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Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Thu, 13 Nov 2025 10:52:32 GMT

Hello all,

I'm toying (pun intended) with ideas in the context of a future challenge and while I'm not settled on what the thing should do, I'm decided on using the MAX32666FTHR2 board that I was gifted with a couple of years back.

The "Light Up Your Life Challenge" gave me a push to finally use the 8*8 WS2812B matrix that I once bought because it looked nice and was not expensive, and so I'm trying to hook it up to the board.

As I can't seem to get the proper output signal to send to the LEDs, I decided to observe what's coming out of the MAX32666 pins.

My first experiment was to use a continuous timer, whose clock is half the system clock (96MHz) and which toggles the output pin every time the counter reaches the comparison value. Here is what I get with two relatively high values:

The signal should be a nice square wave, it does not look that clean, but I know there's always some rise and fall time coming into play.

But things get worse when the comparison value for the timer is even lower, like here with 3:

After further fiddling with the various peripherals on the MCU, I was able to output the 96MHz clock signal, and this is what I get:

I had to stop the oscilloscope because it was not able to see a proper trigger level, let alone compute the frequency from such small signal.

Now, if I read the graph properly, there's about 10ns between two peaks, which is consistent with a 96MHz frequency for the signal.

However, what I'm really wondering about is the fact that the faster the signal, the more distorted I observe it, to the point where it won't even reach full voltage swing between 0V and 3.3V.

Out the top of my head, I can see three reasons for these results:

Reaching the limits of the MAX32666 pin switching capabilities
Reaching the limits of what the oscilloscope can see
Reaching the limits of my own stupidity

For point 1, I tried looking for rise and fall time on output pins in the electrical characteristics for the MAX32666 but could not find anything related to this in the (somewhat confusing) datasheet.

For point 2, the DSO is said to be "100MHz" so as 96 is lower than 100, it should be fine. But maybe Shannon law should be taken into account and 100/2 being lower than 96, the results are expected.

For point 3, well, it could come on top of the two other points, and so I won't rule it out just yet.

I know there is a "Oscilloscope 101" webinar in the works which might actually answer those questions just fine. If that's the case, I'd gladly wait for it.

Thanks for your attention.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

Robert Peter Oakes — Sun, 16 Nov 2025 19:43:34 GMT

one thing I did not see being mentioned is about the scope (And probes). Bandwidth of 100Mhz will be 3db down already but the reference for this is a sine wave, not a square wave.

A square wave is made up of an infinite series of odd-numbered harmonics, including the fundamental frequency. For an ideal square wave, these harmonics are all in phase, with the amplitude of each successive harmonic decreasing by an amount inversely proportional to its harmonic number

so if the square wave is 96Mhz then you will also have signals of 288Mhz, 480Mhz and that's just 3rd and 5h harmonic there are a lot more to get a good square wave but as the scope is max 100Mhz with 3db down on a sine wave then these harmonics needed to shape the wave form are long gone into oblivion, hence you seeing more and more like a sign wave (Triangle ish in your case). even at the much lower frequencies you will see a distorted square wave due to harmonics being filtered . even a 30Mhz signal will have 3rd and 5th harmonics at or beyond the scopes bandwidth 30Mhz primary, 90Mhz 3rd harmonic and 150Mhz 5th harmonic. I hope this help you understand more of what your seeing.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

jc2048 — Sun, 16 Nov 2025 11:28:45 GMT

I quite often improvise 'springy clips', like this

That way, you can solder them, rather than hold the probe with one hand whilst working the controls with the other, though in this case it's quite precarious the way that I just have the tip touching the blob of solder (don't do that in the middle of a valuable board that you don't want to damage). The component between the blobs of solder is the SMD load resistor that I'm trying to probe as cleanly as possible. Sometimes, if I can be bothered, I'll do a little spiral for the probe tip to sit in which can also be soldered in place.

