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Everywhere I encounter either asyncronous (ripple counter) binary counters or syncronous ones, the application uses JK flip flops with the J and K inputs tied together, such as described here: enter image description here

From my understanding this reduces the functionality of the JK flip flop to that of a D type flip flop, so why not just use D-type flip flops? I've seen SIPO shift registers constructed form D type flip flops and not JK ones, so I'm a little confused why this seems to be convention.

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  • \$\begingroup\$ JK flip-flops toggle when J=1 and K=1. if you were to build a counter outa D flip-flops, you would need a full binary adder for each bit. \$\endgroup\$ – robert bristow-johnson Apr 24 '17 at 18:58
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    \$\begingroup\$ JK with inputs tied together is a T-Type not a D-Type. \$\endgroup\$ – Trevor_G Apr 24 '17 at 19:18
  • \$\begingroup\$ @Trevor my mistake, I guess the question still holds for why JK instead of T type though? \$\endgroup\$ – Alex Apr 24 '17 at 19:21
  • \$\begingroup\$ @AlexEshoo, dunno, probably because physical t-types devices are rare to the extreme. \$\endgroup\$ – Trevor_G Apr 24 '17 at 19:30
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    \$\begingroup\$ I think part of the reason is just availability. Because JK flip flops can be configured to act as RS, D, or T flip flops, there's really no reason to mass-produce the other types (save for simplicity, but with modern technology, at least for discrete flip flop ICs, it doesn't really cost any more to add a few transistors). \$\endgroup\$ – Hearth Apr 24 '17 at 21:32
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For some reason, device based on D flops have largely standardized on using the rising clock edge as the capture/output event, but JK flip flops have standardized on using the falling edge. If a ripple counter is supposed to count upward, each bit should change its state on the falling edge of the previous bit, and so using JK flops makes it possible to build an up-counting ripple counter where the data output of each stage can be fed to the next stage.

I personally think it would have been helpful to have a wider range of devices that are based on D-style flops operate on the falling edge rather than the rising edge, but for whatever reason JK flops are just about the only common "jelly-bean" logic chip that uses the falling edge.

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