> You should have a look at the 'input injection current' spec of the ADC.
Whch is in the MCF5213 Electrical Characteristics document, Table 34, Page 37:
IADI Input injection current per pin: 3mA
And read note 7 associated with that spec.
The Maximum (normal) voltages for the pins are Vrefl and Vrefh, but according to the Errata, they had better be connected directly to Vssa and Vdda or there are problems.
> The inputs of the ADC should be protected with on-chip diodes connected to power supply rails.
As indicated in the diagram in section 2.13.
> Thus, the input voltage range extends one diode voltage drop above and below supply voltage.
But the limiting specification is the injection current.
> The input current should be limited to maximum allowed injection current. Use a series resistor if necessary, but consider the possible degradation of ADC performance.
Both the frequency response and the voltage offset the resistor causes. "Figure 8. Equivalent Circuit for A/D Loading" in Section 2.13 should give you enough information to work this out.
There is also a HUGE amount of information on this subject in the MCF5282 User Manual (the one Freescale uses to describe the MCF5313). Read the sections
28.9.5 Accommodating Positive/Negative Stress Conditions
28.9.6 Analog Input Considerations
28.9.7 Analog Input Pins
28.9.7.1 Settling Time for the External Circuit
28.9.7.2 Error Resulting from Leakage
Tom
