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PCF8574A: PCF8574A not responding to 0x71 address for A0, A1, A2 grounded

Rashmitha Ramesh Nair
Intellectual 455 points
Part Number: PCF8574A

Hi,

I am trying to use the IO expander chip PCF8574A to READ ports P5-P7.

I do not have any devices connected on these ports, the pins are just pulled up or pulled down so that they return either 1 or 0 (these help with binary patterns for identifying different boards)

I understand from the datasheet that the device should be given a control address of 0x71 if A0, A1, A2 are grounded and then the device responds with the data P0-P7.

I have been struggling with these for the past few months and now it turns out that the device is responding to 0x38 which is its slave address. When using 0x71 there is either no response at all or P0-P7 appear zero consistently even if some of the ports are pulled up.

I am confused as to how the datasheet can be giving such wrong data. 

Question: Is this a known issue?

I just want to make sure I am using the chip in the right way. I can read the data correctly now however the inconsistency with the datasheet is worrying.

Regards,

Rashmitha

  • +1
    Guru 271010 points

    An I²C address has 7 bits. The first byte of an I²C transaction includes both the address and the read/write bit. This byte might be called the address byte, but it would by wrong to call it the address.

    The PCF8574A datasheet calls it the 8-bit address, which is at least misleading.

    Anyway, what matters is how your software/hardware on the master handles the address. If it is a 7-bit value, you must use 0x38. If it is an 8-bit value and includes the R/W bit, you must use 0x70 or 0x71.