rent switch, etc.
(3) System with weak analog signal circuit and high precision A / D conversion circuit.
2. Measures for increasing the system's anti-electromagnetic interference ability:
Use low frequency microcontrollers
Low external clock frequency microcontrollers can effectively reduce noise and improve the system's anti-jamming capability. The same frequency of square wave and sine wave, square wave in the high-frequency components are much more than the sine wave. Although the wave amplitude of the high frequency component of square wave is smaller than that of the fundamental wave, the higher frequency is easier to emit as a noise source and the most influential high frequency noise generated by the microcontroller is about as three times as the clock frequency.
Reduce the distortion in signal transmission
Microcontrollers are mainly manufactured by high-speed CMOS technology. In the signal input side, the static input current is about 1mA, the input capacitance is roughly 10PF. With high input impedance, high-speed CMOS circuit output has a considerable load capacity, that is a considerable output value. The door output is lead to a very high input impedance input through a very Long line, causing serious reflection problem which will result in signal distortion and increasing system noise. When Tpd>>Tr, a transmission line problem comes into being, so you must consider signal reflection, impedance matching and other issues.
The delay time of signal on the printed circuit board is related to the characteristic impedance of the lead, namely, it is connected with the dielectric constant of the PCB material. It can be roughly assumed that the transmission speed of the signal in the printed circuit board is about 1/3 to 1/2 of the light speed. The Tr (standard delay time) of the commonly used logical telephone elements in the system constituted by the microcontrollers is between 3 and 18 ns.
On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead and the line delay time is approximately between 4 and 20ns. In other words, the shorter lead of signal on the printed circuit is better, and the longest should not exceed 25cm. Besides, the number of vias should be as small as possible, preferably less than two.
When the rise time of the signal is faster than the delay time, it is necessary to follow the rules from fast electronics. At this moment, you should consider the impedance matching of transmission line. For the signal transmission between integrated block on a printed circuit board, you should avoid the emergence of Td>>Trd. Generally, the speed of the printed circuit board system cannot be too fast if the board is greater. In conclusion, when the signal is transmitted on the printed board, the delay time should be less than the nominal delay time of the device used.
Reduce the interference between signal lines:
The rise time of point A is Tr step signal which is passed through the lead AB to B side, while the delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the point A signal, the signal reflection after reaching point B and the delay of the AB line, a page pulse signal with a width of Tr will be induced after the Td time. At point C, as the transmission and reflection of the signal on AB line, it will sense a 2Td positive pulse signal that is twice the width of the delay time of the signal on the AB line. This is the interaction between the signal interference. The intensity of the interfering signal is related to the di/at of the C-point signal, which is related to line spacing. When the two signal lines are not very long, the two pulse superposition can be seen on line AB.
The micro-control of CMOS technology is of high input impedance, noise and noise margin, while the digital circuit is superimposed 100 ~ 200mv noise which does not affect its work. If the AB line is an analog signal, then this interference becomes intolerable. If a four-layer board has
large area of ground or the opposite of signal lines is a large area of the g