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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
random 4bit generator freezes with fast clock
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RingMad



Joined: Jan 15, 2011
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PostPosted: Thu Feb 10, 2011 5:30 pm    Post subject: random 4bit generator freezes with fast clock
Subject description: newbie ask for help re 4006 shift reg + XNOR
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This is the first time I explore a shift register, in this case a 4006B, and followed a schematic a friend got out of a french electronics book (see attachment).

Instead of the 100nF and 100k on the clock oscillator, I put a 10uF and 150k pot plus LEDs on the 4077's outputs so I could play around and see what's happening.

At slower clock rates, I get pseudo-random patterns no problem. When I speed up the clock to the point where the LEDs are constantly on, they remain stuck when I slow the clock back down.

Only if I disconnect certain wires, like from the 4006's pin 1 to the 4077's pin 10 for a few seconds then reconnect, then the LEDs become unstuck.

What's happening? Is this a problem with shift registers, or is just a peculiarity of this circuit?

Any advice appreciated.


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Pseudo-Random 4-bit Generator Schematic
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stolenfat



Joined: Apr 17, 2008
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PostPosted: Fri Feb 11, 2011 3:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

not really sure what could be causing that. i bet it's just an issue with that circuit. Does the 4006 have a reset pin? If so you could ad a push button to connect it to Vdd and get the thing to reset instead of attaching wires.

My initial idea was the add more resistance before the LEDs... but i dont think that would help.

Embrace your glitch and add that button?

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RingMad



Joined: Jan 15, 2011
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PostPosted: Sat Feb 12, 2011 6:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks... There's no reset pin on the 4006, but like I said if I disconnect a certain wire for a few seconds, it effectively resets.

I also tried larger res before the LEDs as well as 100k pulldowns on the 4006's data pins. Nothing changed.

But you are right... just embrace the glitch. Or maybe try out the 4015 instead. And don't always trust schematics in books.

I was mainly asking in order to gain insight into what may be going on... a friend of mine suggested that since the circuit contains feedback loops linking outputs and inputs, perhaps they begin to oscillate out of control, thus always remaining at a high "on" voltage.
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Kabzoer



Joined: Feb 07, 2011
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PostPosted: Fri Feb 18, 2011 9:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Maybe it is the logical outcome of the circuit, so that it 'freezes' after a long while. But if you speed it up it will just do that faster? Just guessing Wink
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richardc64



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PostPosted: Fri Feb 18, 2011 12:00 pm    Post subject: Re: random 4bit generator freezes with fast clock
Subject description: newbie ask for help re 4006 shift reg + XNOR
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RingMad wrote:
Is this a problem with shift registers, or is just a peculiarity of this circuit?


Yes.

You have a 18-bit shift register, which will create a very long sequence before it repeats. It probably also freezes with a slow clock. You just haven't waited long enough.

When it reaches the state when all the bits are 1s -- or 0s -- there's no escape, because it clocks that state into registers that are already in that state.

There's ways to prevent the all 1 or all 0 state, but I've never understood the explanation well enough to tell anyone how to do it. Try NOT feeding one of your XORs back into the 4006. Or try the circuit below. It's the noise generator from the Roland DR110. Use one NAND for the Power On Reset and you'll still have 3 XORs and 2 NANDs you can use to combine 4006 outputs for random patterns.

Where'd you get that schematic? I've always despised those symbols. Without the IC# they convey nothing to me.

Fckn committees.


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Last edited by richardc64 on Sun Feb 20, 2011 11:49 am; edited 2 times in total
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Kabzoer



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PostPosted: Sat Feb 19, 2011 5:53 am    Post subject: Reply with quote  Mark this post and the followings unread

What if you replace theXNOR gates with NAND gates in the first circuit? Maybe you can try this klee sequencer number 1 schematic
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Draal



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PostPosted: Sat Feb 19, 2011 7:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Speaking of the klee v 1:

I notice leds on the quad op amp after some of the outs of the 4006. Might be worth exploring to see what results ya get. A fun circuit to add to the discussion at any rate Smile .


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Kabzoer



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PostPosted: Sat Feb 19, 2011 11:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Maybe something more Lunetta-styled, like this?


