PCB News - Circuit board manufacturers: pull-up resistors and pull-down resistors

Circuit board manufacturers: pull-up resistors and pull-down resistors

This article describes the circuit principle and application of the difference between pull-up resistors and pull-down resistors. Circuit board manufacturers generally hire a group of circuit research and development engineers, and some of them are beginners. For those who are beginners in research and development, as well as some people engaged in software development, they often encounter pull-up resistors when developing microcontrollers. The concept of pull-down resistance cannot be understood literally. So, what are the pull-up resistors and pull-down resistors?

The pull-up resistor clamps the uncertain signal at a high level through a resistor, and this resistor also acts as a current limiter. In the same way, the pull-down resistor clamps the uncertain signal at a low level. The pull-up resistor refers to the input current of the device, and the pull-down refers to the output current. So when do you use up and down resistors?

1. When the TTL circuit drives the CMOS circuit, if the high level output by the ttl circuit is lower than the lowest high level of the CMOS circuit (usually 3.5V), then a pull-up resistor needs to be connected to the TTL output terminal to improve Output high level value.

2. The OC gate circuit must add a pull-up resistor to increase the output level.

3. To increase the drive capability of the output pins, pull-up resistors are often used on some single-chip microcomputer pins.

4. On the CMOS chip, in order to prevent damage caused by static electricity, the unused pins cannot be left floating. Generally, a pull-up resistor is connected to reduce the input impedance and provide a leakage path.

5. Add pull-up resistors to the pins of the chip to increase the output level, thereby improving the noise tolerance of the chip's input signal and enhancing the anti-interference ability.

6. Improve the anti-electromagnetic interference capability of the bus. If the pin is left floating, it is easier to accept external electromagnetic interference.

7. In the long-line transmission, the resistance mismatch can easily cause the reflected wave interference, and the pull-down resistance is the resistance matching, which effectively suppresses the reflected wave interference.

In addition, the selection principles of the resistance of the pull-up resistor include:

1. Considering power saving and current sink capability of the chip, it should be large enough; the resistance is large and the current is small.

2. It should be small enough to ensure sufficient drive current; the resistance is small and the current is large.

3. For high-speed circuits, excessive pull-up resistors may have smooth edges.

Considering the above three points comprehensively, it is usually selected between 1k and 10k. The same applies to pull-down resistors.

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