The working principle of PLL. For example, how can our 8MHz crystal oscillator make MCU work at 48mhz or 72mhz

PLL English

Phase locked loop, So many materials are abbreviated to PLL

Now many chips work at a lot of frequencies , How high the frequency is depends on the crystal oscillator frequency , For example, our computer chips 1.5GHZ For example, our commonly used MCU MCU also 72MHZ, But the crystal oscillator cannot be as high as 1.5GH The etc. , The normal crystal oscillator is reaching 100-200MHZ, So we need to use the big brother of PLL , With it, we can super double ！

Let's first look at the block diagram of PLL

  This is the complete block diagram , Let's analyze , Let's not look at the frequency divider at the bottom

Input signal ： It is the input of our crystal oscillator

Phase detector ： It is to compare the input signal with the output signal of the voltage controlled oscillator

filter ： Low pass filter

VCO ： As the name suggests, our frequency increases with the increase of voltage

Then we will be surprised , We can use VCO directly , Adjust the voltage and control the frequency , But if you want the voltage to drift a little , Then the frequency will change , And the working frequency of our chip should be stable , So obviously, only voltage controlled oscillator is not enough .

Make sure the frequency is consistent , It depends on whether our input and output are consistent , So the phase detector is to compare the input and output feedback signals , Pictured

Compare whether the frequency and phase of both sides are consistent , Inconsistent   The phase detector is made and adjusted N An adjustment , Make the input and output frequency phase equal

Then the low-pass filter is to filter the input , Reduce the impact of high-frequency interference

I understand the working principle here , that 8MHZ Is to become 72MHZ, How can we 8MHZ Than 72MHZ, At this time, we need a frequency divider , take 72MHZ Frequency division 8MHZ It takes a long time to supply the phase detector OK