My improvised ground clip was a lead offcut from a component (some old, cheap, but actually very good LEDs) and is plated steel rather than copper - copper is too soft and won't hold it's shape gripping the ground sleeve of the probe.

If you want to see what I was doing with it, it's in the blog below; re-reading that, it's a right mess - I haven't explained what I was doing at all well. And I was wrong about the risetime. That was done with an old 200MHz Tek 2-series scope and the x10 passive probe that came with the scope, so the risetime you see in the waveforms is actually that of the scope and not the pin drivers on the Arduino Uno, nor the output of the transformer, which are both probably beating the scope, but it does show the clean results, without ringing, you can get in a low impedance circuit where the probe isn't loading it too much, and it does show how good the performance of a transmission-line pulse transformer can be, so not entirely wasted as a blog.

https://community.element14.com/members-area/b/blog/posts/experimenting-with-a-transmission-line-transformer

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Sat, 15 Nov 2025 15:40:13 GMT

Ah!
So that's what this spring is about!
I thought it was a spare for the spring inside the hook...

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

dang74 — Sat, 15 Nov 2025 11:56:39 GMT

Agreed... and the shape change to me is more profound than the 3dB drop. You first notice it in the edges... rather than a nice clean edge they become sine like. And although mathematically they have nothing to do with triangles, to me, in terms of appearance, they do look like imperfect triangular waves... and ultimately when you get really close to the limit, they become miniscule sinewaves. If the signal being probed was a pure sine wave I suppose the amplitude would be 70.7% at the bandwidth limit... but since, as you pointed out, that a square wave is composed of even higher frequency components, the amplitude will be much less than 70.7%.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

Jan Cumps — Sat, 15 Nov 2025 09:11:05 GMT

The oscilloscope has a 100 Mhz bandwidth. If you probe a 100 MHz square wave signal with it, you 'll notice a few things - even with perfect probing techniques

The signal root wave (a 96 MHz sinus) level will be 3 dB down
The rest of the square wave is made up of frequencies above the scope's upper bandwidth level. You will not get those.

So in general: if you use the x10 mode, you'll get an attenuated sinus-ish result at best.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

Gough Lui — Fri, 14 Nov 2025 22:31:17 GMT

Yeah. That's a bad idea. Probe loading is hefty at 1x. You should be using 10:1 for anything above around 8MHz in my experience.

- Gough

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

Gough Lui — Fri, 14 Nov 2025 22:30:14 GMT

Most oscilloscope probe bags have accessories including the spring ground - it should be fitted by removing the ground clip lead, removing the "hook" attachment and pushing over the coaxial ground contact collar which is around the probe tip as shown in the image at this link: https://electronics.stackexchange.com/questions/136123/how-do-you-attach-an-oscilloscope-ground-spring

- Gough

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

misaz — Fri, 14 Nov 2025 16:57:17 GMT

MAX32666FTHR is part number (and name) of board. MCU part number is MAX32666GXMBL+. Schematics is in board datasheet. On board website, there are even gerbers.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

hlipka — Fri, 14 Nov 2025 16:09:20 GMT

I took the MAX32666FTHR for a part number, not the board name :-(

Short calculation (well, I used calculator.academy/.../ ): at 96MHz, a capacitance of 10pF results in an effective impedance of 166Ohm. 100pF (which very well happens when your probe is in 1:1 mode) will be about 16Ohm. 8mA through 16Ohm are about 130mV.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Fri, 14 Nov 2025 15:53:11 GMT

The scope is a Multicomp Pro MP720012

The board, I mentioned it in the original message, it's the MAX32666FTHR

The 96MHz oscillator is internal to the MAX32666 so you can only see it if you output it on one of its GPIO pin, which I did via the Audio subsystem bit clock configuration.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

hlipka — Fri, 14 Nov 2025 15:48:47 GMT

Some other question that come to my mind:

- you never mention what scope you are using (from the probes it seems a Tektronix). This should not matter, but might give hints into what else could go wrong.

