Look at this:



Is this an inherent characteristic of the device and we need to use a capacitor to "filter" it out or what? It is just a 50% duty cycle, timer (interrupt) driven GPIO pin.

 

Is this an inherent characteristic of the device and we need to use a capacitor to "filter" it out or what? It is just a 50% duty cycle, timer drive GPIO pin.

No. Your oscilloscope is showing about 50ns ringing.
Make sure that you use a x10 attenuation probe AND the probe grounding clip is connected to circuit GND.

 

No. Your oscilloscope is showing about 50ns ringing.
Make sure that you use a x10 attenuation probe AND the probe grounding clip is connected to circuit GND.

Ahh, thanks. I have one of these that's attached to the board's ground and the GPIO pin:



Here's the details.

 

that seem to be just a BNC cable with clips - without compensation.
your scope must have square wave test point. do you see same ringing when this is connected to it?

check Figure 5 that shows what normal probe looks like. the trimmer capacitor is used to adjust this:
https://www.allaboutcircuits.com/technical-articles/an-introduction-to-oscilloscope-probes/

 

That is a simple BNC cable with no attenuation and no proper cable termination.

x1/x10 attenuation probe has a switch on the body of the probe to select x1 or x10 attenuation.
I always have my probes set to x10 attenuation. Before using the probe you need to adjust the trim capacitor for flat frequency response.

 

I just had a nightmare with the USB drives, but never mind.

Here's the scopes built in 1 KHz test signal, using a Siglent 200 MHz probe set to 10X and the scope input channel set to 10X.

 

This happens when there is a severe impedance mismatch. A typical 10x scope probe has 10 MegΩ with 15 pF to Ground. the idea is to avoid disturbing the circuit under test.

 

A typical 10x scope probe has 1 MegΩ with 15 pF to Ground.

You mean 10 Megohm.

 

OK I adjusted the probe using the supplied trimming tool and got the test waveform square.

I then re-attached the probe to the board, leaving everything set at 10X, here's the waveform and the original waveform for comparison:

 

I have a question. What is the waveform source and what impedance is the source designed to work into.

Ron

 

I then re-attached the probe to the board, leaving everything set at 10X

Where did you connect the probe's ground lead?
It should be on the circuit ground close to the source of that waveform.

 

The scope has a 20MHz BW limit too, enabling that:

 

Where did you connect the probe's ground lead?
It should be on the circuit ground close to the source of that waveform.

The probe ground is connected to one of several ground pins on the board, almost next to the signal output pin.

 

You mean 10 Megohm.

Indeed

 

Finally there was a 8 inch piece of wire between the probe ground crocodile clip and the board ground pin, I rearranged that and made it 1 inch (because the crocodile clip shorts against other pins):



I think I underestimated the impact of poor connection leads between device and scope, that seems to be the upshot of all this.

 

I have a question. What is the waveform source and what impedance is the source designed to work into.

Ron

It's one of the GPIO pins on the board, this is the board. It's GPIO 11 on port D.

 

I did one more change and altered the GPIO pins slew rate from HIGH to LOW (there are four speed constants)



Here's the waveform now:


Once upon a time I understood this stuff pretty well, but its been many years.

 

Finally there was a 8 inch piece of wire between the probe ground crocodile clip and the board ground pin, I rearranged that and made it 1 inch (because the crocodile clip shorts against other pins):

I think I underestimated the impact of poor connection leads between device and scope, that seems to be the upshot of all this.

8 inches is way too long.
If you want to do high frequency probing, here is what we use. That is less than 1 cm from the probe GND to the circuit GND.

 

8 inches is way too long.
If you want to do high frequency probing, here is what we use. That is less than 1 cm from the probe GND to the circuit GND.

View attachment 314403

That makes sense, but in this case the generated signal is just 27KHz, surely that's not what you'd call "high frequency"?

 

That makes sense, but in this case the generated signal is just 27KHz, surely that's not what you'd call "high frequency"?

No.
But it's not the fundamental repetition frequency that's the problem, it's the rise and fall times that have much higher Fourier frequencies which you must accurately resolve.

For a more-or-less linear rise and fall, the approximate equivalent highest frequency component is 0.35/τ where τ is the 10%-90% rise or fall time of the pulse.
From post #9, It looks like you original pulse fall time is <10ns, giving frequency components above 35MHz.