Connecting a negative voltage to an input pin, even through a resistor quite
clearly violates the "operating" portion of the data sheet. The "maximum"
portion of the data sheet is conditions that the chip can withstand without
damage, but are definitely not part of the operating conditions. hence the
protection diode being able to take 20mA is a maximum rating that is safe for
the chip, but for this to happen the operating condition that Vss<Vin<Vdd is
violated. hence you are causing yourself a whole lot of grief by not observing
the operating conditions clauses. It is like saying that a 100V transistor can
take 10A, but to have 1000W dissapation is violating its 150W rating!!!!!!!