This web page:
states "There are both 7- and 8-bit versions of I2C addresses. 7 bits identify the device, and the eighth bit determines if it's being written to or read from. The Wire library uses 7 bit addresses throughout. If you have a datasheet or sample code that uses 8 bit address, you'll want to drop the low bit, yielding an address between 0 and 127."
I think that should read "...you'll want to drop the high bit, yielding an address between 0 and 127."
Wayne
The low bit specifies whether you're reading or writing, so that's the bit you want to drop (i.e. shift the byte left by one bit). But I realize the wording is somewhat confusing, so suggestions are welcome.
I haven't used the library yet, so I can't say how the code was written. But, as I was studying it with the intent to use it, this note seemed very confusing. Since I2C is a serial protocol, it has no inherent byte alignment issues. It's simply needs a 7 bit address. So, the question I'm trying to answer is, how are these 7 bits aligned in the value passed to a function that takes an address (and, BTW, I assume that an address is a char, or byte value? I'm nit sure, as the docs don't say.)
If the 7 bits are aligned to the LSB end of a char value, then you'd want to leave the MSB clear and you would end up with a value between 0-127 (inclusive.) However, if the 7 bits are aligned to the MSB end and you are to leave the LSB clear, then you can't wind up with a value between 0-127. You'll, instead, get all the even values between 0 and 256 (inclusive.) So, no matter how you look at it, the description is confusing, IMO.
Wayne
I changed it to this:
There are both 7- and 8-bit versions of I2C addresses. 7 bits identify the device, and the eighth bit determines if it's being written to or read from. The Wire library uses 7 bit addresses throughout. If you have a datasheet or sample code that uses 8 bit address, you'll want to drop the low bit (i.e. shift the value one bit to the right), yielding an address between 0 and 127.
I hope it's clearer. Yes, in a seven bit value, the high bit is clear, but in converting from the 8-bit value in a datasheet, you want to drop the low bit (which specifies whether you are reading or writing that address).