Jinx escreveu:
>> It is possible to give up the 74HC251 and achieve almost the
>> same result (each strobe pin reads six keys instead of eight).
>>     =

>
> That's very true Isaac. However I do need 8 for this project (it
> may even go to 10, luckily a couple of I/O is still available). For
> another project I'll need at least 15, so will add a second 74HC251.
>   =


For up to 6 keys my method saves one I/O. For 7 to 8 keys it loses by
one I/O. For 9 to 12 both methods use the same number of pins.For 13 to
16 it loses again by one I/O, and so on up to 24 keys...

But don't forget to count the ICs...


> They're left over from a short run and I was just doodling, basically
> looking for an excuse to use them. The page was put up as an
> example of I/O expansion and possibly a discussion point
>   =

 It is always good to discuss alternative methods for doing things. Your
method is better suitable where the inputs are not keys (no dry
contact), or the signals cannot be strobed or they can only pull high.

>   =

>> Normally all the strobes are at high-impedance (it is good to
>> pull them up with a high value resistor)
>>     =

>
> The LCD's internal pull-ups might be enough for that
>   =


The strobe pins I'm talking about are the key scan strobes, they go only
to the low side of the keys, not to the LCD pins.

Regards,

Isaac
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