Why is the 14-stage SN74HC4020/CD74HC4020 binary counter missing 2 output pins, leaving 2 bits inaccessible?

Question

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.

probably so that it would fit into the number of pins on the IC — jsotola, Jul 12 '21 at 21:50
@jsotola . . There are packages with sufficient terminals in the data sheet. And still missing B and C — Marla, Jul 12 '21 at 21:54
if you need all bits of the counter use a CD4040 (or several in series if you need more than 12 bits) — Jasen Слава Україні, Jul 20 '21 at 01:28

score 5 · Accepted Answer · answered Jul 12 '21 at 21:59

5

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):

(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.

answered Jul 12 '21 at 21:59

Spehro Pefhany

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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. – acker9 Jul 12 '21 at 22:12
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. – Spehro Pefhany Jul 12 '21 at 22:14
+1 for that ancient photo (my copy is buried somewhere) Love those SMT flatpaks, but I wonder why their leads were SO long? – glen_geek Jul 12 '21 at 22:55
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@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. – jonk Jul 12 '21 at 23:53
@glen_geek (or anyone): By the way, if anyone knows where I can find [another one of these flat pack lead-bending](https://i.stack.imgur.com/rj2e8.jpg) tools, please do let me know. I think they are unobtanium now and I'm treating this one a lot better than I did decades ago, now that I am not so sure I can get a good replacement for it. You can see that my only remaining tool is a little worse for wear. But still perfectly functional, luckily. – jonk Jul 13 '21 at 00:15
@jonk Looks pretty similar to the Xcelite [DN54GV](https://i.imgur.com/z3624ij.png) (Digikey, for example) but I'm not sure how wide the tip is on yours. McMaster also has [some](https://i.imgur.com/KSwvtIf.png) with static dissipative handles and spring. Similar ones (sans antistatic handles) should be available from jewellery supply houses. Also the original Diamond Diamalloy DN54 show up on eBay from time to time. Last I looked there was a way to set up auto-notifications upon listing. – Spehro Pefhany Jul 13 '21 at 00:54
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@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.) – jonk Jul 13 '21 at 01:05
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@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. – Spehro Pefhany Jul 13 '21 at 02:50

Why is the 14-stage SN74HC4020/CD74HC4020 binary counter missing 2 output pins, leaving 2 bits inaccessible?

1 Answers1