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I have a really simple circuit, a 2-digit decade counter (0 to 99) using two 74LS190s. It works fine when counting upwards (0 .. 9, 10 .. 19, etc), but when going downwards, it seems to decrement the second digit once when the first loops back to 9, and again on 8. So the sequence looks sort of like this: ..., 22, 21, 20, 19, 08, 07, ...

circut diagram

The inputs ANY, DOWN and RESET are just LOGICSTATEs.

What am I doing wrong, and how can I get the correct behaviour (..., 22, 21, 20, 19, 18, 17, ...)?

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What's wrong is that You're using RCO\ from U2 to clock U5, when you should be using U2 RCO\ to enable U5.

You can fix the problem by connecting U2-11 to U5-4 and parallel clocking both counters as shown in the red box, below.

The legend for Figure 2 is wrong, since what's shown is the way to cascade either '190 or '191 counters using RC\ and CE\ .

enter image description here

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  • \$\begingroup\$ U2-11? That's the reset input. I followed the figure, connecting U2-13 to U5-4, and U5-14 to the ANY input. Now, counting up still works, but counting down has a 1 cycle delay, making the sequence "..., 21, 20, 29, 18, 17, 16, ...". I should also mention that the ANY input will be a button, not a clock per se. \$\endgroup\$ – Kroltan Feb 25 '16 at 11:57
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    \$\begingroup\$ @Kroltan: Sorry about that, I misread your schematic. If you're not using a debounced pushbutton switch, that may be part of the problem. I'll run a simulation with a good clock sometime today and see what happens. Later... \$\endgroup\$ – EM Fields Feb 25 '16 at 12:26
  • \$\begingroup\$ No problem, but right now I'm just using plain LOGICSTATEs, which switch immediately with no noise. This circuit will be controlled by another circuit which sends the correct pulses. \$\endgroup\$ – Kroltan Feb 25 '16 at 12:53
  • \$\begingroup\$ I fixed it by making the clock (ANY) last longer than the D/U (DOWN), by using the method with 2 40106s from this question: electronics.stackexchange.com/questions/30689 \$\endgroup\$ – Kroltan Feb 25 '16 at 13:10
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    \$\begingroup\$ @Kroltan: Great! Now I can take that sim off my plate. :) \$\endgroup\$ – EM Fields Feb 26 '16 at 4:09
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Use the output pin 13 (RCO) to enable count on U5 ( pin 4) and clock both counters together (tie pins 14 together).

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