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Can anyone solve this cmos puzzle?

Attached are two CMOS circuits created in Multisim 10.1 (10.1.197)

The first circuit was created after the second.

I'd created two identical oscillators with both CMOS gates and a CMOS IC, but neither of them responded with anywhere near the correct frequency.  (The oscillator of my design wasn't working right -- the C had an impossible square waveform, which prompted further investigation into the circuit.)  I wanted to investigate this to determine what I needed to do to get the frequency roughly correct.  So I decided to go simple-stupid: start with a simple RC circuit, and slowly, step by step, transform it into a CMOS oscillator, just to see where the frequency went to lunch.  The circuits below resulted from this quest for the answer.

I'm very puzzled.  These CMOS gates don't appear to work at all.  I never could get a legal CMOS output (<50mV). 

As I see it, my RC oscillator has no chance of working if the gates don't work.   This is bad enough that I'm going to have to break out EWB 5.12 now, just to see if all those years ago CMOS didn't work and I'd never noticed.

If anyone out there can solve this puzzler, modify these circuits so that I can get legal CMOS operation, I'd like to know how it's done.


  
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Incidentally, the math you'll see in the second (which was actually the first) circuit was just me being lazy and doing the math where it wouldn't get lost, where I could easily check my Multisim results versus the actual math of the circuit.  After all, this was a simple RC circuit being transformed into an oscillator, and something was clearly going wrong.  It's akin to the logical bridge question.

You come up on two guys at the entrance of a bridge.  You know one always lies and the other always tells the truth.  You want to know if the bridge is safe to cross.  What question should you ask the two guys to know whether it's safe to cross the bridge?

Often the answer is to ask the two guys what the other would say.

I've always said, create a fact and have each confirm the fact.  For example, look at your watch, and ask each what time it reads.  ...which was my approach to my CMOS oscillator problem.

I know how an RC circuit works, and I know somewhat about CMOS operation.  One's lying and the other was telling the truth.  The math was the fact of the matter.

Yeah, I know this forum isn't meant to be about philosophical discussion.  But I just thought I'd explain a little more about where I was going with the circuits, why things look as they do.
 
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First circuit solved. The CMOS gates you have there require the power source VSS to on the schematic somewhere. What you need to do is to examine the components pin requirements under Properties. This may save a lot of headaches when placing/hooking them up. I think the hidden pins need to have something done about them as it seems to me to becoming problematic when placing them. If you don't know to look in the Properties menu under pins to determine their voltage requirements then thing like this are bound to happen.

Hope this helps. Looking at the other circuit now.

Kittmaster's Component Database
http://ni.kittmaster.com

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The first piece of the puzzle is that you don't have VSS tied to GND so they are disconnected.

Second, you don't have the proper timebase set on the scope for your analysis, part of your answer lies here:

http://www.datasheet4u.com/html/4/0/0/4001B_FairchildSemiconductor.pdf.html

looking specifically at Tplh (time propogation low to high) with VDD at 5V.

As I read it, the NOR should have a low based on your dual high condition, BUT, during startup, there is no stablization of during the current inrush to allow the inputs to stablize. During this process, the input ramps the output in a linear fashion, once the inputs reach proper operating level, then gate switchs after the 120ns setup prop delay time(datasheet), at which time, then goes low and remains low for the duration, as it should.

I'm not sure how your trying to measure a frequency out of that since it becomes steady state with DC power supplies.

This comes from the simpleRC scope.....I hope this is what you were looking for???

Chris


Signature: Looking for a footprint, component, model? Might be here > http://ni.kittmaster.com
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O.K. Second circuit. I did manage to get it to simulate, but not the way you would want.  What I did  was to replace all the anolg ground to the DGND and added the VSS. This got it simulating with only one output pulse.

Now the bad news. Since I can rememberI have not been able to get CMOS Oscillators to work, period. This goes back to at least Version 2001. There may be a way to get them working but I haven't found it.

If NI has an answer for this I, for one, would like to know what it is.



Message Edited by lacy on 08-01-2008 07:42 PM
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Whoohoo!  Yeah, I forgot about the Vss side of the digital setup.  That's a weird thing for me, just sticking unconnected supplies up there in space, but I remember going through the Multisim tutorial and remembering them sticking this combination in space.  Thanks Lacy.  But, you say we're still going to have a problem getting a CMOS oscillator to work?   ...I'm assuming you also run with the "real" simulation setting?

