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I purchased a few SN74HC4020 ICs, which is a 14-stage ripple-carry binary counter. The SN74HC4020 datasheet shows output pins for bit 1 (pin QA) and bits 4-14 (pins QD-QN). So in other words the output for bits 2 and 3 are not exposed. Why are these 2 bits not exposed on this IC and is there a work-around if I'm trying to divide by 2^2 sometimes, or 2^3 sometimes, along with 2^14?

I'm guessing there's some binary arithmetic involved using some logic gates that I'm not smart enough to know at first glance. Or perhaps the other answer is that I have to chain another 4-stage counter IC with this one in order to divide by 2^2 or 2^3.

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    \$\begingroup\$ probably so that it would fit into the number of pins on the IC \$\endgroup\$
    – jsotola
    Jul 12, 2021 at 21:50
  • \$\begingroup\$ use another counter for the missing bits \$\endgroup\$
    – jsotola
    Jul 12, 2021 at 21:51
  • \$\begingroup\$ like a 74HC4024 \$\endgroup\$
    – ErikR
    Jul 12, 2021 at 21:53
  • \$\begingroup\$ @jsotola . . There are packages with sufficient terminals in the data sheet. And still missing B and C \$\endgroup\$
    – Marla
    Jul 12, 2021 at 21:54
  • \$\begingroup\$ if you need all bits of the counter use a CD4040 (or several in series if you need more than 12 bits) \$\endgroup\$
    – Jasen
    Jul 20, 2021 at 1:28

1 Answer 1

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It's a marketing decision, cast into stone when the CD4020A was designed ca. 1970.

There was a large cost and size jump to go to more than 16 pins at the time (18 and 20 pin 300mil DIPs didn't come along until later). and 24-pin DIPs such as used for the CD4067 were huge (600 mil wide). These were the packages available in 1973 (the CD4020A was already available then):

enter image description here

(From 1973 RCA Solid State COS/MOS Databook)

As you can see, military and aerospace applications were heavily represented and low-cost commercial 'plastic' packages were limited to 14 and 16 pins.

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  • \$\begingroup\$ Thanks. Not that it matters to my application, but just out of general curiosity, what was a ripple counter with the 2nd and 3rd output missing actually useful for, circa 1970 or thereafter? And +1 for producing catalog pages from 1973. \$\endgroup\$
    – acker9
    Jul 12, 2021 at 22:12
  • \$\begingroup\$ We would use chips like that as frequency dividers or timers. You might only use a single output (with a jumper to select, perhaps) and the first output is useful for calibration or testing if the frequency is low. Now, of course, most everyone would use an MCU in most such situations. \$\endgroup\$ Jul 12, 2021 at 22:14
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    \$\begingroup\$ @glen_geek We needed those long leads so that RF would transmit to our nearby AM radio, so we could hear them working. (We didn't have anything at GHz speeds and everything, or some beat frequency of it, nestled nicely into the AM band. You instantly know when they stop working, as they stop radiating. ;) Frequencies were low, surface mount was new, and it was easy to use a special tool I still have to slightly bend all the leads on one side at a time, then solder one side first with a slight tilt, switch to the other side, and repeat. I needed the length, anyway. Can't speak for others. \$\endgroup\$
    – jonk
    Jul 12, 2021 at 23:53
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    \$\begingroup\$ @SpehroPefhany Thanks. I appreciate the effort. The ones I have narrow down to a tip width of about 5 mm, which I consider "slightly short of perfect." There are some other details about that one (more than I'd like to write about right now) that make it still less interesting, though. (This reminds me of an 1880 scythe handle I used to use to harvest an acre of wheat every so often. Easy, could do an acre in a day with it. Lent it to neighbor who immediately broke the handle in half. Spent 10 yrs buying handles. The best of them was 4 times more work and I couldn't do an acre if I tried.) \$\endgroup\$
    – jonk
    Jul 13, 2021 at 1:05
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    \$\begingroup\$ @jonk I can identify with wanting to preserve a tool that is now rare and works very well. I have a 24"/600mm steel rule with decimal inches 1/10, 1/100 and mm 1, 0.5 (2 edges on 2 sides) that I have not seen available in many, many years. One I lost in a company upheaval was a large bright magnifier with a clear base to let light in and built in flush rule, maybe 20 or 25mm wide and fine divisions. \$\endgroup\$ Jul 13, 2021 at 2:50

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