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I'm currently doing an assignment for school where I have to construct a MOD-56 BCD ripple counter using JK Flip Flops. This requires 2 BCD displays and 7447N drivers to interpret the bits from the flip flops as the clock cycles. After finally figuring out the gates needed to establish the correct MOD value I noticed something weird. When the circuit is initialized in Multisim, the 10s place BCD is initialized to 7 rather than 0, even though the clock hasn't cycled. I put some probe lights on the inputs to 7447N chip to see if maybe I could figure out why, and suddenly it was initializing to 6. I removed one probe, and it initialized to 2. This makes absolutely no sense to me. It shouldn't matter in the grand scheme of things since I only have to show that the counter does indeed reset after 55, and it does that perfectly once it resets to 00 after being incorrectly initialized to 70, but it bothers me that I can't figure out why. Any ideas? Here's the circuit I've been working with:

MOD-56 BCD counter

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  • \$\begingroup\$ Power-up is a moment of chaos until VCC climbs close to its 5V final value. I wouldn't trust a simulator to mimic reality for this event. A power-up reset circuit offers an initializing pulse that can be used to set those flip-flops to a known state. \$\endgroup\$ – glen_geek Dec 10 '17 at 19:34
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    \$\begingroup\$ 7447s don't have states, they're purely combinational logic. It's your flip flops that are powering up in certain states. \$\endgroup\$ – Finbarr Dec 10 '17 at 20:10
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Add a reset pulse in your simulation to set/reset each of those flip-flops to your desired state. Typical reset pulse from a commercial supervisory circuit is 200ms long.

That's why the chips have those inputs!

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