An in-depth understanding of crystal oscillation circuit derived from xtalin pin and xtalout pin of single chip microcomputer

The first step in the process of using single chip microcomputer is to choose the clock source of single chip microcomputer , You can often see the following circuit schematic diagram ：

In this schematic diagram, a crystal oscillator is connected to XTALIN Pins and XTALOUT Pin , Provide clock frequency to MCU , Now let's analyze the working principle of this circuit in detail .

The clock sources of single chip microcomputer generally include the following ：

1、 Through the of single chip microcomputer CLKIN The pin directly inputs a square wave clock to the MCU by an external clock source .

2、 As shown in the above figure, the MCU is connected by an external crystal oscillator XTALIN Pins and XTALOUT Pin , Together with the oscillation circuit inside the single chip microcomputer, it forms a clock source , Usually this clock source is called crystal oscillator (SysOsc), The accuracy of this clock source can reach very high, generally used ppm To represent the error of the external crystal oscillator . The frequency of this oscillator is determined by the external crystal oscillator , The biggest feature of oscillator is high frequency stability . Quartz crystal oscillators are used in systems that require high frequency accuracy and stability , Such as electronic watch 、 The master clock of the computer 、 High precision signal generator 、 Main oscillation source of all communication systems

3、internal RC Oscillator (IRC) Internal clock source , Usually a fixed frequency , such as 12M, The accuracy of this kind of clock can reach ±1.5%. When the communication rate is relatively high , For example, serial port 115200 Baud rate of , At this time, if IRC Error code will appear in clock communication ; When the communication rate is reduced , such as 2400 When the baud rate is reached, the communication will be normal , So in high-speed communication, we still need to use the above article 2 Kind of clock source .

4、 Special clock source of single chip microcomputer , such as dedicated Watchdog Oscillator (WDOsc) Clock dedicated to watchdog circuit , The clock used for the timer of single chip microcomputer wake-up （internal low-power, low-frequency Oscillato）, The accuracy of this clock source is not high , Usually only ±40% The accuracy of the .

The following figure is a single chip microcomputer NXP824 Clock resources ：

Will be inside IRC The clock passes through the switching matrix SWM Switch to output pin foot CLKOUT, The following waveforms can be seen through the oscilloscope , Show IRC Clock bit 12M.

void CLKOutInit (void)
{
    LPC_SWM->PINASSIGN[11] = (LPC_SWM->PINASSIGN[11] & (~(0xFF << 16))) |                             
                             (CLKOUT << 16);                             /* Distribute CLKOUT To the pin P0_7        */
    
    /*
     * CLKOUTSEL To configure
     *  >bit[1:0]       0x0  IRC oscillator             0x1  Crystal oscillator(SYSOSC)
     *                  0x2  WDT oscillator             0x3  Main Clock
     *  >bit[31:2]      Reserved
     */
    LPC_SYSCON->CLKOUTSEL = 3;                                          /* CLKOUT Source selection ：MAINCLK        */
    LPC_SYSCON->CLKOUTUEN = 1;                                          /* to update CLKOUT Clock source             */
    while (!(LPC_SYSCON->CLKOUTUEN & 0x01));
    LPC_SYSCON->CLKOUTDIV = 24;                                         /* CLKOUT Output distribution coefficient ：24       */
}

 

It can be seen from the above that , The clock source of single chip microcomputer is generated by the oscillator , The following information is collected and sorted out from the network , Mainly to deepen the understanding of oscillation circuit .

 

 

Design principle of crystal oscillator

 1.1、 Classification of oscillators

1.2、 The principle of oscillation

1.2.1、 The generation of self-excited oscillation

1.2.2、 Conditions for oscillation

 1.3、 Starting vibration and amplitude

 1.3.1、 Vibration initiation process

1.3.2、 Stability of amplitude