8051 System Clock

    Every microcontroller has some clock frequency.In general cases, a quartz crystal is used to make the clock circuit. The connection is shown in figure (a) and notes the connections to XTAL 1 and XTAL 2.  In some cases, external clock sources are used and you can see the various connections above. Clock frequency limits (maximum and minimum) may change from device to device. Standard practice is to use 12MHz frequency. If serial communications are involved then its best to use 11.0592 MHz frequency.

   The below diagram shows that the 8051 clock circuit.
                                        8051 clock frequency circuit



 The above diagram shows that TTL clock source. Transistor-transistor logic. Clock frequency is very important to execute the program and instructions.

   One complete oscillation of the clock source is called a pulse. Two pulses form a state and six states form one machine cycle. Also, note that two pulses of ALE are available for 1 machine cycle. The waveform of wave pulses. The diagram shows below.

                                    8051 machine cycle waveform
  The above diagram shows that system clock, each clock have state 6. Each state have two poles.

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