Schematic diagram of crystal oscillator clock and PCB Design Guide

In a circuit system , The clock is an essential part . The clock circuit is quite critical , The function in the circuit is like the function of human heart , If the clock of the circuit system goes wrong , The system will be disordered , So in PCB It is very necessary to design a good clock circuit in .

Our common clock circuits are ： Crystal 、 Crystal oscillator 、 Clock distributor . There are some IC The clock used may be generated by the main chip , But back to the source , It is also produced by one of the above three . Next, combined with specific examples , Explain the clock circuit layout 、 Wiring principles and precautions .

1. Crystal

PCB Commonly used crystal packages in ：2 Plug in package and SMD encapsulation 、4 Pin of SMD encapsulation , Common packages are shown in the figure below ：

Although the crystal has different specifications , But their basic circuit design is consistent , therefore PCB Layout 、 Wiring rules are also universal . The basic circuit design is shown in the figure below ：

It can be seen from the circuit schematic diagram , The circuit consists of crystals +2 It consists of two capacitors , These two capacitors are gain capacitance and phase capacitance .

Crystal circuit layout , Two capacitors are placed close to the crystal , The layout rendering is as follows ：

When wiring , A pair of lines in the crystal should be in the form of quasi difference , The line shall be as short as possible 、 And it should be thickened and wrapped , The effect is as follows ：

The above is the most basic and common crystal circuit design , There are also some deformation designs , If adding series resistance 、 Test points, etc , Here's the picture , The design idea is consistent ：

Combined with the above ,

The layout should pay attention to ：

1. and IC Cloth on the same level , In this way, less holes can be punched ;

2. The layout should be compact , The capacitor is located between the crystal and IC Between , And placed close to the crystal , Make the clock line to IC Try to be as short as possible ;

3. For cases with test points , Try to avoid stub Or make stub Try to be as short as possible ;

4. Do not place high-power devices nearby 、 Such as power chip 、MOS tube 、 Inductance and other devices with high calorific value ;

Pay attention to wiring ：

1. and IC Same floor layout , Routing on the same floor , Make as few holes as possible , If you punch , Need to add a return hole nearby ;

2. Class differential routing ;

3. The line should be bold , Usually 8~12mil; Because the crystal clock waveform is a sine wave , So here we deal with it according to the idea of simulation design ;

4. Signal wire package ground processing , And the shielding ground hole shall be made for the ground wire or copper sheet ;

5. The crystal circuit module area is equivalent to the analog area , Try not to have other signals cross ;

2. Crystal oscillator

Compared with crystal circuit , Crystal oscillator is an active circuit , It mainly consists of three parts ： Crystal oscillator + Power filter circuit + Source matching resistance ： The common circuit design is shown in the figure below ：

The layout and wiring effect diagram is as follows ：

Layout 、 Wiring summary ：

1. The filter capacitor is close to the power pin , Follow the principle of big before small , The small capacitor is closest ;

2. The matching resistor is placed close to the crystal oscillator ; If there is no such resistor in the schematic , It is suggested to add ;

3. Do not place high-power devices nearby 、 Such as power chip 、MOS tube 、 Inductance and other devices with high calorific value ;

4. Clock line press 50 Ohmic impedance line ; If the clock line is too long , You can walk inside , Add a reflux hole at the hole changing layer ;

5. Other signals remain the same as the clock signal 4W spacing ;

6. Parcel handling , And add shielding ground hole ;

3. Clock distributor

There are many kinds of clock distributors , In the design, ensure that the clock distributor to each IC Keep the distance as short as possible , Usually placed in a symmetrical position , for example ：

Clock distributor circuit ：

The design is as follows ：

Layout 、 Wiring summary ：

1. The clock generation circuit should be close to the clock distributor , The common clock generation circuit is crystal 、 Crystal oscillator circuit ;
2. The clock distribution circuit is placed in a symmetrical position , Guarantee to all IC The clock signal circuit of should be as short as possible ;
3. Do not place high-power devices nearby 、 Such as power chip 、 MOS tube 、 Inductance and other devices with high calorific value ;
4. When the clock signal line is too long , You can walk inside , Change the layer hole 200mil There should be a backflow through hole in the range ;

The above content comes from official account “ Big talk hardware ” 《 Reference to the original ：PCB Modular layout — Clock circuit design 》