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RingMad



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PostPosted: Sun Feb 20, 2011 5:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Hmmm, interesting stuff, folks. Maybe the "all on" state IS the logical conclusion of my circuit... I'll have to test that.

For the moment, I just put a small resistor in series with the pot so that it staves off the freezing a bit if I max out the pot.

Where I got the schematic?... like I said, my friend had photocopied it from a french electronics book... I had a little trouble reading it at first too, but certain things seem clearer. But I had a bit of trouble with that Roland DR110 circuit too... like, what's C27?

For the other circuits, they could be cool, but I'm not sure I want to get into lots of opamps yet... for one thing, I'm not using a bipolar supply. I've got just a single LM324 on my breadboard, for the R/2R.

Anyhow, I've posted 2 new videos... one of this circuit in action, as well as another random generator circuit... see the "the absolute beginners thread"...
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richardc64



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PostPosted: Sun Feb 20, 2011 11:45 am    Post subject: Reply with quote  Mark this post and the followings unread

RingMad wrote:
But I had a bit of trouble with that Roland DR110 circuit too... like, what's C27?


I should have erased "C27." The two unused XORs form the clock in the DR110. I deleted that part of the circuit, freeing 2 XORs, since you already have 1/4 4093 as a clock. I overlooked C27.

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Draal



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PostPosted: Sun Feb 20, 2011 11:55 am    Post subject: Reply with quote  Mark this post and the followings unread

That klee circuit may have a couple of op amps, but it's no big hurdle. Notice no -15v on the schematic, just pos. and grnd. If not for you right now, no worries; it might aid others that want to play with the 4006 Smile .
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Kabzoer



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PostPosted: Sun Feb 20, 2011 12:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, and i think you can leave the four opamps at the right out.
And if you have some digital, or amplified noise, you don't even need the opamp at the left! Haven't tested that...
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richardc64



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PostPosted: Mon Feb 21, 2011 5:10 am    Post subject: Reply with quote  Mark this post and the followings unread

RingMad wrote:

For the moment, I just put a small resistor in series with the pot so that it staves off the freezing a bit if I max out the pot.


If maxing the pot, thus killing the clock, were the problem, then disconnecting any wires wouldn't correct the problem.

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RingMad



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PostPosted: Tue Feb 22, 2011 2:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

By "maxing the pot", I meant reduce the resistance to 0, thus maxing the oscillator run at its highest frequency.

Actually, a friend of mine raised that question w.r.t. oscillators... if the pot is at 0 ohms, then maybe the chip runs too fast and overheats? He was thinking maybe it was necessary to always have a high-Wattage resistor of some minimum value in series with the pot such that the max voltage of the output (or input?) doesn't exceed the chip's rating? Or something. I don't see any of you doing this, so maybe that's just being paranoid.

Anyway, I changed things a bit, so I have a 4.7uF cap on the 4093 with a 100ohm res in series with a 150K pot. This gives me the fastest speed I would need, and doesn't freeze.

With this configuration, I also tested it for about 3 hours, running at max speed, and it never froze, so I think that disproves the "logical outcome of the circuit" theory. However, it does sometimes freeze when I first power up the circuit (and the pot is at 0).

In any case, it's no big deal. I like the circuit and along with the XOR chain, will probably end up in my finished machine as a pseudo-random generator.
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ezekiel



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PostPosted: Wed Mar 02, 2011 6:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

I would wonder about your clock oscillator running at way-above-audio frequencies. It has a low feedback resistance. The signal it feeds into the shift register might be failing to meet the set-up specs for the shift register. I could imagine problems such as too narrow time high and/or too low of a voltage peak. You might be operating in metastability mode. Nonetheless, at near-zero resistance, I understand the power consumption goes way up. But, the sound is cool.

Try putting an inverter in between the clock oscillator and the shift register. That would buffer the signal, especially with the low resistance feedback path. It should also help in the slow-clock situation.
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RingMad



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PostPosted: Sat Oct 13, 2012 5:00 am    Post subject: Reply with quote  Mark this post and the followings unread

For any who end up in this old thread, analog_backlash has solved this freezing problem by swapping the 4077 with a 4070 and swapping 2 wires. The new schematic can be seen here: http://electro-music.com/forum/topic-52066.html .

James.
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