Clock circuit is the heart of singl

Clock circuit is the heart of single chip microcomputer system, it controls the working rhythm of single chip microcomputer. MCU although the internal oscillator circuit, but to form a clock, must be external additional circuit. The CPU of a microcomputer is essentially a complex synchronous sequential circuit, and all the work is carried out under the control of clock signals. For each instruction executed, the CPU's controller sends a specific set of control signals. The clock signal of ATMSP430F149 is usually generated in two ways: one is internal oscillation mode, the other is external clock oscillation mode.Internal clock circuit, in the ATMSP430F149 microcontroller internal has a high gain inverting amplifier, as long as in the microcontroller XTAL1 and XTAL2 pin external timing feedback loop, the oscillator OSC can self-oscillate, and in the microcontroller internal generation of rectangular clock pulse signal. The timing feedback loop is usually composed of quartz crystal oscillator and fine tuning capacitor, among which the frequency of quartz crystal oscillator is one of the important performance indicators of the single chip microcomputer. In general, the frequency of quartz crystal oscillator is selected as the typical value of 12MHz, which is conducive to obtaining the baud rate without error. Internal clock circuit external capacitor C1, C2 is not strict requirements, they play a stable oscillation frequency, the role of rapid vibration. The typical values of capacitors C1 and C2 are between 60 and 70pF and the oscillator has high frequency stability. When the clock frequency corresponding to 30pF is selected, 12MHz is difficult. When the external ceramic oscillator is connected, the capacitance value is usually selected as 47pF.The external clock circuit USES the external oscillation signal directly as the clock source to access XTAL1 and XTAL2. Usually XTAL2 is directly connected to the internal clock circuit, and the input XTAL1 is grounded. Since the logic level of XTAL2 is not TTL, a 4.7-10k pull-up resistor is recommended.