- you also do not mention what that board is you are using. Is there a schematic for it?

- you mention that you were 'able to output the 96MHz clock signal' - this hints at that this is not the actual oscillator, but another pin at the MAX32666, right? Are you _sure_ that there is nothing else connected to that pin? How do you probe to that pin? At 96MHz even moderate resistors or capacitors can wreak havoc with your signal...

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Fri, 14 Nov 2025 15:36:20 GMT

The limit is set at "full band"

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

hlipka — Fri, 14 Nov 2025 15:20:24 GMT

Another thing to look for: did you enable a bandwidth limit on the scope input? For most scopes that would be 20MHz.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

michaelkellett — Fri, 14 Nov 2025 14:10:34 GMT

Try on the x10 setting - expect much lower input capacity.

The Cal Test cheapo 100MHz 10:1/1:1 probe sold by Farnell claims 90pf on 1:1, 16pF on 10:1.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Fri, 14 Nov 2025 13:10:47 GMT

It is switchable and I'm using it on the x1 setting

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

michaelkellett — Fri, 14 Nov 2025 12:07:30 GMT

Is it a switchable probe, and if so are you using it on x1 or x10.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Fri, 14 Nov 2025 10:49:23 GMT

Thanks for the suggestion, I was under the impression this only applied if I was driving "power hungry" things like LEDs.
I did change the setting from 00 to 11 and this is what I now get:

The difference is not much but it is indeed cleaner with more "swing".

I also tried placing the scope in AC mode for CH1 along with 100 millivolts per division to scope the 96MHz signal and this is what I got:

So, clearly, the 96MHz is here on the pin, it's just that I don't seem to have the proper equipment to measure the full voltage swing. That does not surprise me as I'm near the limits of the scope, but at least I learned quite a few things along the way.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

obones — Fri, 14 Nov 2025 10:45:04 GMT

Thanks for your reply, here are some answers to your questions.

The probes are the T5100 probes that came with the oscilloscope and are rated for 100MHz. Sadly, the manual does not have the graph you mentioned, only a voltage derating according to frequency graph.

As to the "ground clip", I believe you mean the alligator clip small lead that attaches just below the guard as can bee seen here ?

If that's the thing, well, I did not hook it up to get the pictures above, and the situation did not change when I clawed it to a GND pin on the demo board.
I searched for the term "spring ground" and saw various depictions of hand made metal springs that fit to the tip of the probe, but I can't seem to figure out if this would be applicable here.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

michaelkellett — Thu, 13 Nov 2025 13:07:01 GMT

A (very) quick scan of the MAX32666 suggets that they have 4 possible drive current settings for GPIO pins.

1mA drive into a 10pF load will give you a slew rate of 100mV per ns.

(and of course 8mA will get 800mV/ns)

Your 96MHz signal has 5ns up/down time so you would expect a signal amplitude of 500mV pk - pk with the 1mA drive. (And I've assume 10pF but your probe + pcb could be worse)

The scope trace shows about 350mV.

So if you are on the low current setting for your pin that would be about right.

Try setting the pin for 8mA and you might see a rather (8x) better slew rate.

It's often a good idea to check the pin drive on modern processors because quite a few offer options - low currents can reduce noise when you can get away with them, high currents are good for speed but may be handy when you actually want power out of the pin.

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

Gough Lui — Thu, 13 Nov 2025 12:03:59 GMT

This is a clear sign of being slew-rate limited somewhere along the signal chain.

You've mentioned your oscilloscope is "100MHz", but 96MHz is pretty close, so you can expect a slight amount of roll-off (up to 3dB) in your measurement already. This means about a halving of amplitude, but sometimes better.

But have you checked the probes you're using are rated for 100MHz as well? I presume you'll be using "standard" 10:1 style passive probes, but they will have a rating - check in your probe manual. If your probes are cheap "50MHz" probes, well that could be your issue too.

