LakisFM1 wrote:
> I want to use the outputs of the pcf8574 by turning on LED's but there s a
>  problem.
>  As i saw in the datasheet of PCF8574 the outputs are STRONG High only
during
>  the up transition of the pulse and doesn't stay in this status.
>
>  So the Led turns on for a small time and then goes off.
>  So maybe  need an external Latch to keep the outputs of PCF8574's state?.

No!
I actually don't have the original datasheet, but I do use the PCF8574 (I have
read many articles about them).
The device has 8 combined in-/outputs, the outputs are realized as open drain
with pull-ups. So every bit can be pulled low while the output is high. The
pull-up will source something around 0.4mA. The output driver can sink up to
20mA, but is not able to source anything (because it's open drain).
And that's how you use them as inputs: Write FFh to the outputs once (power on
state is FFh, too) and read back the state of the inputs how often you want.
As output: Just connect LED through resistor to VDD. Writing a zero to the
according bit will light the LED, a one will switch it off (negative logic).
To other logic devices: Just connect any CMOS or TTL input you like (positive
logic). All outputs will be latched until the next succesful write cycle to
the PCF8574, no read cycle will modify the output drivers.

Hope it helps,
    Florian