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I have been using the 74LS161 for some tinkering, and then switched to the CD4040 because it had 3 times as many counter stages in the same package size, and I don't need preset for my purpose.

Early on I was intrigued though by how much it is different. A 74LS161 increments on the rising edge of the clock input. Whereas the CD4040 increments on the falling edge of the clock, thus effectively making the clock itself another, the "zero-th" counter stage. This is a nice feature.

However, now I have a problem where the 74LS161 was really good for: the Ripple Carry Output, it's missing with the CD4040. I need something to trigger an S-R-latch, resetting it when the counter overflows. And I do not want to now hook up 12 lines to a 12-input NOR gate cobbled together from three chips that I might need for that. If I could just have that ripple carry output, which is exactly one clock pulse wide. Is there any way I could derive that?

I'm thinking of a falling edge of the last counter stage. How do I make the falling edge trigger an SR latch? Maybe instead of the SR latch I use a single edge triggered D-flip-flop, invert the last counter stage so that have a rising edge with which I can trigger the flip flop while holding D high. Then I could use an asynchronous reset and clear which most of them have, including the 74LS74 that I have on hand.

So is that the way to go or is there any other way of deriving a ripple carry like single pulse from a 4040 counter? Or a general technique to turn a binary counter into a Johnson counter?

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  • \$\begingroup\$ What clock frequency do you want it to run at? \$\endgroup\$
    – Bruce Abbott
    Aug 2, 2020 at 20:02
  • \$\begingroup\$ Hi again @BruceAbbott, I'm starting with 4 MHz and the SR latch was to serve as the gate of that main clock via a NAND gate to an active-low Write-Enable line. \$\endgroup\$
    – Gunther Schadow
    Aug 2, 2020 at 22:42
  • \$\begingroup\$ @GuntherSchadow A falling edge can be used to trigger a 74121 or 74123 one-shot. I didn't read your details too much, yet. But you write something asking about that possibility. So there it is -- a way. If it helps any. \$\endgroup\$
    – jonk
    Aug 3, 2020 at 3:41

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You can use a 74xx73 J-K FF, which is negative edge triggered. There are hack-y ways of doing it with resistors & capacitors etc.

One of the fundamental differences between the 4040 and the 74xx161 is that the latter is a synchronous counter and the former is a ripple-carry counter. It can be a critical distinction in some applications. That's why there is a terminal count output on the '161 — to allow multiple 4-bit units to be cascaded into a wider synchronous counter. Otherwise you could just use the MS bit.

A Johnson counter could be made with a shift register. You require a lot of flip-flops to make a counter with 4096 states. 2048, to be specific.

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