51 MCU clock circuit principle

The clock circuit is an oscillator that provides a beat for the microcontroller. The microcontroller must perform various operations under the control of this beat. Therefore, the microcontroller will not work properly without a clock circuit. The clock circuit itself does not control anything, but you use the program to let the microcontroller do the corresponding work according to the clock.

There is a high-gain inverting amplifier in the MCS-51 microcontroller. The input of the inverting amplifier is XTAL1 and the output is XTAL2. The oscillator circuit and the clock circuit formed by the amplifier together constitute the clock mode of the microcontroller. According to the different hardware circuits, the clock connection mode of the microcontroller can be divided into internal clock mode and external clock mode, as shown in Figure 1.

(a) Internal clock circuit (b) External clock circuit

Figure 1 Clock circuit

Internal clock schematic (a self-excited oscillation circuit)

In the internal clock circuit, a quartz crystal oscillator and two fine-tuning capacitors must be connected across the XTAL1 and XTAL2 pins to form an oscillation circuit. Usually, C1 and C2 are 30pF, and the frequency of the crystal oscillator is between 1.2MHz and 12MHz. For the external clock circuit, XTAL1 is required to be grounded and XTAL2 is connected to the external clock. There are no special requirements for the external clock signal, as long as a certain pulse width is guaranteed and the clock frequency is lower than 12MHz.

The oscillation signal of the crystal oscillator is sent to the internal clock circuit from the XTAL2 terminal, which divides the oscillation signal into two and generates a two-phase clock signal P1 and P2 for the single-chip microcomputer. The period of the clock signal is called the state time S, which is twice the oscillation period. The P1 signal is valid in the first half of each state, and the P2 signal is valid in the second half of each state. The CPU uses the two-phase clock P1 and P2 as the basic beat to coordinate the effective operation of various parts of the single-chip microcomputer.