Oops, sorry, that is correct.

Anyhow, driving 8 CMOS inputs from 1 CMOS output
should not be a problem, which was the initial
question, I think.

Jan-Erik.


On 2010-09-13 00:36, Steve Maroney wrote:
> With the MCP23S17, both the SPI and I2C versions have address pins. On th=
e SPI version, you can put 8 chips on one CS line.
>
>
> Best Regards,
> Steve Maroney
>
> Business Computer Support, LLC
> Mobile Phone:504-914-4704
> Office Phone: 504-904-0266
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>
>
>
> -----Original Message-----
> From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf =
Of Jan-Erik Soderholm
> Sent: Sunday, September 12, 2010 5:02 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] SPI Fanout with large numbers of MCP23S17's
>
> 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 wi=
ll
>>>> 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
>>>
>>> __________________________________________________
>>> Fale com seus amigos  de gra=E7a com o novo Yahoo! Messenger
>>> http://br.messenger.yahoo.com/
>>
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