I2C I/O Expander PCF8574 hanging?

[NavMicroSystems]

Has anyone ever experienced problems with a PCF8574 hanging or blocking the I2C bus?

On one of my dataloggers I found that there had no data been logged for a number of days.

Due to the large number of Digital Inputs I have to deal with there is a number of PCF8574S on Board.

When I checked the system the PIC itself was running fine.
No Watchdog or Brownout Reset had occured during the period of time where data shold have been, but wasn't logged.
(There would have been a log entry, the RTC and Log-Memory are on a separate I2C-Bus)

I flipped some Inputs (which should have caused Log entries) but nothing happened.
(apart from the fact that the PIC responded to requests via RS232)

After a Powercyle everything was fine again.

To me it looks like the PCF8574 was hanging and required a Power OFF and ON.

[mister_e]

Hi Ralph,

did you use the INT output to read them or do you read them in a loop?

I had similar problem in the past but i used the INT pin to read them only when something change. I fix the problem by reading them in a loop.

Well it can be many things but that's what happened to me.

[mister_e]

mmm, i know you do many industrial equipement so i don't know how it could be revelant but, is there's any chance that some 'high voltage noise' came on an input and freez the chip???

I know that there's some diode inside but how efficient they are... i can't tell if they will beat 1N5817, UF1002-T or else like that.

[NavMicroSystems]

There are opto isolators on every single pin, and the PCFs are being polled

[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.