Hi Edson-
Yes, that's doable.  However, you can also use negative logic, if you reverse
the diodes.
David

Edson Brusque wrote:

> Hello Wouter,
>
> > - you have a pin 'left over' (or two, when you don't read). Use them all
> to
> > drive E in and-fashion (you can use a D-R and gate, I think that does not
> > count as active?)
>
>     This gave me an idea, just wait... I need to elaborate it:
>
>     Well, here we go. Sorry for attaching a 33KB bitmap on this email, but
> it's small. :)
>
>     When you're scanning the keyboard, RB3:RB0 are outputs and RB7:RB4 are
> inputs. You'll need the external pull-downs (about 10K). As we'll see below,
> the internal pull-ups wouldn't work.
>
>     Just do the keyboard scanning as usual positive-logic. When you have a
> key pressed, the corresponding input pin will go high when you put the
> corresponding output pin high.
>
>     As you can see, the LCD will ignore these signals, because you need to
> have RB1 AND RB2 HIGH for it to see a HIGH on the enable line.
>
>     I think R1-R8 can be 1K and R9 10K. The diodes can be 1N4148. This will
> make the LCD-ENA line LOW when RB1 OR RB2 are LOW. This is the normal
> keyboard scanning situation, where you turn RB0=HI, read ROW1 on RB7:RB4,
> turn RB0=LO RB1=HI, read ROW2 on RB7:RB4 and so on. You never will have both
> RB1 and RB2 HIGH when scanning the keyboard.
>
>     You can use negative logic for the keys if you invert the LCD-ENA signal
> with a transistor, but then it's not a only passive circuit anymore.
>
>     Have I won the award? So I want divide the price with Wouter. ;^)
>
>     Best regards,
>
>     Brusque
>
> -----------------------------------
> Edson Brusque
> Research and Development
> C.I.Tronics Lighting Designers Ltda
> Blumenau  -  SC  -  Brazil
> www.citronics.com.br
> Say NO to HTML mail
> -----------------------------------
>
>   ------------------------------------------------------------------------
>                 Name: keyb2.bmp
>    keyb2.bmp    Type: Pbrush File (image/bmp)
>             Encoding: base64

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