I've been researching pullup and pulldown resistors after seeing them used in many digital circuits and while I understand what their purpose is, I haven't found any resource on how to determine the resistance values to use in such circuits and how these values affect the circuit's behaviour.
One of the examples I have is...
...which is an A/D converter circuit for a keypad input. I have implemented this and it actually works very well but I can't quite pinpoint why a 1Mohm resistor was picked as the pullup and what exactly happens to it when one of the buttons are pressed - does it affect the voltage of the ADC input in such a small amount that it doesn't register or is it overridden somehow or...?
I know this is probably a very basic question but I can't seem to apply the thoery I have learned to this one.