Saturday, December 18, 2010

【 Weak current College 】 SCM system reliability technology and development 】

Summary: this article describes recent single-chip technology to provide system reliability for its efforts and development. Remind users to type selection, single-chip microcomputer application system design and manufacturing process, what should be noted that, in order to achieve high reliability single-chip applications. Keywords: SCM, reliability, electromagnetic compatibility
With the rapid development of semiconductor technology, the microcomputer itself constantly used in the design of some new anti-jamming technology, the reliability of the MCU. In addition to select strong anti-interference ability, the single-chip microcomputer system reliability of other auxiliary components is also critical that some interference of components use helps improve system reliability. In addition, the SCM system in circuit design, printed circuit board design, wiring and manufacturing technology, system installation has no good grounding, have direct impact on the application of the reliability of the system. SCM's own anti-jamming measure
To improve the reliability of the microcomputer itself. In recent years the manufacturer in single-chip microcomputer designed a series of measures have been adopted with a view to improving the reliability. These techniques primarily in the following areas. 1. reduce external clock frequency
External clock is a high-frequency noise sources, can cause interference to the application of the system, you may have against external interference, electromagnetic compatibility testing cannot be fulfilled. The reliability of the system requires a high level of application of the system, choose the frequency of low noise reducing single-chip is one of the principles of the system. The 8051 MCU, for example, the shortest instruction cycle 1 μ s, external clock is 12MHz. While the same speed of Motorola microcontroller system clock simply 4MHz, better suited for industrial control systems. In recent years, some production 8051-compatible microprocessor manufacturers are adopting new technology, without sacrificing the operation speed of the external clock needs to be reduced to 1/3. But Motorola microcontroller in the newly launched 68HC08 family and its 16/32 bit microcontroller in commonly used internal locking-phase technology, external clock frequency down to 32KHz, while internal bus speed is increased to 8MHz and even higher.
2. low noise series MCU
Traditional integrated circuit design, power supply, ground leads on often arranged symmetrically on both sides. As in the lower left corner of the Earth, is the power supply. This enables Silicon power noise across the whole. Improved technologies will power, and arrangements in the two adjacent pin, this reduces electricity through the entire wafer, on the one hand, the external decoupling capacitor in PCB design easier to reduce system noise. Another in integrated circuit design to reduce the noise of the example is driving circuit design. Some SCM provides several high current output pins, from dozens to hundreds of Ma Ma per channel. The power of the driver circuit integrated MCU internal undoubtedly increased the noise source. While hopping along the softening technique to eliminate this effect by a high-power tube made of several small tubes in parallel, then for each pipe output string equivalent resistance on different resistance. To reduce the di/dt.
3. the clock monitoring circuit, watchdog technology and low-voltage reset
Monitoring the system clock when it finds the system clock stop mode when the system reset signal to restore the system clock is a microprocessor enhances system reliability. While the clock supervisor electricity directive effectively and STOP is a rightness of Antinomies. You can only use one of them.
Watchdog technology is monitoring the application of a timer interrupt service routine health, when this program does not work when the judgment as a system failure, resulting in a system reset.
Low voltage reduction technology is monitoring the microcontroller supply voltage, the voltage falls below a certain value produces a reset signal. Due to the development of chip, the chip itself on the power supply voltage range of requirements and more wide. Power supply voltage 5V from original to 3.3V and continue down to 2.7V, 2.2V, 1.8V. On whether to use low voltage reset functions should be based on specific application to weigh.
4. EFT techniques
The newly launched Motorola M68HC08 series monolithic integrated circuits using EFT (Electrical Fast Transient) technology further enhances microcontroller anti-interference ability. When oscillation circuit of sine wave signal from outside interference, the wave will overlay some Burr. To its plastic Schmitt, the Burr becomes trigger signal interference with normal clock signal. Alternating use Schmitt and RC filter can make this class Burr does not work, this is the EFT techniques. With the continuous development of VLSI technology, anti-interference circuit internal technology is constantly evolving.
