# 74LS90 chip

Discussion in 'General Electronics Chat' started by nismospecv, May 5, 2007.

1. ### nismospecv Thread Starter Member

Mar 14, 2007
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Does anybody have any information on how to make a divide-by-2 counter and a divide-by-4 counter using 7490 chips?

I'm not really sure how to even go about making a table for this to get me started.

If you already know the pin layout for creating it, that would be helpful also.

May 16, 2005
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3. ### nismospecv Thread Starter Member

Mar 14, 2007
10
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This doesn't make sense to me. What do you mean by use pin 12 for divided by 2?

Also, what would a divide-by-3 look like?

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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The signal on pin 12 will be 1/2 the frequency of the signal on pin 14.

Look at one of the datasheets in the link I provided.

Divide-by-three will require more than just a 7490. You'll have to reset the chip every third clock cycle with external logic.

5. ### Salgat Active Member

Dec 23, 2006
215
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Couldn't you do an approximate divide-by-3 by tying QA and QB to the ANDing MR0? That way when it hits 3 it resets to 0 and then has to count 3 times again to get back to 0(a pulse every 3 pulses for QB)?

6. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
I think that would work! It should be quite precise at lower frequencies, too - only a few dozen nanoseconds too long of a pulse at pin 9. Similarly, one could get approximate divide-by-five or divide-by-nine from using pin 8 or pin 11, respectively.

One could also divide-by-seven by ANDing pins 12 and 8 in bi-quinary mode.

Congratulations, Salgat! Its a divide-by-n counter!

7. ### Iommi New Member

Jun 3, 2007
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I apologize, but I don't understand Salgat's answer to the "divide by 3" question. What exactly must I do in order to use 7490 IC's in a divide by 3 configuration. Thank you so much for your patience.

8. ### Salgat Active Member

Dec 23, 2006
215
1
Tie the pins QA(Q1) and QB(Q2) to the pins MR0, which has 2 inputs. By tying each output to the different MR0 inputs, the counter will reset when the counter reaches 3. A schematic might be needed if you don't understand :\

May 16, 2005
5,072
6