Electrical – I2c slave address confusion

Per your comment, your GPIO pins are not open-collector but are forcing both high and low. I2C requires open-collector on both the clock and data lines. Without this, the slave cannot, for example, ACK by pulling the data line low (since the master is actively pulling it high).

If you cannot set open-collector mode, another way is to leave the output data bit low and switch the pin mode between output (low) and input (effectively open-collector).

I know what you mean. I have seen these ambiguities in documentation too. That doesn't make it right, though.

I personally only communicate the address as a 7 bit value, whether in binary (0-1111111), hex (0-7F), decimal (0-127) or some other scheme. The R/W bit is NOT part of the address. It and the address are two separate fields that happen to be crammed into the same byte.

Unfortunately, even though the 7-bit address interpretation is correct and does seem to be the more common, there is a lot of bad documentation out there. This means you can't count on the next datasheet you read to do it one way or the other. When reading documentation, this is a issue you just have to be aware of and be extra careful to see how exactly the address is being presented. So far I haven't come across a datasheet where this couldn't eventually be figured out.

When writing your own documentation, please do the rest of the world a favor and express the "address" as a 7 bit value. If you want to show it with the R/W bit, call it the "address byte" or something to make this clear.