ok it works, but LED's are very dim
As far as I can tell the difference lies between the PCF8575 used in the demo and the PCF8575C which I'm using. Although I'm not exactly sure how to compensate for that difference just yet.
There is in fact a small difference in the schematic diagrams for PCF8575
and PCF8575C. The PCF8575C lacks the weak pull-up in the form of the 100 ?A
current source in the output I/Os. This is the reason that PCF8575's I/Os remain HIGH
after power-on and after being written HIGH. Without this current source, PCF8575C's
I/Os will be in high-impedance state after the initial strong pull-up transistor is OFF (it
is ON during the acknowledge phase)
quote from: http://www.electro-tech-online.com/general-electronics-chat/94967-i2c-pcf8574-pcf8575-i-o-expander-i-cant-drive-pin-output-high.htm and
• Difference between PCF8575 and PCF8575C are:
– No internal I/O current source (open-drain)
– TTL input levels
from http://ics.nxp.com/support/documents/interface/pdf/an469.pdf