microcontroler circuit design question / new digital scope

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samt_32
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microcontroler circuit design question / new digital scope

Post by samt_32 » Tue Aug 05, 2014 4:42 pm

I have a adjustable duty cycle 555 timer circuit that has a 25% duty cycle signal with a 83Hz frequency at around 12 Volts peak to peak. I am inputting the 555s square wave into a Atmel micro through a 100Mhz ferret bead, 4.7K resistor, 1uF ceramic cap to ground and a 2.4K resistor to ground. The 4.7K resistor and cap form a low pass filter. This setup occasionally causes the output of the micro to go high. I am wondering if my scope would be fast enough to catch the occasional pulse to the micro input that would cause it to trigger or would I need a digital storage scope to catch the pulse? If I need a digital scope how many samples per second and memory would I need to catch this event? Everything is well decupled and was working fine with a older pcb board layout and the same value components but the manufacturer of the cap was different. The new board layout and different manufacturer but same value cap I think are to blame. I cannot easily swap the cap with the old board due to the SMT footprint been different. The 555 timer circuit is just on a breadboard and is indapendent of the micro pcb. I can hook up the 555 timer to the old board and it does not cause the output to occasionally go high.

sghioto
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Re: Tektronix 465 scope / micro input question

Post by sghioto » Tue Aug 05, 2014 6:17 pm

samt_32,

Welcome to the Forum!
I don't have an answer for the type of scope that may be required but have you tried adjusting the duty cycle of the 555 circuit?

Steve G

samt_32
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Re: Tektronix 465 scope / micro input question

Post by samt_32 » Tue Aug 05, 2014 7:43 pm

That is not an option. The 555 timer circuit is supposed to simulate a can-bus tail light signal so it can't change.

Dean Huster
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Re: Tektronix 465 scope / micro input question

Post by Dean Huster » Wed Aug 06, 2014 6:12 am

I'm here, too! One thought you need to consider regardless of what scope you use to try to capture the glitch is this: how will you trigger the scope (regardless of whether it's an analog or digital one) to catch the glitch? Does an output somewhere do something totally different when the glitch occurs so that you can trigger on that? The advantage of the digital scope here is that you can look backward in time from the triggering event to see the glitch. With any analog scope, whether it has high-speed storage or not, you can't look back in time. The only way an analog scope would capture the glitch is if it was the initial edge of the glitch that caused the event and the system response was fast enough that the scope could be triggered by the response BEFORE the glitch finished its transition back to "normal". So, for the most part, an analog scope is not going to work unless it's a Tek 7000-series with either a logic analyzer (7D02) plug-in or a higher-speed digital plug-in installed. I don't think the 7D10 would be fast enough. And even at that, it may take a high-speed analog storage model to do it, such as a 7623/33, 7834/7934, 464 or 466.

A digital scope probably wouldn't have to have blazing speed to capture the glitch itself. You're working with a 555 timer which, if it produced the glitch, would be limited in how short the glitch would be. Don't know about the speed characteristics of the Atmel processor since I'm one of those guys who treats nearly any processor design (especially simple ones that can be implemented with simple digital logic) with extreme distain. Even Tek's cheapest digital models made since around 1997 will have have an ample sample rate .... the TDS210/220 are rated at either 1 GS/s or 2 GS/s as I recall. Beware the import DSOs that feature fantastic data depth and other nifty things and connect to your computer or are teensy little handheld things, for they almost always lack in the sampling speed, typically less than 200 MS/s. Now, for a slow glitch, 200 MS/s may capture it, or at least a rough image of it.

Still, the rub is, how will you trigger the thing to capture the glitch? If the processor output that is affected by the glitch is always changing anyway, you have to sort out the change that was normal from the one that was glitch-induced. See the problem?
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

samt_32
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Re: Tektronix 465 scope / micro input question

Post by samt_32 » Wed Aug 06, 2014 10:10 am

Thanks for the info. The Atmel micro (a 90usb162) runes at 16Mhz from a external crystal oscillator and I can see a lower frequency micro square wave of it on the input lines as well as the 5 Volt regulated and filtered input. This signal is more prenounced on the new board layout then the old one. The output signal of the micro only changes in duty cycle not frequency when ir goes high (it is driving some LEDs) so I presume that this would be harder to trigger on. I have been looking at some HP 5400 series scopes and I found one that is 100Mhz with 20Msa/s for $100 no probes and the auto set button is broken and it is not calibrated from 1997. Would this work or should I be looking at one of the $350 china made scopes with 1 to 2Mb of memory 1Gsa/s and 100Mhz instead? I could also get a Tek TDS320 for $300 but there are no pictures of it working nor probes. These are all scopes that I have found on Kijiji or Allsold.ca in Calgary so the price may change. Can I use my 100Mhz Tek probes on the other models of scopes assuming that I stay within the frequency range of the probe?

Dean Huster
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Re: microcontroler circuit design question / new digital sco

Post by Dean Huster » Sat Aug 09, 2014 7:53 pm

Scope probes will transfer to another scope just fine.

I'm not much for buying a new scope over a used one. For the same price, a good, used one will have better performance, usually be of better pedigree and be more robust. The Chinese import will have horridly-written instructions and little to no service data. That seems to be the problem with most Asian imports, save for Japan. 20MS/s is pretty lousy compared to the 2GS/s you can get for the same price from a used Tek scope. Memory depth isn't an issue ... sampling speed is. I understand your problem of being antsy about getting a hangar queen instead of a gem from a used equipment sale. It would be great if you could find an internet sale item that would be close enough for you to visit to check out the equipment first. I've seen several invitations from sellers like that here in the U.S., and would expect to see the same in Canada. Myself, I tend to buy something decent looking and then buy one or two even cheaper hangar queens for fixing up the first one to prime condition. But that's also why I shy away from high-end DSOs, because there isn't much cheap out there yet compared to the analog models.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

samt_32
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Re: microcontroler circuit design question / new digital sco

Post by samt_32 » Sat Aug 09, 2014 10:36 pm

I have yet to see a used 1Gsa/s scope for around $350 in my neck of the woods (Calgary). That is why I brought up the Rigol or other similar China made scopes. I am wondering if I could try to catch the glitch with my current scope by using a camera set to a long exposure time and then see if there is a higher trace that is not normally visible to the human eye?

samt_32
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Re: microcontroler circuit design question / new digital sco

Post by samt_32 » Wed Aug 13, 2014 12:05 am

Well I think I figured out the problem. The two resistors that I mentioned were 10 times to small (4.7 K instead of 47K and 2.4K instead of 24K). The problem seems to be fixed now and the circuit does not respond to the 555 timer circuit at all. Thanks for the help.

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