The crystal oscillator circuit prov

The crystal oscillator circuit provides a clock signal for the operation of a single-chip AT89SSI, and has a high gain inverting amplifier used to form an internal oscillator on the chip, and pins XTAL1 and XTAL2 are the input and output of the amplifier, respectively. This amplifier forms a pre-running oscillator with a ceramic resonator as an off-chip quartz crystal or feedback element. The external crystal of the circuit and the capacitor are connected to the feedback loop of the amplifier to form a parallel oscillation circuit. The size of the external capacitor slightly affects the level of the oscillation frequency, the stability of the oscillator, the difficulty of starting the vibration and the temperature stability, so when using quartz crystals, the capacitance of the capacitor ranges from 30pF to 10pF. When vibrating using ceramic, the capacitance capacity is 40pF10pP, in this design a quartz crystal is used and the capacitance value of the capacitor is set to 30pP.The basic function of the reset circuit is to provide a reset signal when the system is powered on and the reset signal is canceled until the system power supply is stable. For reliability, it is necessary to remove the reset signal after a certain delay after the power supply has stabilized so that the jitter caused by the power switch or power plug is opened and closed and does not affect the reset. Single-chip microcomputers need to be reset when they start, so the CPU and various components of the system are in a determined initial state and start working in the initial state. The reset signal of the AT89S51 is input from the chip's REST pin to the Schmitt trigger. If the system is in normal operation and the oscillator is stable, the CPU can respond and reset the system by having a high level on the REST pin and maintaining more than 2 machine cycles (24 oscillation cycles). The reset method of single-chip system is as follows: manual button reset and power-on reset This design uses manual button resetManual button reset requires artificially adding a high level to the reset input terminal REST. The method used is a REST terminal and a positive power supply Vcc. Request button. By artificially pressing the button, a Vecc of + 5V level is added directly to the REST terminal and the system is reset. Human behavior keeps buttons faster for tens of seconds, so designs can fully meet reset time requirements. SW-PB is the dynamic reset switch of the reset circuit, and the capacitor Chl can avoid the interference of high frequency harmonics in the circuit.