Re: Special XOR for half-adder and full-adder

Posted by cadet1620 on Mar 04, 2016; 12:24am
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Special-XOR-for-half-adder-and-full-adder-tp4029600p4029620.html

CMOS does prefer Nand over Nor. INACD^*, but my reading says that the issue is one of transistor size. Because holes have about 1/3 the motility of electrons, PMOS transistors need to be 3 times the width to have the same on resistance as their matching NMOS transistors.

Given 1 as the smallest size, then the Not gate will have a 1-sized pull-down and a 3-sized pull-up. A Nand will have 2 3-sized pull-ups in parallel and 2 2-sized pull-downs in series for total size 10. A Nor will have 2 1-sized pull-downs in parallel and 2 6-sized pull-ups in series for a total size of 14. The ratio gets worse as the number of inputs goes up.

For the transmission gate Xor you need both A and /A as well as B and /B. There are some tricky Xors that use pass transistors that can be used internally in a chip, but need to be buffered with a Not if you need full output drive.

--Mark

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^* I'm not a CMOS designer. This is all from book learning.