5. software measures
Single-chip design in the directive itself also has some interference. Illegal instruction reduction or illegal instruction interrupt is encountered when running the program illegal instruction or illegal addressing space can produce a reduction or interruption. SCM application system program is written in advance, there can be no illegal instruction or addressing. Must be a system disturbance, CPU error reading instruction.
The above mentioned is currently widely used SCM should have internal anti-jamming measure. In the selection of single-chip, to check if there are these properties to design a high reliability of system.
In application design, the designer has their own experience. Here to remind of the last on the ROM to do without. The principle is the event procedure to the recovery can be from here.
For single-chip system interference suppression components
1. decoupling capacitor
Each IC power, should be configured between a decoupling capacitor, it can filter out high frequency noise from power supply. As storage components, it absorbs the IC's internal transistor breakover, globe induced current change (di/dt), thereby reducing system noise. To selected high-frequency characteristics of monolithic capacitor or porcelain capacitor for decoupling capacitor. Each PCB power into the place you want to place a large-capacity capacitor energy storage. Because of the electrolytic capacitor winding type structure, its distribution and inductance is large, to filter out high frequency interference signal almostHas no effect. When used with decoupling capacitor doubled. Tantalum capacitor electrolytic capacitors is compared.
2. Suppression of high frequency inductors
Use coarse wire into the axial several holes of ferrite cores, constitutes a high containment devices. The string into the power cord or ground in blocks high-frequency signal from the power supply/ground. This component is particularly applicable to separate a printed circuit board, circuit, digital circuit, as well as the supply of high power driven areas. It should be noted that it must be placed in the area between the power capacitor energy storage and not be placed in the storage capacitor and electric parts.
3. Since the restoration of the fuse
This is a new polymer material component, when the current lower than their nominal value, it's only a few zero DC resistance in Europe. While the current to a certain extent, its resistance soared, causing fever, and the more thermal resistance, the greater the thus blocking the supply current. When the temperature drops down to automatically return to normal afterwards. This device prevents the CMOS device in the face of strong shock-type interference caused by the so-called "thyristor trigger" phenomenon. This phenomenon means integrated circuit silicon substrate become breakover, thus giving rise to current increases, resulting in fever and even burnt CMOS integrated circuit. 4. lightning protection devices
Outdoor use of single-chip system or power cable, signal cable from the outdoor overhead into the Interior, to consider the issue of lightning. Commonly used lightning devices are: gas discharge tube TVS (Transient Voltage Supervention), gas discharge tube is when power supply voltage is greater than a value, usually a dozens of Volt or hundreds of volts and gas breakdown discharge, the power cord on a strong shock pulse import Earth, TVS can be viewed as two parallel and opposite Zener diode, when electric voltage higher than a certain rating shidaotong. Its characteristics are transient and through hundreds to thousands of amps of current. Such components to and common-mode and anti-differential mode with the use of the inductive interference to improve anti-jamming performance.
Improved SCM system anti-interference ability of the principal means of
1. grounding
Here's the Earth grounding finger-jointed, also known as a protectorate. For single-chip system provides good ground, to improve the system anti-interference ability is extremely useful. Especially for lightning systems, good grounding is important. The above mentioned a antijamming series is intended to be a symbol, lightning stroke, surges, jamming and fast burst noise removal, removal methods are introduced will interfere with the Earth, if the system is not grounded, or despite the Earth but the Earth resistance is too large, then the component will not be able to play a role. For single-chip power of land known as logically, they and the Earth's land can communicates, floating null, or received a resistance, depending on applications. The ground just add in the heating pipes. Never put the Earth wire and power line of FireWire, zero line zero line confusion.
2. isolation and shielding
Typical signal isolation is photoelectric isolation. The use of opto-isolation devices are single-chip computer input and output isolation open, on the one hand, the interference signal will not be allowed to enter single-chip system, on the other hand the noise of the SCM system itself will not be spread by conduction. Shield is used to isolate the space radiation, noise, especially large parts, such as switching power supply, with a metal box cover can reduce the noise sources on-chip system. On special scared of interference of analog circuits, such as the high sensitivity of weak signal amplification circuits can be screened out. Whereas it is important that the metal shielding itself must take real.
