I'm using a MC9S08LC60 MCU and I have VREFH and VREFL pins available externally in the micro-controller.
I want to connect this pins to different values than VDD and VSS, but I read the following instruction in the data-sheet:
"When available on a separate pin, VREFL must be connected to the same voltage potential as VSSAD."
Why is that? Why would it be available externally and not be connected internally in the MCU if the user has no control over it?
And why VREFH can't be lower than 1.8V?
Having the VrefH and VrefL pins externally accessible would permit additional noise filtering to be applied - potentially very useful for 12-bit resolution mode.
The lower limit on VrefH is similar to the lower limit of Vdd. I presume that, for an even lower VrefH level, the ADC accuracy would deteriorate to an unspecified level.