74HC390: Ripple Counter is Counting Rubbish

[rstofer]

Only ground the unused INPUT pins.

It might be fair to say that some input pins need to be held high.  Grounding wouldn't work well.
Nevertheless, every input needs to be a some defined voltage within the specified range of either logic 1 or logic 0.

I don't prefer to tie inputs directly to Vcc - in the bad old days of TTL, this was considered poor practice and a resistor was almost always used to tie signals high.  OTOH, a solid ground, as appropriate, was good practice.  Page 2 bottom left paragraph

http://alt.ife.tugraz.at/datashts/nsc/f6732.pdf

[Brumby]

Fair point.

High or low ... just don't leave unused inputs floating.  But leave the outputs alone.

[amyk]

The oscillation on the optocoupler in the original circuit is interesting, does anyone have a good intuitive explanation? Ringing usually happens only at the edge, but here there's not much of an edge at all and the signal oscillates all the way up. Perhaps it's a combination of high source impedance and inductance?

Side quest: Why is it that with the SN74HC14N (a CMOS device) the signal is faster but more jittered?

Let's use the correct precise technical term here - "jitter" refers to cycle-by-cycle variance in the period of a periodic signal, whereas what you're seeing is more accurately described as "ringing".

Why does the faster edge cause more ringing? A simple analogy can help explain. Remember that every element in a circuit, including the wires and breadboard, contain a combination of inductance, capacitance, and resistance. The capacitance and inductance act mechanically like a weight on a spring; imagine holding one end of a spring, and a weight hanging from the other end. If you move your hand slowly up or down, the weight will just follow the movement. If you move your hand quickly, the spring will stretch more, and then the weight will oscillate up and down several times before settling in its new position. The quicker you move your hand, the more the weight will oscillate.

You can already see the effects of parasitics here, but at much higher frequencies (RF, microwave) they become so significant that they are effectively the only way to design a circuit!