Electrical and Computer Engineering 2277A/B Lecture 23: Binary Adder Circuits
20 Jan 2021
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Adder/subtractor circuits perform calculations in signed two"s complement format. Half adder: a digital circuit that adds two one-bit numbers and outputs a sum bit and a carry bit. Full adder: a digital circuit that adds two one-bit numbers to an incoming carry bit and outputs a sum bit and an outgoing carry bit. Let the two one-bit numbers be x and y and the incoming carry bit be z . A full adder circuit consists of two half adders and an or gate. Adds two 4-bit numbers, and , and an incoming carry bit, c0. 9 inputs, so complete truth table has 29 = 512 rows. Assuming , , and c0 are updated simultaneously, outgoing carry bit, c4, does not update until carry results propagate through all 4 full adders. Assuming , , and 0 are updated simultaneously: 1 will update after xor gate delay + and gate delay + or gate delay.
