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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