Trying to see clock signal on an oscilliscope

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@OP

What scope do you have? 100MHz scope's are pretty decent and is beyond the "cheap" stuff.

Something like a Rigol / Hantek / Uni-T ?

"cheap" scopes nowaday's cost USD20 or so ( jyetech dso138 ) and those are toys.

If they would have taken the effort to write bit better software for it then it would have been useful however for signals upto 200kHz or so.

Unfortunately they completely bodged the triggering on that thing.

 

The AVR's have a crystal oscillator, which runs (if configured correctly) at the same frequency as the crystal.

Cristal should have reasonably sine wave on it's legs. (But the probe capacitance changed the frequency a bit while measuring).

Amplitude is between a few 100mVpp an 4Vpp for the "full swing" configuration of mostly older AVR's.

In the AVR itself this sine is "boosted" / amplified to a square wave, and very likely also phase shifted to generate delay's for internal timing, but those signals can only be found on the silicon itself.

 

There is a "fuse" in the avr which can divide the clock frequency by 8 (Or is that fuse only for the RC oscillator).

So you AVR runs at the Crystall frequency or at 1/8 of that.

 

Yow Neil:

your lines are squiggly.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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On 'scope probe grounding:  (Just to follow clawson's take on making the subject wander)

 

Yeah, weird sinusoids where you expect a square wave is typically lousy grounding.  Whenever I get a signal on my 'scope that doesn't look like it should, the first place I look is to the grounding.

 

When I lay out a circuit board I like to include two thru-holes at 0.1" (w/pads), connected to ground, in an otherwise underused corner of the PCB, and solder in a little arch of leftover resistor lead.  Why two and the arch?  I found that one would let the grounding clip spin wildly and the lead itself would bend dramatically, both coming far too close to other circuits for my comfort.

 

S.

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dbrion0606 wrote:
Your 100 Mhz samples, a priori, at a 10 ns rate

A scope is generally specified by its bandwidth, not sample rate (although digging into the spec should reveal the sample rate).

 

For example, a Rigol DS1052 50MHz 2 channel scope samples at 500MSa/s when using both channels.

David (aka frog_jr)

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Analog oscilloscopes don't have a sample rate.  S.

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frog_jr wrote:
A scope is generally specified by its bandwidth, not sample rate

For DSOs, both are key parameters.

 

BW is traditionally the "headline" number, but the sample rate should be right up there in the key specs

 

 

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DocJC wrote:
Plus who knows how many gates the USART's final data transmission module's signals have to go through, perhaps many. The Clock Out signal likely has relatively few gates to go through from the original master clock signal to the I/O pin.

 

Thank you for explaining that.

 

 

Thank you everyone for trying to help me on this. I will step back and just view the clear square waves I get from pin 3. Thats good enough for now for my exploring.

 

Thank you all again :)

 

 

PS: this has not been a 'waste'  Although I do not have exactly what i wanted to see, I have learned alot from all the comments in this thread. On the whole a win for me :-)

 

I am a new AVR programmer. I am learning alone out of books, the Internet, etc. Please excuse me if I ask simple questions. Thanks.

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