Click the input switches or type the '0'..'9' and 'a', 'b' bindkeys to play with the circuit. See the next applet for an animated demonstration of the Hamming weight adder-tree.
To keep the circuit layout readable, the carry-in inputs of the multi-bit adders are tied to GND (logical 0) in the example. Naturally, these carry-in inputs could be connected to additional input pins, which would allow for 15-bits of input with the same number of adders. As 15 is also the highest number that one three-bit adder can generate, a 16-bit bitcounter would obviously require another adder, etc.
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