Question: LM75-A has a product ID that can be read but the LM75B and LM75C do not. If we read this product ID on LM75-C w/ I2C reading process, because LM75-C does not have this register, how will the LM75-C respond? Will I2C be locked by LM75-C?
Answer: For the LM75C there are keeper cells on the internal bus. So what I suggest you first do is write to pointer register 0x7 – some value other than 0xA1 as this is the product ID for the LM75C. Let’s say 0xFFFF. If it’s an LM75A you will get back 0xA1 and if it’s an LM75C the result will be 0xFF for the first byte. I suggest you do a double byte write and single byte read. If you write 0xFF00 then the LM75C will appear to lock the bus, but that only happens because it will transmit a 0 for D7. (please refere to the LM75 data sheet at www.national.com/ds/LM/LM75A.pdf)