74LS90 Counting odd numbers only - Electronic Circuits Projects Diagrams Free

Joel Sparkman · 8th August 2007, 06:27 PM

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?

RODALCO · 8th August 2007, 08:36 PM

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 ?

Roff · 8th August 2007, 08:40 PM

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.

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8th August 2007, 06:27 PM	(permalink)
Joel Sparkman	74LS90 Counting odd numbers only 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?

8th August 2007, 08:36 PM	(permalink)
RODALCO	7490 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 ? __________________ There are more ways to get to Rome. Electricity, Electric clocks, Meters and Trains are great.

8th August 2007, 08:40 PM	(permalink)
Roff	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. __________________ Ron (aka Rube)