A while back a reader contacted me and asked why the 4017 has such a strange pin out.
Time for some industrial archeology!
1st things 1st… what’s a 4017? It is a decade counter. Every time a clock signal is made the counter makes active a single output from a set of 10.
The design is old. The part that was analyzed was from 1983…. but I think the design goes back into the 1970’s! A good example of early CMOS design (a semiconductor which has but P-FET and N-FET structures).
The schematic of this part is like this:
And now the mystery is solved. Note that there are 5 flip flops and they then go to a bit of logic above (a bunch of NAND gates). The flip flops are in the inside of the chip. The design appears to have a single metal layer. Note that outputs 1 and 5, for example, need the signal from the same flip flop. In order to do this the designer put the logic near the pads and then went for the easiest possible routing. This results in what appears to be very non logical outputs being next to each other.
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