Perhaps a viable solution to how NI could fix this digital-supplies-in-space oddity is to just add the pins to the gates and then do just like is the case with multiple op amp packages: once you've covered the supplies for one gate all are covered for that package.

BTW, I downloaded the multiple page circuit you left for me (thank you!), but I haven't been home long enough to even look at it, but I will tomorrow.

 
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I don't understand.  Tying VSS to Gnd gives "Warning: Cannot connect digital power to analog ground."

The timebase on the scope is correct.  One RC is 1 ms (67% of Vs stored in C), and 5 RC is 5ms (99% of Vs stored in C.)

I think you misunderstand what I was doing based on what you said later.  A CMOS oscillator is just a simple RC circuit that causes the gates to switch and cause charge or dicharge of the capacitor.  My original reason for investigation was I did not see the exponential function; instead I saw a square wave and a very wrong frequency.  Hence, I decided to start with the basic RC circuit (you were still looking at the progression to, not the destination) and then gradually tweak one piece at a time until I either arrived at a working oscillator or a reason for what I'd seen.

But please do explain this grounding thing to me, because I don't understand what you're saying yet.  
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I believe I could get a CMOS oscillator to work if I could just get all the parts to act correctly.  As I see it, Multisim simulated the RC correctly, but it just didn't go right for me (and it was my fault based on what's been said) when it came to the CMOS gates.  I just need to get the gates to simulate like CMOS gates, and it should be all downhill.  (I was, frankly, stunned when my oscillator wasn't working right, as I've built and used the circuit many times and was considering it a given of the design.  But when I put it together with what it'd be driving, something using the same gates!, it just fell apart.  This is what brought on my quest to figure out what had happened.  I clearly had the CMOS working correctly (Multisim was happy), but then adding the oscillator circuit wrecked my same CMOS gate-using formerly working circuit.  ...was my first circuit only looking like it was working???  I had to investigate pronto.)

If I do get a CMOS oscillator working, you bet I'll upload it for you. 
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Lacy, you know that Phase Locked Loop circuit I built and then uploaded to kitt's new site the other day.  Why does it work?  I've never used VSS or DGND.  That circuit used the same CMOS did it not?  So why did it work?

Perhaps I should investigate it by dismantling it into my oscillator... 😉  (For example, I could drive it with my oscillator and then dismantle everything around the oscillator.)

You know, like I'm gradually dismantling my working RC circuit into the oscillator. 

Remember when I was working on that board tester a bit back?  Remember how doing something unrelated caused something previously working to fail?  That's where I'm going with this.  Maybe we have a similar situation this time?  (By the way it dawned on me that I could be able to make a fake connector real by installing 0.01 ohm resistors.  Yeah, you remember that connector thing we were working on?  But anyway...)

I'm now really puzzling just why my PLL works.  I'm thinking I must have made Multisim happy somehow.

 
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Here's the reason why your PLL worked and this particular set of circuits won't. In the PLL you had the digital simulation setting set to IDEAL. You don't need to have the power sources on the schematic under these circumstances. Also you are clocking the 4011 IC's with a 4013 Flip Flop that is tied to a clock source. In the PLL circuit you are not trying to get it to self-oscillate. Even the VCO is a not made up of these CMOS IC's rather a Voltage controlled square wave oscillator.

One other thing. I told you about the connector in your board tester numerous times and apparently I either didn't make what I was saying clear or you just didn't grasp it at the time.

CMOS Oscillators will not work in Multisim. It is hard to even get a regular oscillator using discreet components to work. Maybe you can find something that I am missing here, but I have tried a number of CMOS oscillators from books and from the web and they just do not function. I was trying once to design a model for an IC that used an internal CMOS oscillator. I could get the rest of the circuit working using an external clock frequency, but I never could get the on-board CMOS Oscillator to work and without that the chip would have been useless. So I finally said screw it and gave up on the project.

I really hope you can get these to work. I would encourage you to keep trying and not give up like I did. Who knows, maybe you will be the one to finally figure it out.

Kittmaster's Component Database
http://ni.kittmaster.com

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