Can i use ADC in mcf5282 for voltage comparator

[tama]

Hi Guys

I am bit new to micro controller based designs.  in my current project i am using a mcf5282 processor. it has a inbuilt adc in it, with 8 analog input and a ref high(VRH) and low voltage (VRL) port. on my board i have 3.3 and 5v supply and i need to implement a window comparator for these supplies.

As the adc has only one ref high and low vlt port. How do i implement this window comparator using this ADC.

as there is only one ref low and high vlts port can i simply connect the upper and lower thresholds for the window comparator and 3.3v as 3 inputs and make the software compre the conversion internal. Can do something like this. if so then wht should be connected to VRL and VRH

Pls let me know

Thanks in advance

[macl]

I recommend setting VRH=VDDA & VRL=VSSA and keep VDDA between 3.3 - 5V.  See Table 32-6 (QADC Electrical Specifications) in the users manual for details.  You can implement your comparator in software.

[Kremer]

 VRL should be connected to VSS. If you direct connect VRH to any of those supply voltages you´ll not be able to measure the window of one of those voltages, because, if the level changes (5V or 3.3V) the reference voltage will equality change, and you´ll not measure the right window. 

 Or even worse. Let´s suppose you connect VRH to 3.3V and you are measuring the window for 5V. If the 5V remains unchanged and 3.3V suffer some variation, you´ll detect a 5V window error but the cause was the VREF variation.

 That´s why VREF on ADC´s is the key for measuring things. I suggest you to develop a voltage reference circuit to this part if it is a very important feature of your project.

Regards