Never mind, I found it. In binary, a standard 2 bit (+ carry) full adder is
also counting the number of ones. To expand that, one must simply add the
results from more than one full adder.

http://www.circuitcellar.com/library/eq/144/5.htm

A full adder is shown here:
http://et.nmsu.edu/~etti/spring98/electronics/tapper/img00052.gif and
explained in dizzying detail here:
http://et.nmsu.edu/~etti/spring98/electronics/tapper/bcdpaper.html

Basically, for inputs A B and C, the outputs S0 and S1 are:

S0 = (A XOR B) XOR C

S1 = ( C AND (A XOR B) ) OR (A AND B)

---
James.
 


> -----Original Message-----
> From: James Newton, Host [mailto:jamesnewton@piclist.com] 
> Sent: 2007 Jan 29, Mon 18:46
> To: 'Microcontroller discussion list - Public.'
> Subject: RE: [EE] Digital circuit that counts 'ones'.
> 
> All I seem to have is at:
> 
> http://www.piclist.com/techref/microchip/math/bit/countbits.htm
> 
> But that is programmatic rather than discrete logic.
> 
> I seem to remember such a thing but I also can't find it with 
> a short dogpile (.com) so I will be watching to see if anyone 
> comes up with it.
> 
> ---
> James.
>  
>  
> 
> > -----Original Message-----
> > From: piclist-bounces@mit.edu
> > [mailto:piclist-bounces@mit.edu] On Behalf Of Carlos A. Marcano V.
> > Sent: 2007 Jan 29, Mon 17:30
> > To: Microcontroller discussion list - Public.
> > Subject: [EE] Digital circuit that counts 'ones'.
> > 
> > This one is making me go crazy. I know there is a 
> well-known solution 
> > for detecting the number of 'ones' in a byte using logic 
> gates (just 
> > as you might use XOR's for parity check) but I just can't simply 
> > remember!
> > 
> > Do any of you guys do remember it?
> > 
> > Regards,
> > 
> > *Carlos Marcano*
> > -Guri, Venezuela-
> > --
> > http://www.piclist.com PIC/SX FAQ & list archive View/change your 
> > membership options at 
> http://mailman.mit.edu/mailman/listinfo/piclist

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