We already know that the process of executing instructions by the microcontroller is to sequentially execute the instructions from the ROM program memory one by one, and then perform a series of micro-operation control to complete various specified actions. It must be coordinated in various internal actions. In a certain order, in other words, this series of micro-operation control signals must have a strict sequence in time. This order is the timing of the single-chip microcomputer. This is like the electric bell used in our school to ensure the classroom order. Under the unified coordination of ringtones, how to arrange the timing of each MCU and the activities of each MCU? Then look down. Every time the computer accesses the memory, we call it a Machine cycle. It is a time base. Just like the seconds we use in our daily life, one machine cycle in a computer includes 12 oscillation cycles. What is the oscillation cycle: one What is the oscillation period? The oscillation period is the period of the oscillation source, that is, the time period of the crystal oscillator we use is a 12M crystal oscillator. What is its time period? Friends of electronic technology should not be difficult to calculate T = 1 / f, which is 1 / 12 microseconds, then a single machine cycle using a 12M crystal oscillator should be equal to 12 * 1 / 12 microseconds is 1S. Inside the MCS-51, there is a high-gain inverting amplifier. The input is pin XTAL119 and the output is XTAL218. We only need to connect two capacitors and one crystal to form a stable self. Excited oscillator. The working principle of its internal circuit is not introduced. Here we mainly talk about the choice of capacitor and crystal. See the above picture crystal oscillator size and the oscillation frequency of the microcontroller. We will explain the size of the capacitor in detail when we go to the serial interface. The stability of the oscillation of the oscillator and the fastness of the start-up, usually choose 10-30P ceramic capacitor or correction capacitor. In addition, when designing the circuit, the crystal oscillator and capacitor should be as close as possible to the chip to reduce the distributed capacitance of the pcb board, and ensure the stability of the oscillator operation to improve the anti-interference ability of the system. In addition to the internal clock mode, the microcontroller can also use the external clock to introduce an external clock. When our system consists of multiple microcontrollers, we must introduce a unique common to ensure the synchronization of the clock signals between the microcontrollers. The external pulse signal is used as the oscillation pulse of each single-chip microcomputer. At this time, XTAL2 should be left floating without the external pulse signal introduced by XTAL1 as shown in the right figure. This is the work of the prawn. Some commands in the 89C51 MCU only require one machine cycle and some instructions require two or three machine cycles. In addition, there are two instructions that require 4 machine cycles. It is not difficult to understand that it is always longer to wipe the floor at home than to wipe the table. However, I am a masculine man who rarely does housework. How to measure the length of instruction execution time, we need to use a new concept instruction cycle - that is, the machine cycle required to execute an instruction. INTEL Company stipulates each instruction execution. Of course, this does not require us to remember it. But here is the DJNZ instruction. We have to remember that it is a two-cycle instruction that requires two machine cycles, ie 2S12M crystal, and then our experimental delay time should be It is calculated that 62500*2S=125000S is 125mS, so the number is 0.125S. It is no wonder that LED1 flashes so fast, this delay will be used later. . . Everyone already knows that the MCU works under a certain timing control. Then what is the relationship between the timing and the clock? The clock is the basis of the timing. The MCU itself is like a complicated synchronous sequential circuit. In order to ensure the synchronous working mode, the circuit must be unique. How the clock in the microcontroller is generated by the timing of the clock signal control. Texture:
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