I2C I/O Expander PCF8574 hanging?

[Dave]

NavMicroSystems, I have had this problem in the past and I found that if before reading the PCF8574's I reset the pins used for inputs to a high state the problem went away. I beleive the output register internal to the PCF8574's is being corrupted.

Dave Purola,
N8NTA

[NavMicroSystems]

Dave,

many thanks for the hint.

Can you remember in detail how you solved the problem?

The only way I see is to write $FF to the PCF right before reading it.

[Dave]

Yes, I used a scope and looked at the port I/O pins of the PCF's in question and found that pin's used as inputs and programmed for high states were indeed low and after removing the optoisolators from the sockets that were feeding the inputs the port I/O pins were still low. This prompted me to re-write the port I/O pins used for inputs to high states before reading there state's. I have had no problems since....

Dave Purola,
N8NTA

[NavMicroSystems]

Dave,

many thanks!

[mister_e]

That's interesting. Kinda TRIS refresh. Well it make sense, if we RTFM they say clearly

...The I/Os should be high before being used as inputs

Something to remeber for all here!

Thanks Dave.

[NavMicroSystems]

That's interesting. Kinda TRIS refresh. Well it make sense, if we RTFM they say clearly

Originally Posted by TFM
...The I/Os should be high before being used as inputs

Something to remeber for all here!

Thanks Dave.

Steve,
you are right,
that's what the datasheet says,
but it is not really clear.

When initialising the PCFs I do first write $FF to it.
From that point on there are only READs.

So the the I/Os have been set HIGH before being used as inputs.
(The System has been running for about six months without any error, last week was the first time I found a gap in the log.)

The DataSheet doesn't tell that you have to set the I/Os HIGH before every single READ.

However,
if that solves the problem I can live with it.
(I have already changed my READPCF-Routine)

Thanks again to Dave for pointing that out.

EDIT
OK, that takes some extra clock cycles, so I had to recalculate the timing.

I'll put the new FIrmware Live on Monday and let you know wether or not the problem occurs again.

[badrad]

I am trying out the PCF8575 - 16 bit expander in my application. The high byte will be connected to LEDs for output, and the low byte is connected to switches.

For your application using the PCF8574 - did you use the I2CREAD and I2CWRITE commands control the bit expander? Would you mind posting a snippet of your code example on how you control the device?

Thanks.