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STM8S208CB STM8S Unique ID significant bytes?

Question asked by dowdell.john on Jun 5, 2012
Which of the Unique ID bytes actually get used?

I'd like to use the unique ID of the microcontroller in a project to identify uC units on a comms bus. And as a product we would be using many of them. We have tried to avoid having to program IDs into them during production. I'd like to use the minimum number of bits in my application to ID the device. At the same time I'm trying to avoid having the same ID used twice.

At the moment I'm using four bytes. If there's bits I'm reading from the Unique ID bits that never gets used, I'd like to be able to ignore them but the datasheet doesnt mention anything about parts of the ID that dont get used. I suspect that its a production-time process that gets applied to all chips and so for the chip we are using some bits may not get used.

I notice that only the high byte of the x and y wafer coordinate bytes change between devices so I dismiss using the least significant bytes of the coord bytes. so my two least significant bytes of my ID are the x and y high bytes.
For the most significant byte of my four byte ID i'm using the least significant byte of the lot number. For the next byte I use the wafer number byte.

I'm looking to shave as many bits as I can here.
So, what is the highest and lowest value of coord byte that I can expect?
What is the highest value of wafer number? does it actually get up to 255 per Lot?

I'd be grateful for any assistance in this matter.
Thanks, in advance,
John Dowdell