Clock circuit of 8051 Microcontroller Family

Clock circuit of 8051

The 8051 has on chip oscillator pin XTAL1 and XTAL2 are provided for connecting a resonant network to from an oscillator crystal frequency ranges from 1 MHZ to 24 MHZ.

Ceramic resonators may be used as a low-cost alternative to crystal resonators but due to decrease in frequency stability and accuracy, ceramic resonators are not preferred for high-speed serial data communication with other system.

The oscillator formed by the crystal, capacitors, and on chip inverter generates pulse train at the frequency of the crystal as shown in fig.

The clock frequency & establishes the smallest interval of time within the microcontroller, called the pulse, p, time. The smallest interval of time to accomplish any simple instruction, or part of a complex instruction, however, is the machine cycle. The machine cycle is made up of six states. A state is the basic time interval for discrete operations of the microcontroller such as fetching on encode byte decoding encode, executing an encode, or writing a data byte. The oscillator pulses define each state.

Program instruction may require one, two or four machine cycle to be executed, depending on the type of the instruction. Instruction are fetch and executed, by the microcontroller automatically, beginning with the instruction located at Rom memory address ooooh at the time the microcontroller is first reset.

We can calculate the time any particular instruction will take to be executed as follows. The time to executed that instruction is found by multiplying by 12 and dividing the product by the crystal frequency.

         

T(instruction)  =    (c * 12d)/Crystal frequency

A 12 MHZ  crystal results in convenient time of 1 microsecond per cycle. An 11.0592 MHZ crystal frequency of 921.6 kloherts, which can be divided evenly by the standard communication baud rates of 19200, 9600, 4800, 2400, 1200 and 300 HZ.