Alessandro Zummo wrote:

>   why don't we have a 4066 who activates the switches only when
>  the lcd isn't selected?

 (Nice diagram)

  Well, the long and short of it is keeping it simple.  You use a whole
4066 to connect/ disconnect four switches, and you use an extra PIC pin
function to perform the enable.  If you are going to devote these
resources, you could do it a lot better, such as using a shift register
chip to read up to eight switches (needing pull-up/ pull-down resistors)
with three connections to the PIC (latch data and shift may be commoned
to the LCD data pins and serial data, being an output may either go to a
separate PIC pin or be commoned to a third LCD data pin using a 33K
resistor).

  Or, continuing with the theme of the analog switches, a 4051 could be
connected to eight switches (no pull-ups here), a pull-up (4K7) on its
common and the ubiquitous 33K resistor to one data line while the other
three select the 1-of-8 on the 4051.

  The theory here is that an extra IC, be it 4066, 4051 or 74HC165
(shift register) COSTS - money, space and PCB holes (though if you are
using SMD, the latter are trivial).  However, the manufacturing cost for
those three ICs is almost identical, so IF you are going to use an extra
IC, you might as well get the maximum functionality.  Otherwise, the
discussion was about avoiding extra ICs.

  Here's a question to the professionals; am I correct here on costs?

  Board space: 1 16-pin IC = 14 resistors (through-hole)
  (& stuffing) 1 16-pin IC = 8 diodes (through-hole)
               1 16-pin IC = 4 resistors (SMD) = 4 diodes

  Money:       1 16-pin IC = 16 resistors = 8 diodes

  Cheers,
        Paul B.