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Question about oscilloscope output.
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Question about oscilloscope output.

by analogpixel on Sun Jul 28, 2019 2:09 pm

I was trying to get an attiny85 to work with a neopixel ring, and nothing was working (although I was able to push code to the chip just fine) , and then I finally swapped out the chip, and it started working (so bad chip.) I took a reading with both the bad chip and the good chip on the oscilloscope, and on the bad chip I had "blocky" waves, (btw I'm not very experienced with the oscilloscope) I was just wondering what the "blocky" waves coming out of the chip would be indicating?

This is the bad waveform:
bad2.png
bad2.png (7.66 KiB) Viewed 95 times



this is the good waveform:
good.png
good.png (8.7 KiB) Viewed 95 times


thanks,
matt

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Re: Question about oscilloscope output.

by adafruit_support_mike on Sun Jul 28, 2019 2:19 pm

Neither of those look like the control signals for a NeoPixel.

Could you post a photo showing your hardware and the oscilloscope probe connections please? 800x600 images usually work best.

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Re: Question about oscilloscope output.

by analogpixel on Sun Jul 28, 2019 2:31 pm

The scope is hooked to the ground of the tiny, and the pin used for the signal (pin 0).

Screen Shot 2019-07-28 at 12.25.30 PM.png
Screen Shot 2019-07-28 at 12.25.30 PM.png (902.54 KiB) Viewed 86 times

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Re: Question about oscilloscope output.

by analogpixel on Sun Jul 28, 2019 6:38 pm

I was fiddling with the scope, and got better output between the good and bad, maybe this is the scope output you were expecting to see:

good:
good.jpg
good.jpg (33.85 KiB) Viewed 70 times


bad:
bad.jpg
bad.jpg (29.61 KiB) Viewed 70 times


looks like the bad chip just wasn't triggering high enough.

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Re: Question about oscilloscope output.

by adafruit_support_mike on Mon Jul 29, 2019 2:08 pm

That’s more like what I’d expect, yes.

The noise in the previous traces was probably the result of triggering at or near 0V, with the timebase cranked down to the nanosecond range.

The first of the newer traces shows some ringing (decaying high/low oscillation) on the rising and falling edges, which will mostly be due to the jumpers shown in the connection photo. All wire has some parasitic inductance and capacitance, and fast-changing signals will echo in the 20MHz to 50MHz range until the energy is burned off as heat by the parasitic resistance. You can trim that down by putting a resistor in series with the probe that reads the signal.

Start around 100 Ohms and adjust up or down as necessary. Too little resistance will leave some ringing, too much will round off the rising and falling edges.

The second set of new traces shows voltage loss and some rounding on the rising and falling edges, which is characteristic of a blown input pin. Most CMOS inputs are the gate of a mosfet, which is basically one side of a small capacitor. The voltage on the input pulls electrons close to the other side of the gate insulation, creating a conductive path.

The gate insulation is a layer of glass a few atoms thick, so excess voltage can arc through it.. for 5V CMOS, that only takes about 7.5V.

A hole in the gate puts the input pin in physical contact with the silicon under the insulation, which behaves like a resistor with some capacitance on the far side. You get voltage attenuation of the input signal, and the capacitance limits how quickly the voltage can change, producing rounded rising and falling edges.

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Please be positive and constructive with your questions and comments.