well I understand it now, thanks a lot.. I'm using PIC16F877A.. because I'm actually doing a line following robot.. and I'm using PID as the algorithm so I could only make sense of the PID if I could get the required input high or low at the right place...
Anyway, I would also need to change the threshold voltage accordingly depending on the surrounding environment that's why I need to vary the threshold voltage for the internal ADC of the PIC16F877A.. that's why I'm thinking if I want to vary the threshold voltage, I'll have to use the internal ADC and but if I use the external hex schmitt trigger 74LS14, I can't vary the threshold voltage, so I'll have to use the internal ADC but I duno how..