3. filtering
Filtering refers to various types of signal by frequency characteristic classification and control of their direction. Commonly used with a variety of low-pass filter, high pass filter, band-pass filter. Low-pass filter used in the access on the AC power cord to the AC for 50 weeks passed, other import high-frequency noise. Low-pass filter configuration indicator is the insertion loss, select low-pass filter insertion loss is too low to suppressing noise, but too high insertion loss can cause "leakage", affects the system's personal security. High-pass, band-pass filter is based on the system for signal processing requirements.
The wiring and printed circuit board technology
Printed circuit board design on the SCM system anti-interference ability is important. To try to control the noise source, minimize noise propagation and coupling, and minimize noise absorbing these three principles for the design of printed circuit boards and wiring. When you design for printed circuit board microcontroller, do not imitate controlled the following bars check.
· PCB to rational distinction can generally be divided into single-chip system, i.e. the three areas of analog circuits (fear interference), digital circuit area (that is afraid of interference, and interference), power-driven district (interference).
· Printing plate by a single point of power, single-point grounding principles of transmission. Three regional power supply, ground wire from the point of three road leads. Noise components and non-noise components to some from far.
· Clock oscillation circuit, a special high-speed logic part of the coil. Let the surrounding farm approach zero.
· I/O drive parts, power amplifier as close as possible to the printing plate edge, close to the pin connector.
· Can slow do not have high-speed, high speed devices only used at key places.
· Use met the system requirements of the minimum frequency of the clock, a clock generator to as close as possible to use the clock of the device.
· Quartz crystal oscillator Shell to ground, clock lines to short and not made everywhere.
· Use 450 polyline wiring, do not use the 900 line to reduce the emission of high-frequency signal.
· Single-panel, dual panel, power cord, rough ground as possible. Signal line through-hole to as little as possible.
· 4-layer Board than 20dB double panel low noise. 6 layer Board than 4 laminates 10dB lower noise. Economic conditions allow to use multi-layer boards.
· Key lines to short and try to rough, and both sides plus protection. Be sensitive to signals and noise field band signal through a flat belt cable leads, to use ground-signal-ground ... Leads the way.
· Quartz oscillator noise sensitive devices below, to the followingIncrease space instead going other signal cables.
· Any signal lines do not form loops, such as the inevitable, the loop should be small.
· Clock line is perpendicular to the I/O thread is parallel to the I/O line noise to clock line to stay away from the I/O thread.
· On the A/D class devices, the digital section and some prefer to bypass the simulation also do not cross. Noise-sensitive cables dont and high-speed line, parallel to the current line.
· Single-chip and other IC circuit, if there are multiple power supplies, and end, each end to a decoupling capacitor.
· Single-chip unused I/O port are defined as output.
· Each IC to plus a decoupling capacitor to selected high-frequency signal good monolithic capacitor ceramic capacitor as decoupling capacitor. Decoupling capacitor welding in printed circuit boards, PIN to short.
· From high noise to signal you want to add filtering. Relay coil must be sealed with discharge diode. Can be used with a resistance to soften I/O line jumping along or provide certain damping.
· Large-capacity tantalum capacitor or polyester capacitor instead of Electrolytic Capacitor for circuit charging capacitor energy storage. Because the electrolytic capacitor inductance is large, high-frequency is not valid. When using electrolytic capacitors with high characteristics of good decoupling capacitor doubled.
· If need be, the power cord, ground can be combined with copper wire wound ferrite of HF choke devices blocking the transmission of high frequency noise.
· Weak signal pin, high frequency, high power led cable to Canadian Shield. Pinouts and ground wire to wire together.
· Printing plate is too big, or too high frequency signal lines, line delay time greater than or equal to the signal rise time, the line to the transmission line processing, to increase Terminal matching resistor.
· Try not to use IC socket, the IC direct welding in printing plates, IC-greateropportunities distributed capacitance.

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