First of all, one of the necessary conditions for the normal operation of the single-chip microcomputer is the clock circuit, so the single-chip microcomputer needs a crystal oscillator.
Crystal oscillator, the full name is quartz crystal oscillator, is a high-precision and high-stability oscillator. Through a certain external circuit, a sine wave with stable frequency and peak value can be generated. When the single-chip microcomputer is running, it needs a pulse signal as the trigger signal to execute the instruction itself. It can be simply imagined as: the single-chip microcomputer executes one or more instructions when it receives a pulse.
When the microcontroller works, it fetches instructions from the RoM one by one, and then executes them step by step. The time for the single-chip microcomputer to access the memory once is called a machine cycle, which is a time reference. One machine cycle includes 12 clock cycles. If a single-chip microcomputer selects a 12 MHz crystal oscillator, its clock cycle is 1/12us, and one of its machine cycles is 12×(1/12)us, which is 1us.
Among all the instructions of the MCS-51 single-chip microcomputer, some are completed relatively quickly and only need one machine cycle, some are relatively complete, which requires 2 machine cycles, and two instructions require 4 machine cycles. In order to measure the length of instruction execution time, a new concept is introduced: instruction cycle. The so-called instruction cycle refers to the time to execute an instruction. For example, when you need to calculate the time required for the completion of the DJNZ instruction, you must first know the frequency of the crystal oscillator. If the crystal oscillator used is 12 MHz, one machine cycle is 1 us. The DJNZ instruction is a two-cycle instruction, so it takes 2us to execute once. If the instruction needs to be executed 500 times, it is exactly 1000us, which is 1ms.
The machine cycle is not only of great significance for the execution of instructions, but also the time base of the single-chip timer and counter. For example, a single-chip microcomputer selects a 12 MHz crystal oscillator, then when the value of the timer increases by 1, the actual elapsed time is 1us, which is the timing principle of the single-chip microcomputer.
The crystal oscillator provides the working signal pulse to the microcontroller. This pulse is the operating speed of the microcontroller. For example, 12 trillion crystal oscillator. The operating speed of the microcontroller is 12 megabytes per second. There is also a crystal oscillator inside the microcontroller. Connect an external crystal oscillator to get a more stable frequency.
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