On the MCU hardware anti interference

Development Zone processor electronic products, how to improve the anti-interference ability and electromagnetic compatibility?One, some of the following systems with special attention to anti electromagnetic interference:
 In 1, the microcontroller clock frequency is very high, particularly fast bus cycle system.
 2, system with high power, high current drive circuit, such as a spark of the relay, high current switch.
 In 3, including weak analog signal circuit and high precision A / D converter system.Two, in order to increase the system electromagnetic interference resistance ability to take the following measures:
 1, choose low frequency micro controller:
  Selection of the external clock frequency low micro controller can effectively reduce the noise and improve the anti-interference ability of the system. The same frequency square and sine wave, square wave, sine wave is much larger than in the high frequency components. Although the square wave frequency components of the wave amplitude, higher than the wave is small, but the higher the frequency emitted easily become the source of noise, the microcontroller generates the most influence of high-frequency noise is approximately 3 times the frequency of the clock.
2, reduce the signal distortion in
 Micro controller mainly adopts high speed manufacturing technology CMOS. Signal input terminal input current of about 1mA, input capacitance 10PF, the input impedance is very high, high-speed CMOS circuit and the output end of a load capacity, namely the considerable output value, a gate output end through a very long line into the input impedance is high input, reflection the problem is severe, it may cause signal distortion, increasing the system noise. When Tpd > Tr, became a transmission line problems, must consider the signal reflection, impedance matching problem. Signal on the printed circuit board in the delay time and the characteristic impedance of the lead, with the printed circuit board of the dielectric constant of the material. Can loosely think in printed circuit board, signal wire transmission speed, at 1 / 3 to 1 / 2. Micro controller systems are commonly used in logic telephone element Tr ( standard delay time) for between 3 to 18ns. In the printed circuit board, signal via a 7W resistor and a 25cm long lead, online delay time approximately in 4 ~ 20ns. That is to say, the signal in the printed circuit leads on the shorter is better, the length of not more than 25cm. And the number of holes should be as little as possible, preferably not more than 2.
  When the signal rise time faster than the signal delay time, according to fast electronics processing. At this time to consider the transmission line impedance matching, for a printed circuit board integrated block between the signal transmission, to avoid the Td > Trd, printed circuit board the system speed is not too fast.
 Use the following conclusion of the printed circuit board design of a rule:
 Signal transmission in a printing plate, the delay time should not be greater than the nominal time delay device.
 3, reduce the interference between signal wires:
 A a rising time of step signal through the Tr lead AB to B end. Signal on the AB line delay time is Td. At point D, because the A signals in the forward transmission, arrived at the B point after the signal reflection and AB line delay time, Td will induce a width Tr of the page of pulse signal. At point C, because AB signal transmission and reflection, can induce a width of a signal on the AB line delay time two times, namely 2Td positive pulse signal. This is the cross interference between signals. Strength of the interference signal and C signal by di / at, and the distance between lines on. When the two signal line is not very long, seen on AB is actually two pulse superposition. CMOS process for the manufacture of micro control by high input impedance, high noise, noise margin is very high, the digital circuit is the superposition of 100 ~ 200mV noise does not affect their work. If the graph AB line is an analog signal, the interference will be tolerated. Such as a printed circuit board as four plates, one of which is a large area of land, or double panel, a signal line is the opposite of a large area of land, this kind of signal interference will be reduced. The reason is, a large area of the signal line characteristic impedance, signal in D end reflection is greatly reduced. Characteristic impedance and signal line between the dielectric constant of the medium is inversely proportional to the square of the natural logarithm, and dielectric thickness is directly proportional to. If AB lines into an analog signal, to avoid the digital circuit signal line CD to AB interference, AB under the line to have a large area of land, AB line to CD line distance greater than AB line and the distance of 2 ~ 3 times. Available locally shielding, in the lead node side lead on both sides of cloth to the ground.
 4, reduced from power supply noise
 Power supply to power supply system at the same time, also the noise added to the power supply. Circuit of microcontroller reset line, line, and some other control lines are most vulnerable to external noise interference. Network interference through power into the circuit, even if the battery power supply system, the battery itself also has a high frequency noise. Analog circuit analog signal more withstand from power disturbances.
 5, pay attention to the printed wire board and components of the high frequency characteristics ofIn high frequency condition, a printed circuit board to lead, vias, resistance, capacitance, inductance and capacitance connector distribution can not be ignored. Capacitance inductance can not be ignored, the inductance of the distributed capacitance can not be ignored. Resistance to high frequency signal reflection, the distributed capacitance of the leading wire will play a role, when the length is greater than the noise frequency corresponding wavelength 1 / 20, on the antenna effect, noise transmitted outward through the lead. Printed circuit board through-hole about cause 0.6pf capacitor. An integrated circuit package material itself introduced 2 ~ 6pF capacitor. A circuit board connector, 520nH distributed inductance. A double straight stick 24 pin integrated circuit socket, introduced in 4 ~ 18nH distributed inductance. These small distribution parameters on the low frequency micro controller system is negligible; while for high-speed system must pay special attention to.
