Crystal oscillator for the single - chip microcomputer has what important role
I. what is crystal vibration
Crystal oscillators are generally called crystal resonators, which are electromechanical devices. They are made of quartz crystal with very small power loss through precise cutting, grinding, electroplating and welding of leads.
Crystal oscillator is very important for the microcontroller, can be said to be no crystal oscillator no clock cycle, no clock cycle can not execute the program code, then the microcontroller will not work.Next to understand the chip crystal oscillator circuit principle and role.
Second, the necessity of single chip crystal vibration
When MCU works, it is to take instruction from ROM one by one, and then execute step by step.MCU access a memory time, called a machine cycle, this is a time benchmark.A machine cycle includes 12 clock cycles.If a single chip chooses 12MHZ crystal oscillator, its clock cycle is 1/12us, and one of its machine cycles is 12x(1/12)us, namely 1US.
Of all the instructions on the McS-51 MCU, some are quick enough to achieve one cycle of the machine, some are relatively good enough to achieve two cycles of the machine, and two instructions need four cycles of the machine.In order to measure the length of instruction execution time, | is introduced into a new concept: instruction cycle.An instruction cycle is the time it takes to execute an instruction.For example, when it is necessary to calculate the time required for the completion of the DJNZ instruction, the frequency of crystal oscillations must first be known. Let the crystal oscillations used be 12MHZ, then a machine cycle is 1US.The DJNZ instruction is a double-period instruction, so execute 2US once.If the instruction needs to be executed 500 times, it is exactly 1000us, or 1ms.
Machine cycle is not only important for instruction execution, but also the time reference of microcontroller timer and counter.For example, a single chip microcomputer chose 12MHZ crystal oscillator, so when the value of the timer plus 1, the actual time is 1us, this is the timing principle of the single chip microcomputer.
Third, the role of single - chip crystal vibration
Crystals are there in each single chip microcomputer system, the whole is called crystal oscillators, crystals in the single chip microcomputer system is very big, he combined with single chip microcomputer internal circuit, necessary to generate MCU clock frequency, all the instruction execution of single chip microcomputer is established on the basis of the crystals to provide the higher clock frequency, the faster the running speed of the single chip microcomputer is also.
Crystal oscillations work in a resonant state with a crystal that converts electrical and mechanical energy into each other to provide stable, accurate single frequency oscillations.Under normal working conditions, the absolute accuracy of crystal oscillator frequency can reach 50 parts per million.Higher precision.Some crystal oscillators can also be applied voltage in a certain range of frequency adjustment, known as voltage controlled oscillator (VCO).
The crystal oscillator provides the basic clock signal for the system.Usually, one crystal oscillator is Shared by one system, which makes it convenient for each part to keep synchronization.Some communication systems use different crystal oscillations for base and rf frequencies, which are kept in sync by electronically adjusting the frequency.
Crystal oscillators are usually used in conjunction with PLL circuits to provide the required clock frequency for the system.If different subsystems need clock signals of different frequencies, they can be provided by different phase-locked rings connected to the same crystal oscillator.