Pull-up resistors (or pull-down resistors for active high gates) are almost always used in high speed logic circuits. Their purpose is to force a definite logic state of “1” (high) or “0” (low), after the active logic signal is removed, rather than allowing parasitic leakage currents and PCB capacitances to affect the logic state in a random manner. In bipolar (TTL) logic, where 5VDC is a nominal “1” state, a typical value for a pull-up resistor would be between 1K and 10KΩ. In CMOS logic, much higher resistor values are typically used, because of the higher impedance of the logic gates, and the lower currents involved. Some micros do have weak pull-up resistors on several gates, but most prudent designers will augment these with additional parallel resistance (though not so much as to slow down the gates by demanding too much pull-down current).