What is the timing of the microcontroller? The timing of the timing

Crystal oscillator

Quartz Oscillator 3225 20M OSC

The origin of the timing We have already known that the process of executing instructions by the microcontroller is to sequentially fetch the instructions one by one from the ROM program memory and then perform a series of micro-operations to complete various specified 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. It is necessary to arrange the courses and activities under the unified coordination of the ringtones. How is the timing of the MCU specified? Then look down.
The time of the cycle every time the computer accesses the memory, we call it a machine cycle. It is a time reference. Just like the seconds we use in our daily life, one machine cycle in the computer includes
12 oscillation cycles What is the oscillation period? What is the oscillation period? The oscillation period is the period of the oscillation source. It is the time period of the crystal oscillator we use. What is the time period of a 12M crystal oscillator? The friend of the electronic technology should not be difficult to calculate. T=1/f is also 1/12 microsecond. Then a single-chip microcomputer using 12M crystal oscillator should have a machine cycle equal to 12*1/12 microseconds or 1S.
Internal clock circuit:
Inside the MCS-51, there is a high-gain inverting amplifier whose input is pin XTAL119. The output of the pin 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 oscillator. The size of the crystal oscillator above is related to the oscillation frequency of the single-chip microcomputer. When we go to the serial interface, the size of the capacitor is explained in detail to affect the oscillation of the oscillator. The stability 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 to ensure the stability of the oscillator operation. The anti-interference ability of the system. In addition to the internal clock mode, the MCU can also adopt the external clock mode when the external clock is used. When our system is composed of multiple MCUs, in order to ensure the clock signal between each MCU. Synchronization should introduce a unique common external pulse signal as each microcontroller Oscillation pulse time should be
XTAL2 is suspended 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. . .
The clock circuit of the two single-chip microcomputers We already know that the single-chip microcomputer works under certain timing control. Then what is the relationship between the timing and the clock? The clock is the basis of the timing. The single-chip microcomputer itself is like a complicated synchronous sequential circuit to ensure the synchronous working mode. It is necessary to operate in time series under the control of the unique clock signal. How is the clock in the microcontroller generated?