----- Original Message -----
From: Daniel Serpell <daniel@ATMLAB.UTFSM.CL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, June 26, 2003 4:33 PM
Subject: Re: [PIC]: 8-byte vertical adder?


> On Thu, Jun 26, 2003 at 05:43:23AM +0100, Sergio Masci wrote:
> > If I understand correctly you are clocking in all bit 7's followed by all
bit
> > 6's down to all bit 0's
> >
> > Try the following:
> >
> >         acc = 0
> >
> >         for (int j=0; j<8; j++)
> >         {        acc = (acc << 1) + count_one_bits(read_port())
> >         }
> >
> > count_one_bits() could be implemented as a lookup table
>
> It's more efficient to do directly. Each loop iteration can be
> done in 15 cycles, if you unroll it and add the "acc=0" at the
> begining, you get 15*8+1 = 121 cycles (and 121 program words).
>
> Seems that it's the faster version.
>
>     Daniel.

Assuming that this is actually giving you what you want and that acc is 16 bits,
the first 3 iterations of the loop can be perfomed on 8 bit acc and the
remainder on 16 bit acc. Also your lookup table will require 256 entries

Regards
Sergio Masci

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