James,
I think that if you set them high & then use them as inputs you are
OK. If you only read from them then the high current pulse output is
not enabled. The only time it is enabled is if you are writing to them
as outputs and are going through a low-high transistion.

"An additional strong pullup to V CC (I OHT ) allows fast rising edges
into heavily loaded outputs. This device turns on when an output is
written high and is switched off by the
negative edge of SCL. The I/Os should be high before being used as
inputs. After power on, as all the I/Os are
set high, all of them can be used as inputs. Any change in setting of
the I/Os as either input or outputs can be
done with the write mode. If a high is applied externally to an I/O
that has been written earlier to low, a large
current (I OL ) will flow to GND."

RP


On 02/11/05, James Newton, Host <jamesnewton@piclist.com> wrote:
> Has anybody used this TI PDF 8575 chip?
>
> http://focus.ti.com/lit/ds/symlink/pcf8575.pdf
>
> I'm working on a very low power application where we need a few more IO pins
> than what we can get on the microcontroller and the project lead is
> recommending this. It's basically an I2C addressable shift register, but it
> claims to allow any pin to be an output OR an input without having to set
> any direction bits. (!!!)
>
> I can't get my head around how it can have pins that are inputs, and others
> that are outputs without any direction registers in it. As far as I can see,
> it drives any pin that you write out a 1 to with 1mA to the Vcc rail. So
> they are fairly easy to overcome with an input that may need to drive that
> pin low. Sort of the standard open collector with a pull up deal. No problem
> so far.
>
> But then they have something called IOHT that drives any pin that set high
> hard to Vcc until after the last clock.
>
> "An additional strong pull-up to VCC (IOHT) allows fast rising edges into
> heavily loaded outputs. This device turns on when an output is written high
> and is switched off by the negative edge of SCL"
>
> So what are we saying? That its ok for some other device that is trying to
> drive a low into one of these pins to drive against the supply rail as long
> as you only do it for a little while?
>
> His statement is that this will require less power over all than will any
> standard available shift registers. Isn't even the 1mA pull up going to suck
> down the juice for any input that happens to be a zero? Not to mention the
> ton of juice whenever we write out new values to the output pins and that
> zero gets driven high at full power for the time between the bit and the
> last clock?
>
> Am I missing something here?
>
> I've been looking for a set of good low power shift registers (one in and
> one out), and I have to admit, I can't find anything that will pull less
> than about 100uA in normal operation (not including what ever it is driving
> as an output). This PDF 8575 has an Icc of 75uA or less in operating mode
> and will standby down to around 3uA.
>
> Help?
>
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