 In 6, the arrangement of the elements to be reasonable partitionComponents on a printed circuit board arrangement should fully consider the problem of anti electromagnetic interference principle, is one of the components between the lead to as short as possible. In layout, the analog signal, high speed digital circuit part, noise source (such as a relay, large current switch) this three part reasonably separately, so that mutual coupling of the signal to a minimum.
 7, handle good grounding wireThe printed circuit board, power line and ground the most important. To overcome the electromagnetic interference, the main means of grounding. For double panels, wire layout in particular, through the use of single point grounding, power and ground from both ends of the power supply to a printed circuit board to the power supply, a contact, a contact. Printed circuit board, to have multiple ground return, these are converging to power the point, is called the single point grounding. The so-called analog, digital, high power devices to open a branch, is refers to the wiring separately, and finally come together at the ground. And the printed circuit board is connected outside the signal, usually using shielded cable. For high frequency, and the digital signal, and both ends of the grounding shield cable. Low frequency analogue signal shielded cable, one end grounded for good. To noise and interference is very sensitive to the circuit and the high frequency noise is particularly serious circuit should use metal shielding.
 8, make good use of the decoupling capacitor.
  Good high frequency decoupling capacitors can be removed to 1GHZ high frequency component. Ceramic chip capacitors or multilayer ceramic capacitor frequency characteristic is better. Design of printed circuit boards, each integrated circuit power, between to add a decoupling capacitor. The decoupling capacitor has two functions: one is the integrated circuit energy storage capacitor, supplying and absorbing the integrated circuit open door charge and discharge energy; on the other hand bypassed the device high frequency noise. A digital circuit in typical decoupling capacitors for 0.1uF decoupling capacitor 5nH distributed inductance, the parallel resonance frequency at about 7MHz or so, that is to say to 10MHz below the noise has good decoupling, above the noise of the 40MHz almost has no function. 1uF, 10uF capacitance, parallel resonance frequency is more than 20MHz, removing the high frequency noise effect is better. In the power into the printing plate place and a 1uF or 10uF to high frequency capacitance is often beneficial, even in a battery powered systems also need this kind of capacitor. Each about 10 integrated circuit to add a charge and discharge the capacitor, or storage capacitance, capacitance of 10uF optional. Not the best electrolytic capacitors, electrolytic capacitor is a two layer thin film roll up, roll up the structure in high frequency performance for the inductance, preferably using gall capacitance or poly carbonate oil capacitor. The decoupling capacitor value selection is not strict, according to C = 1 / F computing; 10MHz 0.1uF, on micro controller system, take 0.1 ~ 0.01uF can.Three, reduce the noise and electromagnetic interference with some experience.
1 can use a low chip would not have high-speed, high-speed chip used in key areas.
2 available strings of a resistance method, lower control circuit up and down along the hopping rate.
3 as far as possible for relay to provide some form of damping.
4 meet the demand of the system of the lowest frequency clock.
5 clock generator as close as possible to the clock device to use. Quartz crystal oscillator shell to ground.
6 wire circle clock zone, the clock line is short as soon as possible.
7 I / O drive circuit as close to the printing plate edge, to leave as soon as possible printing plate. On entering the PCB signal to filter, from high noise area to signal to the filter, at the same time series termination resistors approach, reducing the signal reflection.
8 MCD useless end to pick up high, or ground, or defined as output, integrated circuit connected with the power to the end are connected, not hanging.
The 9 idle door circuit input end is not vacant, unused amplifier is connected with the input end, a negative input end is connected with the output end.
The 10 printed circuit board to use 45 lines instead of 90 lines to reduce the high frequency signal wiring external transmission and coupling.
The 11 printed circuit board according to the frequency and current switching characteristics of partition, noise components and noise components to distance farther away.
12 panels single and double panel with single point connected with a power supply and a single point grounding, power line, ground wire to coarse, economy is able to bear the words laminated to reduce power, the Rong Sheng inductance.
The 13 clock, bus, a chip select signal to stay away from I / O lines and connectors.
The 14 analog voltage input, the reference voltage terminal line as far as possible from the digital circuit signal line, especially the clock.
15 on the A / D device, digital and analog part rather uniform don't cross.
16 clock lines perpendicular to the I / O line than the parallel I / O line interference small, clock component pin away from the I / O cable.
17 element pins as short as possible, the decoupling capacitor pin as short as possible.
18 key lines should be rough, and on both sides and protection. High speed line should be short to be straight.
19 sensitive to noise line not with large current, high speed switching lines parallel.
20 quartz crystal below as well as on noise sensitive devices below do not walk the line.
21 weak signal circuit, a low-frequency circuit around instead of forming a current loop.
22 any signals do not form a loop, such as inevitable, let the loop area as small as possible.
Each of the 23 integrated circuit a decoupling capacitor. On the edge of each electrolytic capacitor to add a small high frequency bypass capacitor.
24 high capacity tantalum or polyester capacitor without electrolytic capacitor for charging and discharging of the energy storage capacitor circuit. The use of tubular capacitor, shell be grounded.