Hi Pandi,
The built-in Schmitt Triger circuitry is the regular circuit for digital IO. Based on the image that you sent, it just applies to the Digital Input, as the Digital Output and Analog Inputs are on separate routes.
You could check the hysteresis voltage (Vhys) on Table 9 “Static characteristics” of LPC2478 Datasheet, available on the following link:
http://www.nxp.com/docs/en/data-sheet/LPC2478.pdf
Hope this will be useful for you.
Best regards!
/Carlos
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