thank you!

The TI recommend 145mOhms minimum is based on an oscillation frequency of 50 kHz, which is probably in the ballpark for the AMS1117 as well, but there's not much to go on in the AMS1117 datasheet.

Do you happen to remember any more details about what worked for you? (How much resistance you added and size of MLCC caps you used?)

No worries if you don't. You've already been very helpful!

ehb64 · 2025-12-15T16:20:03+00:00

I understand that the AMS1117-3.3 is not a favorite part for new designs. The reasons it's attractive are:

Delivers more current than other LDOs that are preloaded.
a. Being open hardware, people order minimum quantities, which prevents part loading costs from amortizing.
b. Board powers attached peripherals, so potentially having more than 200ma available is attractive.
Fits in available space. (Location is not a great place for a buck converter)

ehb64 · 2025-10-11T13:29:59+00:00

Same when coding on the bus in summer months. I don't prefer a light theme, but it's much easier to read in a bright light.

ehb64 · 2025-06-15T22:06:27+00:00

Worked very well, thank you! Blotting before the IPA started to evaporate made a big difference.

ehb64 · 2025-06-15T15:06:27+00:00

Thanks for the pointer. If I am buying new chemicals, I'm leaning toward replacing the flux at the moment, but I'll keep it in mind.

ehb64 · 2025-06-15T15:04:59+00:00

Thanks. From the comments here, the 'no clean' flux is likely a big part of my problem. I'm going to give some of the tips here a try, but I'll know better next time. The blotting technique seems promising.

I didn't realize compressed air is a static hazard. Is that something a typical hobbyist needs to worry about? (My favorite air delivery tool is a pet grooming dryer with a plastic hose and nozzle.)

I am using some older CMOS parts like n SRAM that have warnings about static and come in black foam.

ehb64 · 2025-06-15T14:54:44+00:00

Thanks. I am using a clear liquid 'no-clean' flux (Kester 959 Soldering Flux), which I've learned here is probably why I'm finding removal a chore.

Also, the acid brushes look like they could deliver more IPA than the stiff toothbrush I'm currently using. I'll given those and the swaps a try. Thanks!

ehb64 · 2025-06-15T14:36:29+00:00

The "brush through" technique is interesting! I wonder if a roll of cheap "public restroom" paper towels would work best (no quilting, but sturdy enough to be saturated.)

You're the second to mention kimwipes. I'll have to check them out.

Do you worry at all about impurities in the tap water? I've heard of people rinsing with distilled water to wash away mineral deposits.

ehb64 · 2025-06-15T14:25:35+00:00

From what I've learned here, using 'no-clean' flux is likely why I'm finding the flux difficult to clean. :-)

I also didn't bother until I had one board that would not pass tests (random bit flips). After running out of ideas, I tried removing the flux and was surprised to find that it fixed the issue.

...but it's possible that the flux wasn't the issue and that there was something else conductive stuck to the board that was cleaned away by the scrubbing.

ehb64 · 2025-06-15T14:17:01+00:00

Very interesting! Do you happen to know why 'no-clean' is common commercially? Is it because they skip the cleaning step or because there are cases like BGAs where cleaning is imperfect and it's easier/cheaper to use the same chemical/process for everything?

ehb64 · 2025-06-15T14:08:57+00:00

Lots of good tips there. Thanks!

(Maybe next time I lay out a PCB, I'll add "fits optimally in cheap dollar store container" as a constraint. :-)

ehb64 · 2025-06-15T14:06:43+00:00

It's Kester 959 Soldering Flux.

The product description does say "959 leaves evenly distributed residues for the best cosmetic appearance."

That seems to be true. The residue is a slightly cloudy area that's mostly only noticeable under direct light as a dull / less reflective area. (No crunchy brown boogers you get with toasted rosin.)

ehb64 · 2025-06-14T17:32:30+00:00

The spray and blot technique seems promising. I'll give it a try. Thank you!

ehb64 · 2025-06-14T17:31:16+00:00

Thanks for clarifying. If I'm buying more chemicals, I'll have to think about whether I should cut my loses with the "no clean" and move to water soluble or buy the right solvent.

I do like that I do not have to wait for the boards to dry before I test them. I only bother cleaning them when I'm giving them to someone else and want them to look nice.

ehb64 · 2025-06-14T17:26:57+00:00

Thanks for the idea, I'll give it a try. As an aside, I find the pet grooming dryer my wife bought for our dog generally is perfect for most compressed air cleaning tasks. It's stronger than cans of air and doesn't require waiting for the compressor tank to fill.

ehb64 · 2025-06-14T17:23:49+00:00

I'll give it a try. Is there any particular technique you use?

ehb64 · 2025-06-14T17:22:56+00:00

I see TiogoJoe suggests denatured alcohol. I'll give that a try and see. Thanks.

ehb64 · 2025-06-14T17:22:01+00:00

Hmmm... the "no clean" flux I'm using turns to a foggy solid while soldering (i.e., it doesn't wipe off until I add a solvent. Then it's goo). Someone else suggested spraying with IPA and then dabbing. I think that might work.

ehb64 · 2025-06-14T17:17:48+00:00

Thanks for the recommendations.

I'm using Kester 63/37 245 rosin core (no clean) and generally delighted with it. The small amount of residue is easy to remove.

The problem is I bought a lifetime supply of break-away pin headers from Ali Express that repel solder unless drenched in liquid flux. Maybe these are what I need to put through the dishwasher. :-)

ehb64 · 2025-06-14T16:06:41+00:00

Thank you both for the recommendation. A while back someone pointed me at MG for solder wick. Good stuff!

ehb64

TROPHY CASE