To let 1 CMOS output drive (only!) 8 CMOS inputs
should never be any real problem. I saw a circuit
with at last 100's of driven CMOS inputs when I saw
your questions at first. You never told us that it
was only 8 inputs you was looking at, did you ? :-)

Note also that the I2C version has 3 address lines,
so you can run 8 devices on the same I2C bus without
separate address och chip-select lines from the PIC.

In the SPI case you need to have a separate CS line
for each MCP23S17, either as separate I/O pins
or through some extra address decoder chip.

Jan-Erik.




On 2010-09-12 23:45, Forrest Christian wrote:
>    Unfortunately I need the pins to behave much like a pin on a
> traditional PIC.. that is, being able to be individually driven high or
> low or read as an input.
>
> Basically I have weak pullups/pulldowns on everything - and then I set
> everything to a weak high pullup then pull one pin low and see which
> other pins went with it.  Then do the same for a pulldown, except I pull
> one pin high.   Repeat for each of the 128 pins...
>
> I think I'm just going to try this with the eight I need for this design
> and see how it works.  I realized I actually have two SPI devices on
> this pic so perhaps I'll be ok...
>
> -forrest
>
> On 9/12/10 7:10 AM, Isaac Marino Bavaresco wrote:
>>    Em 11/9/2010 22:36, Forrest W Christian escreveu:
>>>     I am building a test apparatus (ok, it's a cable mapper) which will
>>> require around 256 bits of I/O on a pic system.  Maybe even more.
>>>
>> If you don't need the pins to be bi-directional you could use the
>> 74HC595 (output only) and 74HC597 (input only). They work serially and
>> can be daisy-chained, you would save all the chip-selects.
>>
>> Isaac
>>
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