Electronic Design - SMT or DIP materials and up or down resistors in PCB design

The difference between SMT materials and DIP materials

SMT materials are mainly surface mount components, such as resistors, capacitors, and single-chip microcomputers on the internal surface of mobile phones. They are soldered using surface assembly technology. Usually, they go through the following processes during production:

Solder paste printing: The solder paste is printed on the surface of our PCB board by hand or machine.

Component placement: use manual or placement machine to mount SMT components on the printed PCB board.

Reflow soldering: through the process of gradual heating, the solder paste is melted at a certain temperature, and the mounted components and the PCB board are effectively soldered together to achieve reliable electrical performance.

Advantages of this processing method These components have a small footprint, a very high production efficiency, and less problems.

DIP materials are mainly in-line components, such as electrolytic capacitors on computer motherboards, power transformers, triodes, etc., which are mainly processed by manual soldering or wave soldering. Compared with SMT materials, the processing technology is different. And the cost is much higher than the patch.

At present, most of the welding industry is mainly based on SMT material welding, with only a small number of DIP components, and some special products will basically use DIP components, such as power supplies.

Pull-up resistors and pull-down resistors in PCB design

The PCB design pull-up resistor is: connect an uncertain signal (high or low level) to the power supply VCC through a resistor, and fix it at a high level;

The PCB design pull-down resistor is to connect an uncertain signal (high or low level) to the ground GND through a resistor and fix it at the low level.

Where are the pull-up resistors and pull-down resistors used in PCB design?

Answer: Used in digital circuits where there are high and low levels.

How to wire the pull-up resistor and the pull-down resistor in PCB design?

Answer: Pull-up resistor: one end of the resistor is connected to VCC, one end is connected to logic level access pins (such as MCU pins)

Pull-down resistor: one end of the resistor is connected to GND, one end is connected to the logic level access pin (such as the single-chip pin)

The role of pull-up resistors and pull-down resistors

1) Improve the drive capability of the output pin:

For example, when the CPU pin of STM32 outputs a high level, but due to the influence of the subsequent circuit, the output high level is not high, which means that the VCC cannot be reached and the circuit operation is affected. Therefore, it is necessary to connect a pull-up resistor (in fact, it is to increase the output current of the wire). The situation of the pull-down resistor is the opposite. Let the CPU pin of the STM32 output a low level. As a result, the low level of the output cannot reach GND (in fact, it is to reduce the output current of the wire) because the subsequent circuit affects the output, so a pull-down resistor is connected.

2) When the pin level is uncertain, let the back have a stable level:

For example, take the example of connecting a pull-up resistor. When the STM32 is just powered on, the pin level of the chip is uncertain, especially when the pin is connected to a button, a certain level must be given to it. The function of the pull-down resistor is if If the level of the previous pin is uncertain, force the level to remain high.

3) Prevent the pin from floating, otherwise it will easily generate accumulated electric charge and static charge, which will cause the circuit to be unstable.

Why is the pull-up resistance of the button 10k ohms?

Answer: The PCB pull-up resistance of the button can be 3.3k, 4.7k, 5.1k, or 10k, but the smaller the resistance, the greater the power consumption. In the current smart ecology, we are pursuing low power consumption and high efficiency., 10k is the pin current that most smart product chips can recognize. If the resistance is too large and the current is too small, the pin cannot be recognized, so 10k is a compromise solution.