GPRSCDMA series application 2

hscims

GPRSCDMA series application 2 [Copy link]

　Comprehensive measures for GPRS/CDMA wireless equipment to resist electromagnetic interference Author: Lin Xiangjian , 2005-3-16 23:42:00 Published in: " GPRS/CDMA Professional Forum " 　　There are three main sources of electromagnetic interference to industrial systems: one is space field interference, which penetrates into the system through electromagnetic wave radiation; the second is power supply interference, which directly damages the system; the third is signal interference channel, which enters the system through the front and rear channels connected to the host. In order to ensure that the instrumentation itself operates accurately, reliably, stably and continuously, it is necessary to take some comprehensive measures for the single-chip microcomputer application system so that the system can automatically resume normal operation after being disturbed, ensure its long-term reliable and stable operation , and meet the electromagnetic compatibility standards of the instrumentation . 1. Select a low-frequency microprocessor 　　Selecting a microprocessor with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability . For square waves and sine waves of the same frequency , the high-frequency components in the square wave are more than those in the sine wave . The higher the frequency , the easier it is to emit and become a noise source . Therefore , low-frequency microprocessors should be used as much as possible under the premise of meeting the system performance requirements . 2. The layout of the printed circuit board and the reasonable arrangement of components The arrangement 　　position should fully consider the anti-electromagnetic interference problem, and the leads between the components should be as short as possible . The signal is transmitted on the circuit board , and its delay time should not be greater than the nominal delay time of the device used . In terms of layout, the three parts of the analog signal part, the high-speed digital signal part, and the noise source part (relay switch) are reasonably separated to minimize the signal coupling between them. 　　A high-frequency choke ring is connected in series in the external interface circuit of the important and easily interfered circuit board to suppress high-frequency interference and also play a role in resisting static electricity carried by operators. 　　Optocouplers are used for photoelectric isolation at the signal output and input ends. Separate power supplies are used at both ends of the optocoupler to effectively suppress the interference of sharp pulses and various noises . At the input end of the optocoupler , even if the interference is of high amplitude , it will be suppressed because there is not enough energy to make the light-emitting diode emit light . 　　The single-point grounding method is adopted. A 0.1μF decoupling capacitor is added to the power supply end of each chip. This capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, providing and absorbing the charging and discharging energy of the integrated circuit when opening and closing the door; on the other hand, it bypasses the high-frequency noise of the device. Add a 10μF high-frequency decoupling capacitor where the power supply enters the circuit board. 　　Try to use 45- degree fold lines instead of 90- degree fold lines to reduce the external emission and coupling of high-frequency signals. No signal forms a loop. 3. Use shielding technology 　　A closed metal box made of copper or aluminum and other metal materials with good conductivity can destroy the path of electric field interference and has an electrostatic shielding function . This shielding box can also destroy the path of magnetic field interference after grounding , and has an electromagnetic shielding function . 4. Use hardware " watchdog " The English name of the " watchdog " timer circuit is Watchdog Timer , or WDT for short . Its function is to monitor the operation of the microcontroller. Once a " freeze " is found, a reset signal is sent to restore the normal operation of the program. There are many types of WDT circuits, but the basic principle is the same. The WDT circuit is an independent timer. The microcontroller will continuously send a timing reset signal to the WDT during normal operation to clear the WDT timer, so that the WDT cannot generate a reset signal. If the microcontroller freezes, the WDT timer cannot receive the reset signal. When the timer is full, an overflow signal is generated to reset the microcontroller and restore the normal operation of the program. 　　However, the hardware watchdog is not perfect. Sometimes the system freezes without being reset. Therefore, it is considered to combine software anti-interference methods to deal with it. 5. Improve software reliability. 　　In the process of system software development, in order to improve system reliability, try to avoid errors and failures.