PCA9535: PCA9535 power up anomaly. Registers do not always default to values specified in data sheet.

We recently experienced some problems which were tracked to the power up behavior of the PCA9535's.

For many years we have been using these parts on at 2 dozen different products and we have never

had to manually initialize the Polarity Inversion Registers which, according to the data sheet (Page 9), default to 0's

(inputs NOT inverted) on powerup.   The recent problems manifested themselves as random occurrences of

unusual data (usually 10010001 ) on Input Port Register 1.  I subsequently determined that the bits which

showed 1's when they should have been 0's were actually being inverted - changing the actual input to 1

would restore the expected zero value.  This lead me to wonder if there was a problem with Polarity Inversion

Registers. I modified the code in one product to initialize the Polarity Inversion Registers to 0x00 and the random

problems seem to have gone away. Apparently the Polarity Inversion Registers are not being cleared on powerup all

the time. In this particular case the label on the IC's is as follows:

0CA5LHT

PCA9535

G4

Is this a known problem and has it been addressed? We risk having to revise many pieces of firmware and potentially offer free firmware upgrades for many customers who already have units which may experience these random problems.

Thanks.