I have an T89C51CC01 with AD inputs. The Datasheet says Voltage ADin max = Uref +0.2V, in my case REF is 2.5V, due to an error in the wiring I had about 4V on an AD input pin. I had a series resistor of about 1k. Now the AD on this CPU (this pin) does not work anymore (it always reads 0xff). That seems to me very strange, because its not in the absolute max ratings specified. I changed the CPU module, then it worked correctly, so its not a software issue either.

Has anybody experienced similair things? This would mean that AD inputs would have to be protected with schottky diodes towards Uref

Werner