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EMF pickup from amplifier in I2C line causing glitches. (Now with scope trace!)
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2N3055:
I understand...

If you just have to make it work, you should try active pullup.. It will clean up transition edges, and actually will clean up some of interference..

I also agree, what you see is probably conducted interference.. So you should play with grounding things differently...

And yeah, datasheet said it was OK...  |O I heard that one before.... Believed it too..  :palm:
As I said, that's a 20W AM transmitter... It will spray all kinds of nastiness.. And  it is doing it without any audio input, it's a PWM scheme.. It might be a bit more efficient that older chips, but it does it by keeping switching pulses short when idle... Meaning higher frequency spectra and better coupling to everything around..

But as I said, I would be suspicious about noise into ground injection, so play with the grounding setup...
Take care!
Monkeh:

--- Quote from: Starlord on July 03, 2016, 10:55:54 am ---Well, I followed TI's reference design.  They said the filter part was optional, so I took them at their word.  :/

--- End quote ---

It may well be optional.. if you're not attaching a fricking antenna.

A datasheet is not a step-by-step tutorial on building a device.
Someone:

--- Quote from: Starlord on July 03, 2016, 10:55:54 am ---I spent a year on this thing, so I've got no choice but to make it work with minimal changes.
--- End quote ---
Why does it have to work?


--- Quote from: Starlord on July 03, 2016, 10:55:54 am ---Well, I followed TI's reference design.  They said the filter part was optional, so I took them at their word.  :/
--- End quote ---
It probably works wonders if you couple it directly to a speaker without filtering.
Fungus:

--- Quote from: Starlord on July 03, 2016, 10:38:45 am ---
--- Quote from: Fungus on July 03, 2016, 09:51:07 am ---It would severely limit the data rate. Is that a problem?

--- End quote ---

Considering I2C is already super slow and I need to update something like 32-64 bytes worth of data 60 times a second for smooth animation?  Probably.

--- End quote ---

64*60*10bits=38400bits.

Back-of-the envelope math suggests you can go a lot lower than 400kbps.

Try the tinfoil wire shield first. Its quick to try it, it might be enough. If it works then you can look for something prettier.

Starlord:

--- Quote from: Someone on July 03, 2016, 11:21:57 am ---
--- Quote from: Starlord on July 03, 2016, 10:55:54 am ---I spent a year on this thing, so I've got no choice but to make it work with minimal changes.
--- End quote ---
Why does it have to work?
--- End quote ---

Because it's a product that I spent the better part of a year working on and have invested thousands of dollars into. I bet everything on it - it's a revision of an earlier design that had its own share of completely different issues but sold well enough to keep me in business, barely - and I've run out of time and money. 

I couldn't even afford to buy this scope, but last week I was in a panic when everything stopped working as soon as I moved it from the workbench, where it had been working fine for the last three months, into the final enclosure.  I spent a day running 50 different tests, mostly code changes since there wasn't much I could think to test electrically without a scope, and when I ran out of ideas for tests I ordered the scope, and the next morning I realized there were a few electrical tests I could try, like different speakers, isolating the speakers from the metal portion of the chassis, and shorter speaker wires.  It was then that I found disconnecting the speakers resolved the issue, and from there I figured out that the speakers I had on the bench worked, and then I discovered those only worked because I hadn't cut their leads short. 

Anyway, it's not like I chose I2C without being certain I could make it work over long runs.  I did my research, and if what should work in theory didn't work in practice, I could always use one of these chips:
http://www.nxp.com/documents/data_sheet/PCA9615.pdf

Of course I hadn't counted on this issue only rearing its ugly head in the 11th hour.  Probably should have invested in the scope sooner, but I was working with an extremely limited budget.
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