3v pic outputting 4v, or not.....

[cloudscapes]

Hey all,

I feel a little stupid. I bet this is one of those 'trap for young players' that I just can't figure out. I'm not an EE mind you, just hobbyist. I'm missing a lot of theory that might no doubt explain this.

I have a PIC32 powered by a pickit3 (3.2v). Probes say it is indeed properly powered at around 3v, and I can run code on it no problem. If I hold a pin high, 3v, as expected. Both my cheap scope and multimeter probe 3v. However, if I toggle said pin, or generate any kind of clock via SPI or PWM, a little over 4v peak to peak. Huh?



That's with bandwidth limited to 20MHz. Turning off bandwidth limit and it's nearly 5v. Now, probing the clock, my multimeter reports around 1.45v, which suggests a 3v clock with 50% duty. That's what it SHOULD be, but my scope says 4v or more.

Is it my cheap scope/probes? The hantek MSO isn't the best scope, for sure, but it's not owon cheap at least, or those pocket scopes. I wouldn't expect hantek to make that huge a mistake. DC or AC coupled, same thing. There's no extra hardware. This is straight from the pin.

Is it the scope? Is it some normal electrical behavior seen in clock signals that I'm not aware of? Some kind of weird ground trick? Will I damage 3v parts hooked up to this clock?

I'm sure the answers obvious. 

[Andy Watson]

Have you trimmed your probe compensation to match the scope?

Edit: Good video/description here:
https://www.eevblog.com/forum/beginners/often-requested-video-the-importance-of-10x-scope-probe-compensation/msg708498/#msg708498

[Ian.M]

+1
The most likely cause is the probes have the input compensation capacitor misadjusted.  The give-away is that the top and bottom of the squarewave sag after the leading edge.

[cloudscapes]

+1
The most likely cause is the probes have the input compensation capacitor misadjusted.  The give-away is that the top and bottom of the squarewave sag after the leading edge.

Have you trimmed your probe compensation to match the scope?

I've never touched those, and pretty much forgot about that. Again, lacking a lot of knowledge.

Did the trick, thanks! I figured it was something obvious.