seems all to be correct!
the moment you write a 1 to a pin, the pin pulls up with max. 1ma, while =
the second fet is on. then, after a short time, this fet is turned off =
and the pin only sources the 100ua from the other fet.
so after writing a 1 to the pin, you can also use it as an input (with =
integrated pull-up)
tino

>-----Original Message-----
>From: Jan-Erik Soderholm [mailto:jan-erik.soderholm@TELIA.COM]
>Sent: Friday, February 13, 2004 7:32 PM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [PIC:] I2C with Philips PCF8574 remote 8-bit I/O expander
>
>John Plocher wrote :
>
>> I am starting to play with I2C and the 8574 I/O expander chip,
>> and I am running into some of those "I don't think I understand
>> the manual" types of problems.
>>
>> Manual: http://www-us.semiconductors.philips.com/pip/PCF8574.html
>>
>> I think I grok the cases where the chip is used as either 8 bits
>> of input or 8 bits of output, but I am clueless about section 7.3,
>> where it talks about using some pins as inputs at the same time that
>> other pins are used as outputs.  The reference to Figure 15 isn't
>> helping.  It sounds like the chip is mainly intended to be used as an
>> "all inputs" or "all outputs" device, but it has some voodoo that
>> can make it pretend to do mixed I/O if you line up various planets...
>
>It looks like when you want a pin to act as an input, you must
>set it HIGH (which also is the power on state). In that case an =
external
>device may pull the pin HIGH or LOW (and when LOW, sink the
>current from the 100uA currect source. You probably should take
>care not to write a LOW to that pin in this case...
>
>When using a pin as output, you just send a HIGH or LOW to that
>pin as you want. But note that when HIGH, it can only source 100uA
>(I think)...
>
>Now, the thing I don't understand, is the extra FET that, when ON,
>shorts the 100uA current source and ties the pin hard HIGH. But
>this might be an missunderstanding...
>
>Anyway, on the URL you quoted above, are also AN469, which
>better explains this. There are also a few very similiar devices
>(from Philips) but with separate I/O-direction control registers
>(also described in the AN469 app note).
>
>And, there are also the MCP23016, which is a very similar
>device from Microchip. You cold get three free samples to play
>with from them :-)
>
>> The other bit of confusion has to do with the "how are i2C slaves
>> intended to be used" department.  The timing diagrams imply that
>> you can read a slave's data by either
>>
>>         START, address-a-chip, READ, STOP
>> or
>>         START, address-a-chip, READ, READ, READ, READ, ..., STOP
>>
>> In the second case, I'm assuming that since the 8574 only has a
>> single 8-bit chunk of data to read, it simply resends the current
>> (possibly changing) states as the READs are processed.
>
>As I understands it.
>
>> Or (and
>> thus my confusion), is the multi-read case only for devices that
>> have more than one byte of data to read?
>
>No, I don't think so.
>
>> Obviously, I'm missing something basic here - can anyone share a
>> clue or two to help?
>
>I've never used the chip, just wild guessing :-)
>
>Regards
>Jan-Erik.
>
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