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I was looking for an 8-bit parallel to serial shift register to use in a project I am making. Initially, I thought that the 4021 would work perfectly for what I need, however, after looking at the datasheet I am a little confused.

Below is a screen-grab of the TI Datasheet (http://www.ti.com/lit/ds/symlink/cd4021b-q1.pdf) (pg 6). I am confused as to why the maximum clock input frequency is listed as a minimum of 3 MHz.

Picture from TI Datasheet

I have never heard of an IC having a minimum input frequency so wondered if the problem is more just my understanding of what the information is telling us. At the top of the datasheet it also talks about the clock being run at a typical frequency of 12 MHz, which is a lot higher than I was planning to run it at.

As an alternative I found the 74HC165 (https://assets.nexperia.com/documents/data-sheet/74HC_HCT165.pdf) and any insight into if this is better or not would be appreciated as well.

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  • \$\begingroup\$ For 5V supply the typical minimum input frequency is 6MHz. But for some 4021 the frequency is too high. So to ensure that your 4021 will work with all 4021 the input frequency should not extend 3MHz \$\endgroup\$ – G36 Nov 21 '18 at 19:20
  • \$\begingroup\$ What are you connecting it to? That might matter more. \$\endgroup\$ – CrossRoads Nov 21 '18 at 19:20
  • \$\begingroup\$ There historically have been parts which would not work below a minimum frequency, for example some early processor designs. But this is not that, but rather a worst-case maximum. \$\endgroup\$ – Chris Stratton Nov 21 '18 at 19:29
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You need to be very carefully reading this: it says "the maximum input clock, under all conditions that we specified as OK, is at least 3 MHz".

That says, when you use 2 MHz, under all condition, that's below the max frequency, and hence, fine.

Under some conditions (e.g. high supply voltage, small capacitive output load), you might be reliably achieving more than 3 MHz, but that will require you to make sure these conditions are met.

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They are expressing that the maximum clock frequency is a function of VCC and temperature. At 2V you won't get parts that max out below 6 MHz, but they could work higher, 17 MHz is typical, and they don't spec a max which means if that chip "works" at 18 MHz then great, but they won't guarantee it. So at 2V your max clock that you can design for is 6MHz.

All that is at 25C, if you need harsher temperatures, then use the appropriate temperature column.

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