Power Supply and Clock Frequency
there are following pins for power supply and clock frequency signals.
VCC + 5 V power supply.
VSS Ground reference
X1 and X2 these two pins are used to connect an external crystal oscillator ( generally RC clock Frequency) which is responsible to generate clock frequency for the systems. For the clock frequency of the microprocessor to be 3 MHz the crystal should have the frequency twice of it i e 6 MHz as it is internally divided by two .
CLK output - output clock this is used to provide clock signal to other ICs used in the same systems to synchronise them with the microprocessor