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74LS90 Counting odd numbers only

Thread starter #1
I'm building a timer with eight 74LS90's in series counting down by ten from either 100KHz or 1KHz. For some reason, the fourth digit counts 1,3,5,7,9,1...
I have several .1 bypass caps at several points on the CB across +5V and ground, but not one on each '90. Has anyone seen this strange behavior?


Well-Known Member

Check that there is no contact bounce on the clock input, even though it is the 4th digit.
Check soldering between output of 3rd digit and input to 4th digit.

Check the jumper between terminal 1 and 12 to make it count as a ten instead of a five counter.

Are all your R9 1-2 and R0 1-2 at the correct level ?


Well-Known Member
I think this could be caused by applying the clock from the 3rd chip to the B input of the 4th chip, instead of the A input. If the A input is not clocked, the A output could be stuck at a logic "1", causing the count to be 1,3,5,7,9, as described.
Hi Ron, treating the IC like a pipeline of JK flip-flops probably that the first flip-flop was tilted ["burned" (crashed)] with a bad usage of IC like high difference of potential (high supply voltage) when using the 74ls90.


Well-Known Member
Most Helpful Member
Otay, I agree but considering the thread is 11 years old? :)

Navigating in Internet this thread was showed (googling), read the thread and concluded the subject, not saw the date at begin :). The old 74ls90 won more information about it, in the old thread :). And maybe I win a friend, Ron feel free for questions, you are welcome.

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