Which design principles should be followed for the selection of the pull-up resistor of the 51-chip microcontroller p0?

1, 51 single-chip microcomputer P0 port in order to achieve the quasi-3 state, the use of OC output, that is, the collector floating output, there is also called totem pole output. This kind of circuit structure has only pull-down capability, high-level output has no current, and it shows high resistance state at high level; plus pull-up resistor, it will lose high resistance state and become 1,0 two states.

2, P0 port pull-up resistor selection, should follow the basic circuit design principles, can not be blindly applied, depending on the situation of the latter:

For example, the latter stage drives an NPN transistor. The magnification of this transistor (β) = 100 times, the load current of the transistor (Ic) requires 100 mA, when the power supply is 5V:

a, the transistor Ib = Ic / β = 1 mA;

b, the pull-up resistor is approximately = R = 5V / Ib = 5kΩ;

c, in order to make the triode into the conduction (on) state after injecting Ib, R should be smaller, such as 4k3;

3. It can be seen from the above calculation that the load size and amplification factor of the triode directly affect the selection of the pull-up resistor of the pre-stage; the selection of the pull-up resistor is unreasonable, or the post-drive is weak or cannot be driven at all; or the power consumption of the MCU port is too large. Especially battery-powered devices should pay attention!

More importantly, the pull-up resistor must not be less than 250Ω! Otherwise it will damage the port!

4, the above circuit design should be said to be unreasonable; we can often see such circuit design:

The microcontroller port controls the relay through pull-up resistor > NPN transistor. This design always needs to pick up the relay when power is on. At this time, the port should be cleared to 0 immediately after the program is started, so that the relay is released; The short-term suction of electricity can not be overcome.

A better use is to use a negative logic design, such as port-driven LED lights, should use port > LED > resistance > power, avoid using port > LED > resistance > ground + pull-up resistor; then for example, the microcontroller output to drive the transistor, most Good to use PNP type! In this way, there is no trouble of pulling up (the base resistance can't be less!); when the IC is in the latter stage, that is, it is to be pulled up, it is also symbolic, and it is completely possible to use 5~10k.

5, in addition to P0 port, other ports are not special design, no need to consider pulling up!