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Dimitree
Reged: 05/11/13
Posts: 3
Loc: Italy
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simulation of a digital circuit
#319267 - 12/26/13 05:15 PM


Hi everyone, my name is Dimitri,
this is a bit off-topic since it's not related to MAME, but I believe you can help me here.
I'm trying to understand how a circuit I own works. It uses many 7400 and 4000 series logic, and my aim is to emulate its functions using a microcontroller. I'm a programmer but quite poor with digital electronic.
I was wondering if there were a simple way to simulate this circuit on my computer in order to understand the logic involved.
Essentially it is an audio echo machine from '80s: the analog audio is converted to digital (with a SAR + DAC), then stored into DRAM and converted back to digital after some time. The part that I'm trying to simulate is the DRAM controller and the logic needed to start-stop the conversion.
I tried LTSpice and Proteus ISIS but I wish there was a simpler and more friendly software.
Thanks and sorry for my english.
Dimitri



Phil Bennett
L'Emuchat
Reged: 04/20/04
Posts: 888
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Re: simulation of a digital circuit new [Re: Dimitree]
#319422 - 12/29/13 03:02 PM


> Hi everyone, my name is Dimitri,
> this is a bit off-topic since it's not related to MAME, but I believe you can help me
> here.
> I'm trying to understand how a circuit I own works. It uses many 7400 and 4000 series
> logic, and my aim is to emulate its functions using a microcontroller. I'm a
> programmer but quite poor with digital electronic.
> I was wondering if there were a simple way to simulate this circuit on my computer in
> order to understand the logic involved.
> Essentially it is an audio echo machine from '80s: the analog audio is converted to
> digital (with a SAR + DAC), then stored into DRAM and converted back to digital after
> some time. The part that I'm trying to simulate is the DRAM controller and the logic
> needed to start-stop the conversion.
> I tried LTSpice and Proteus ISIS but I wish there was a simpler and more friendly
> software.
> Thanks and sorry for my english.
> Dimitri

Sounds interesting!

Could you post a circuit diagram?

Thanks,
Phil



RetroRepair
MAME Fan
Reged: 12/21/09
Posts: 259
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Re: simulation of a digital circuit new [Re: Dimitree]
#319424 - 12/29/13 04:32 PM


You have already listed the best software to do this but there is also Atanua which is pretty basic if not a bit limited: http://sol.gfxile.net/atanua/index.html

I'd probably persevere with Proteus, it gives you a good grounding with digital logic and has an extensive IC library. There's loads of tutorials on Youtube if you are stuck.

There's a lot of good digital logic primers out there too. You will probably want to get some of that under your belt before diving into DRAM controllers.



http://www.youtube.com/retrorepair



Dimitree
Reged: 05/11/13
Posts: 3
Loc: Italy
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Re: simulation of a digital circuit new [Re: RetroRepair]
#319500 - 12/31/13 12:01 AM


thanks for the support I really appreciate it.
I'm indeed trying again with Proteus, the only problem was that I can't find any model for the DRAM and for the ADC.

here there is my schematic (I don't attach it here since is really big):

http://www.ed-sounds.com/board2.png

so far, what I undestood is (hoping that I'm not wrong, but it's possible):

1) the heart of the circuit is the 74LS624 VCO, reading the Service Manual of my unit, it should produce 64kHZ as clock, but this can be modulated (varying the VCO input voltage).
2) On the top of the page, analog signal passes through a Sample and Hold circuit, and then through another set of switches (IC12) (don't know what for..), but this condition is stored in 1 bit of DRAM (IC34).
3) analog signal is then converted using a SAR+DAC, clocked by 74LS624, and the parallel 12bit output is stored in 12x DRAM 1bit 64k chips, and then read back using a DAC.
4) other "user inputs" include:
- bit PC0, don't know what's for.
- 10 bits (PA0,..,PA7 + PB0, PB1) on the bottom of the page. I think these 10 bits sets the delay time (incoming audio is stored in DRAM and then read back after x milliseconds), since the delay time can be anything from 0 milliseconds to 1023 milliseconds, with 1 millisecond step, and 10 bit ^2 = 1024 possible values. This 10 bit value is then converted to useful DRAM address by all that logic. This logic should also take care of the refresh procedure for the DRAM.
5) RAS and CAS bits looks like they got the same value all the time..since there's a bunch of inventers but logically they are useless (maybe they used them to buffer the bits when long pcb traces are used?)
6) Everything becomes overwhelming for me when I think that VCO input voltage is not fixed so that it always produces 64kHZ clock: indeed (if the user wants) it could be modulated like this:
VCO input voltage is a triangle wave (2V min, 4V max) with frequency from 0.1hZ to 16hZ. That would produce a variable clock that ranges from 40kHZ to 78kHZ according to the Service manual.

are my assumptions correct? and what else could be said about this circuit?
many thanks again



sz72
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Reged: 08/20/07
Posts: 78
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Re: simulation of a digital circuit new [Re: Dimitree]
#319514 - 12/31/13 08:49 AM


the bank of dram chips could be simulated by substituting it with a static ram chip, a register (or a set of d-type flip-flops) and some logic



Dimitree
Reged: 05/11/13
Posts: 3
Loc: Italy
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Re: simulation of a digital circuit new [Re: sz72]
#319779 - 01/04/14 03:56 PM


I would but this is far from my competence.. And also, I would miss the ADC simulation that I would need more..since without the EOC signal, the whole logic can't work.

Do you know any forum/blog/place where this kind of circuits is the main topic?



italieAdministrator
MAME owes italie many thank yous, hah
Reged: 09/20/03
Posts: 15229
Loc: BoomTown
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Re: simulation of a digital circuit new [Re: Dimitree]
#319781 - 01/04/14 05:28 PM


> I would but this is far from my competence.. And also, I would miss the ADC
> simulation that I would need more..since without the EOC signal, the whole logic
> can't work.
>
> Do you know any forum/blog/place where this kind of circuits is the main topic?

This is a little specialized for a single forum...you are most likely going to have to pull in thoughts from several places. Chip manufacturer forums might be of some help, ST micro, TI, etc...

Been meaning to take a detailed look at what you are doing and comment, just short on time as usual. Please keep posting here with your progress if you would, very interesting.



sz72
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Reged: 08/20/07
Posts: 78
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Re: simulation of a digital circuit new [Re: Dimitree]
#319848 - 01/05/14 10:24 PM


> I would but this is far from my competence.. And also, I would miss the ADC
> simulation that I would need more..since without the EOC signal, the whole logic
> can't work.
>
> Do you know any forum/blog/place where this kind of circuits is the main topic?

That's the problem about simulators, you need models for all the components in the circuit, and if they are not available you are on your own.

Try your luck here http://www.edaboard.com/forum15.html


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