The heart of 8051 is the circuitry that generates the clock pulses by which all internal operations are synchronized.

Pins XTAL1 and XTAL2 are provided for connecting resonator to form an oscillator. The crystal frequency is the basic internal frequency of the microcontroller. 8051 is designed to operate between 1MHz to 16MHz and generally operates with a crystal frequency 11.04962 MHz.

The oscillator formed by the crystal, capacitor and an on-chip inverter generates a pulse train at the frequency of the crystal. The clock frequency f establishes the smallest interval to accomplish any simple instruction.

The time taken to complete any instruction is called as machine cycle or instruction cycle. In 8051 one instruction cycle consists of 6 states or 12 clock cycles, instruction cycle is also referred as Machine cycle.

Instruction cycle of 8051 (Instruction cycle has six states (S 1 - S 6). Each state has two pulses (P1 and P2))