How do I convert this truth table into a Prop-compatible chip or logic circuit?

ElectricAye

Good morning,

I would like to independently control 4 LEDS but I can budget only 2 Prop pins to do so. I've been trying to reinvent the wheel on this with NAND gates and XOR's etc, but I just can't figure it out and I'm sure there's some kind of simple Prop-compatible (counting?) chip that can easily do this. But what da heck is it?

Suggestions will be greatly appreciated.

Truth table is attached.

Dr_Acula

A 74HC138 can do that with one chip, but the outputs will be active low. Or half a HC139.

Is the challenge to do this with one chip?

If you really want active high outputs, I think a 74HC4051 can do it with one chip. (or 4051). Feed in a high and it passes it through to one of the outputs. Though 'low' would be tristate. Won't matter if driving leds.

Active low driving leds won't matter either - the aim is just to get the led to light up.

138 or 139 if you have them in the parts drawer.

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Post Edited (Dr_Acula) : 6/18/2010 2:35:30 PM GMT

ElectricAye

Dr_Acula said...
...

Is the challenge to do this with one chip?

Doing this with one chip would be nice but I'd settle for almost any combo of gates right now. I'll have a look at your above suggestions.

thanks, Doc

Dr_Acula

See addits above re 4051

If two chips, you can do pretty much any logic with, say, an AND gate and hex inverters eg 74HC08 and 74HC04. Use the 4 AND gates and invert one or both of the inputs for each output state.

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Rayman

Check out this datasheet:

http://focus.ti.com/lit/ds/symlink/cd74act139.pdf

YOu could use this chip, or copy the logic diagram...

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bill190

Just say it like it is...

NOT a AND NOT b· -> IR 1
a AND NOT b -> R 1
NOT a AND b -> G 1
a AND b· -> B 1

ElectricAye

Doc, Rayman, bill,

this is excellent. It seems like somebody told me years ago: you can design any logic circuit with enough NAND gates. As a point of pride, I took it upon myself to design this from scratch... but simply choked on it.

Thanks for helping me out today.

jazzed

@ElectricAye, I want to add a little extra foundation to the topic.
Look at this image and think about how many states are represented.
It is a little more complicated than the 2 variable diagram.



Just count the number of color spaces. Don't forget the area where there is no color.
Use English to describe the conditions for each color "space". Don't use set theory, just logic.
Once you have a list of statements, translate them to equations.

That's just one way to design a 1 of 8 decoder from scratch with confidence.

Cheers,
--Steve

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Post Edited (jazzed) : 6/18/2010 4:45:43 PM GMT

Beau Schwabe

This looks like a good 'Golf Challenge' exercise

I'll post a circuit later, but I was able to get this functionality with 4 transistors, 2 diodes, and 5 resistors.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 6/18/2010 7:00:53 PM GMT

ErNa

There is a way to control two leds with one pin. That makes it easy to drive one of for leds by 2 inputs. Don't remember the threat, was about 4 weeks ago

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heich

This is rare

I was sure i posted an answer here. But i don't see it.
Hope this one works.

ww1.microchip.com/downloads/en/appnotes/00234a.pdf
Page 2. Fig 4

Good look

Regards

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Beau Schwabe

Ohhh C'mon... do it with a little analog discrete logic

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ElectricAye

ErNa said...
There is a way to control two leds with one pin. That makes it easy to drive one of for leds by 2 inputs. Don't remember the threat, was about 4 weeks ago

Yes, it was this:

http://forums.parallax.com/showthread.php?p=910828

But then somebody mentioned that perhaps some little bit of current might flow through both LEDs??? and that might emit a little bit of light from the undesired LED???? See...

Peter Jakacki said...
.... I used a red and a green in series and at 3.3V there was only a bit of leakage current flowing which means that the leds just glow slightly.....

I'm not sure if that's true for what I wanted to do and, if so, whether or not it would mess up my measurements, so I decided to be chicken and use 2 pins to drive 4 LEDs using pure Spock logic.

EDIT: added Peter's quote.

Post Edited (ElectricAye) : 6/18/2010 9:07:13 PM GMT

ElectricAye

Beau Schwabe (Parallax) said...
Ohhh C'mon... do it with a little analog discrete logic

Man, are you trying to fry my brain?

Bobb Fwed

ElectricAye said...
Doc, Rayman, bill,

this is excellent. It seems like somebody told me years ago: you can design any logic circuit with enough NAND gates. As a point of pride, I took it upon myself to design this from scratch... but simply choked on it.

Thanks for helping me out today.

I believe both NAND and NOR logic share the title of being "functionally complete". I too have tried to build even mildy complex logic with just NOR's or NAND's and gave up after short order. Using all of the available standard logic is just too easy.

@Beau
I always love the discrete logic solutions. I am trying to get more proficient with such things. But alas, I find my endeavors are in vain. When I am laying out my PCBs I find the parts for discrete logic circuits are almost never as small as some IC I can find on the market that also can do the same thing at higher frequencies , use less power while doing it, and ultimately are cheaper (after the cost of PCB sqaurefootage and more assembly costs). I will not be downtroddened; exercising your brain in new and exciting ways will keep you forever young.

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ErNa

There will be no current, if another si-diode is connected in serial. But, instead of logic, another propeller may do the job

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Dr_Acula

I believe both NAND and NOR logic share the title of being "functionally complete".

I guess just for the intellectual challenge, see below for a solution using just NAND.

This would be 3 chips to replicate your logic. But if you want to light up a led, use active low and that saves the 4 output inverters, and so now it would only be two chips.

I do like Beau's transistor solution.

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zoopydogsit

I recall years ago using Karnaugh maps when designing stuff in nand gates. A quick google search found the following. Enjoy :-)

http://www.allaboutcircuits.com/vol_4/chpt_8/6.html
http://en.wikipedia.org/wiki/Karnaugh_maps



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ElectricAye

Dr_Acula said...
...

I guess just for the intellectual challenge, see below for a solution using just NAND.....

That's very impressive. No wonder I couldn't figure it out by myself. That fries my brain, too. I'm sure glad you guys helped me out with a chip that does the job!

bill190

FYI - For those learning about logic gates, in the above NAND only example, and the bottom two NAND gates shown...

The right NAND gate has both inputs tied together.·This makes it a NOT gate...

That along with the NAND gate to its left makes the combined two NAND gates function as an AND gate (clever use of the NAND gates).

ericball

ElectricAye said...
I would like to independently control 4 LEDS but I can budget only 2 Prop pins to do so. I've been trying to reinvent the wheel on this with NAND gates and XOR's etc, but I just can't figure it out and I'm sure there's some kind of simple Prop-compatible (counting?) chip that can easily do this. But what da heck is it?

That's a basic 2 in 4 out binary decoder/demultiplexer with active high outputs.

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Beau Schwabe

Here is a cool link to interactivly build and play with logic gates...

joshblog.net/projects/logic-gate-simulator/Logicly.html

... you can knock two of the NAND gates off of Dr_Acula schematic by combining the inverter function of two of the input NANDs.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.