Introduction
Due to the rapid development of microelectronics technology , digital electronic system consists of an IC chip toward large-scale , small , fast direction of rapid development , and the development of faster and faster . Application of the device causes a new modern EDA design layout density, and the frequency of the signal is high , the device with the use of high-speed , high-speed DSP ( Digital Signal Processing ) system design will be more and more high-speed processing in DSP applications signal design issues become an important issue , in this design , which is characterized by the system data rate, clock rate and the circuit density are increasing , the PCB and the PCB design exhibit different behavior characteristics low Speed , signal integrity problems that arise , aggravating the problem of interference and electromagnetic compatibility problems and so on.
These problems can cause signal distortion caused directly or timing errors , incorrect data, address and control lines and system errors and even system crashes , resolved not seriously affect the system performance and brings immeasurable loss . The main way to solve these problems by circuit design. Therefore, a very important quality PCB design PCB , which is the optimal design concept into reality the only way . Several issues are discussed below for the PCB design in high-speed DSP system reliability should pay attention to .
Power Design
High-speed PCB design DSP systems first need to consider is the power supply design problems. In the power supply design , usually the following methods to solve signal integrity problems.
Consider the power and
To decoupling
With the improvement of the operating frequency of the DSP , DSP and other IC components tend to miniaturization, packaging intensive , consider using multilayer circuit design is usually recommended when the power and ground layer can be used exclusively , and for a variety of power , such as DSP 's I / O supply voltage and core supply voltage is different , you can use two different power level , considering the high cost of multilayer processing , you can put more or relative critical junction with a special layer of power , other power supply and signal lines can be as wiring , but the width of the line should be enough to pay attention .
Regardless of whether there is a dedicated circuit board ground and power planes , and must add some reasonable distribution of capacitance between the power supply and ground. To save space, reduce the number of through-holes , the proposed multi- use chip capacitors. Chip capacitors can be put back on the solder side of the PCB that is , chip capacitors connected to the through-hole and connected with wide lines and power through the through-hole formation .
Consider the power distribution wiring rules
Separate analog and digital power planes
High-speed high-precision analog components for digital signal is very sensitive. For example , the amplifier amplifies the switching noise , bringing them closer to the pulse signal, the analog and digital sections of the board , the power level is generally required separate.
Isolate sensitive signal
Some sensitive signals (such as high-frequency clock ) are particularly sensitive to noise interference , their high level of quarantine measures to be taken . High-frequency clock (20MHz clock above , or flip time of less than 5ns clock ) must have a ground convoy , clock width of at least 10mil, escort protective earth ground at both ends of the line width at least 20mil, high-frequency signal lines must be over good contact hole formation , and each is connected with the formation 5cm punched ; clocks side must send a 22Ω ~ 220Ω series damping resistors. To avoid signal noise caused by these lines produced .
Software and hardware anti-jamming design
Average speed DSP applications are PCB board by the user according to the specific requirements of the system design , because design capabilities, limited laboratory conditions , such as not taking perfect and reliable anti-jamming measures , in the event of the working environment is not ideal, electromagnetic DSP program flow disturbance will cause disorder , when the DSP code can not resume normal operation , there will be a runaway process or phenomenon of death , even damage some components . Should be taken to the appropriate anti-jamming measures .
Hardware anti-jamming design
High anti-jamming hardware efficiency, system complexity , cost, volume can be tolerated under the circumstances , the preferred hardware anti-jamming design . Common hardware anti-jamming technology can be summarized as the following:
( 1 ) hardware filtering : RC filter can be greatly reduced all types of high-frequency interference signals. If you can suppress the "glitches" interference.
( 2 ) a reasonable ground : rational design of grounding system for high-speed digital circuits and analog systems , having a low impedance , large areas of the ground layer is very important. Formation that can provide a high frequency low impedance current return path , and make the EMI, RFI becomes smaller , but also the shielding effect of the external disturbance . PCB design to separate analog and digital ground .
( 3 ) shielding : spark AC power , high frequency power , high power equipment , the arc will generate electromagnetic waves , become a source of electromagnetic noise interference , the metal case of the above device can be used to surround , and then ground , which shield interference caused by electromagnetic induction is very effective.
( 4 ) Optical Isolation: Optical isolators can effectively avoid mutual interference between different circuit boards , high-speed optical isolators commonly used in DSP interfaces and other devices ( such as sensors , switches , etc.) .
Software anti-jamming design
Software, hardware jamming interference can not be replaced advantage in DSP applications software systems should fully exploit the anti-jamming capability , which will affect the interference suppression to a minimum. Here are several effective methods of software interference .
( 1 ) Digital filter : noise analog input signal can be eliminated by digital filtering. Digital filtering techniques commonly used are: median filter, the arithmetic mean filtering.
( 2 ) set up a trap : the area is not used in the program to set some boot program , when the program jump to the area affected by the interference , the bootloader will be forced to capture program directed to the specified address , where the program with a special program error processing.
( 3 ) instruction redundancy : After double-byte instruction and the three-byte instruction insert two three bytes of air operations command NOP, to prevent interference with the system when the DSP program running , the program automatically track.
( 4 ) Set Watchdog Timer : If the control program into the " endless loop ", usually a " watchdog" technology to make the program from the " infinite loop ." Its principle is to use a timer , it generates a pulse at a set period , if you do not produce this pulse , DSP should be set within the period of time less than the timer is cleared ; But when the DSP program running , do not will be required by the timer is cleared , then the pulse timer generates a reset signal as the DSP , the DSP reset and re- initialize .
Electromagnetic Compatibility Design
Electromagnetic compatibility is the ability of electronic devices can still work properly in complex electromagnetic environments. EMC design purpose is to make electronic devices can inhibit various external interference and reducing electromagnetic interference with other electronic devices electronic devices . In the actual PCB board more or less exist between adjacent signal interference that electromagnetic crosstalk. Distributed capacitance and inductance of the loop size and crosstalk between related. Solve this mutual electromagnetic interference between the signals may take the following measures:
Choose a reasonable wire width
Because of the impact of transient interference currents on the printed lines printed wire is mainly generated due to the inductance component , and the inductance is proportional to the length of the printed conductor and inversely proportional to the width . Therefore, the use of short, wide wire for interference suppression is beneficial. Clock lead signal line bus driver often large transient current, the printed wire as short as possible . For discrete component circuits , printed wire width that is about 1.5mm
To meet the requirements ; the integrated circuit , the printed conductor width between 0 2mm ~ 1 0mm choice .
Using well-shaped mesh wiring structure .
Specifically, this transverse wiring layer PCB printed circuit board , the next layer of vertical cabling.
Thermal Design
In order to facilitate heat dissipation , PCB is best self- installation, plate spacing should be greater than 2cm, while paying attention to components on the PCB cloth row rules. In the horizontal direction, the power devices are arranged as close to the edge of the PCB , thereby reducing the heat transfer pathway ; power device in the vertical direction is arranged as close to the top of the PCB , thereby reducing its impact on the temperature of the other components . Temperature- sensitive components as much as possible the cloth on a relatively low temperature region , but not on the right above the heat capacity of the device.
Conclusion
In the design of high-speed DSP application system, how to improve the design from theory into reality depends on the quality of the printed circuit board PCB , the operating frequency increasing DSP circuits , pins and more dense , interference increase, how to improve the quality of the signal is very important. Therefore, the system performance is good, the quality of PCB and PCB designers are inseparable. As can be rational layout design to reduce noise , reduce interference, avoid unnecessary mistakes , played on the system performance of the play is not underestimated.