For the MAX32666 - how clean the output is really depends on the "load" on the pin (both resistive and capacitive). You may just have a scope probe on the pin - but scope probe impedance is a function of frequency. You can see this if you look at your scope probe manual - it will look something like this link. If your probe's loading graph looks like that, then you're basically loading your MAX32666's output pin with just 600 ohms, which could be a bit heavy but still perhaps acceptable. Usually heavier loads means logic voltages that are further from zero and VDDIO due to the internal resistances of the sourcing/sinking MOSFETs in the totem pole driver, resulting in lower voltage swing.

Furthermore - are you using the "spring ground" or are you using a "ground clip"? Use of the "ground clip" lead is bad news for high frequency signals - it adds inductance and loop area causing "ringing" (i.e. ripples) that also affect the "squareness" and accuracy of overshoot/undershoot measurements. Shortening the ground return to the shortest length possible - i.e. using spring ground is definitely recommended for a clear image.

- Gough

RE: Observing 96MHz clock signal and its divisions leads to "strange" results

dang74 — Thu, 13 Nov 2025 11:29:01 GMT

I suspect that seeing the 'triangle type' shapes instead of a square wave is because you are reaching the bandwidth limits of your scope. I have a purely analog scope, so sampling theory doesn't even come into play for me, but I've seen this very same thing where the square waves become more triangular as I get closer to the scope's bandwidth limits.

RE: Hantek HT20COP probe safe to use or not?

Pappy19 — Sun, 09 Nov 2025 20:09:16 GMT

Hi Michael thanks very much for your input. Hantek rep in china insists the HT20COP they manufactured is a non contact probe with a 5:1 built in attenuation and is perfectly safe to use directly in a 20v max input scope without their attenuator, non contact being the key word, a 20:1 attenuator will only add noise to the waveform and used for back probing injectors and ignition primary

The newer HT25COP has 10:1 attenuation built in. As u can see iv got a few posts here saying I could still damage my scope by using it. I don’t know wat the go is mate. Surely if it wasn’t safe 1000s of users would have already fried their scopes by now, they’re a budget friendly probe still marketed today and I’d assume they’re being used on budget friendly low voltage input scopes. A Pico cop probe is 5 times the price

Hantek HT20COP probe safe to use or not?

Pappy19 — Thu, 02 Oct 2025 04:19:54 GMT

I brought the PicoScope 2204a recently that has the +/- 20v max voltage input. The HT20COP secondary ignition coil probe has a 10:1 built in attenuator.
some direct injection engine secondary coils can output up to 40kV even with a 20:1 attenuator added into the line I can potentially still damage this 20v max scope?
Voltage max protection spec is 100v but I’d rather not use a cop probe if there’s any risk. I have the hantek HT25 ignition lead clamp pickup also that has its own ground clamp. Maybe that be safer to use. Thanks very much if anyone can point me in the right direction if I wanna use this in an automotive application. Maybe il just stick to 20:1 attenuated primary voltage patterns. I assumed a secondary COP probe touching a coil only picks up a waveform pattern until I found out it’s attenuated. I guess it just comes down to myself not knowing much about scopes in general

RE: Hantek HT20COP probe safe to use or not?

michaelkellett — Thu, 02 Oct 2025 15:59:14 GMT

The Hantek probe is a non contact probe - and probably magnetic because it's intended for Coil On Plug ignition systems.

This Youtube shows you how the probes work, and the guy deserves kudos for doing it really cheap !

https://www.youtube.com/watch?v=Y3KNT4uHnRU

Waving the Hantek probe near ignition cables probably wont do much harm but may not measure much useful either.

I wouldn't recommend attempting a 40kV probe at home - mine looks like this:

there should be a picture of it here but although I easily found the empty box where it's kept the probe wasn't in it 😢

It's about 30cm long and has a big plastic guard on it - you need a lot of 500V resistors in series to stand off 40kv - and good luck finding a 40kv capacitor - although you can put one in // with each resistor.