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In the P0 port of the 51 MCU, when operating in normal IO mode, it functions as a quasi-bidirectional IO port. However, when the second function is enabled, it becomes a standard bidirectional IO port. For bidirectional IO ports, it's necessary to output both high and low levels, which typically requires a complementary push-pull circuit.
In its second functional state, the P0 port of the 51 MCU uses a complementary push-pull configuration. What exactly is a complementary push-pull? Here's an equivalent circuit diagram to illustrate the concept.
When the P0 port is used as an output in its second function, switches K1 and K2 are alternately activated. When K2 is closed and K1 is open, a high level is output. This has strong driving capability because the resistance of the electronic switch is small, much less than that of a pull-up resistor. Conversely, when K1 is closed and K2 is open, a low level is output.
These two switches work in a complementary manner, with one "pushing" and the other "pulling" current at the IO port. Hence, this setup is called a complementary push-pull configuration.
The main advantage of this structure is its strong driving capability and stability. However, it comes with a drawback: it’s more complex to implement. During the transition from a low to a high level, for example, K1 must turn off before K2 turns on. If both switches are turned on simultaneously, a short circuit could occur, leading to serious consequences. Therefore, careful coordination between the two switches is essential.
For the input function of a bidirectional IO port, the port operates in a high-impedance state. This means the input resistance is very high, allowing the port to detect external signals without interfering with them. Unlike the output stage, which requires a push-pull circuit, the input can simply be left in a high-impedance state by disconnecting both switches.
What does a high-impedance state mean? It refers to a floating input where the voltage level is not fixed. Imagine the microcontroller's IO port as a voltmeter with a very large internal resistance—like 100MΩ. This high resistance allows the port to sense external signals without drawing significant current.
If you touch the P0.0 pin, your body's resistance (which is quite high) may cause the detected voltage to fluctuate. This makes the reading unstable, and even without touching, electromagnetic interference might change the level. Later, we'll conduct an experiment using the 51 MCU to observe this behavior firsthand.
Why is a high-impedance input important for a bidirectional IO port? Let's consider a device connected to the IO port. If the device has a high output resistance, it may not drive the IO port effectively. In such cases, a high-impedance input ensures that the microcontroller doesn't interfere with the device's signal.
Imagine a scenario where a device outputs a low level through a 100kΩ resistor, and the 51 MCU's P1.0 port is connected. Due to the voltage divider effect, the MCU might read a high level instead of the intended low. This shows how critical it is for the IO port to have a high input impedance.
In general, a high-impedance input prevents the microcontroller from affecting the output of weak devices. The higher the input impedance, the less interference it causes.
To experience the high-impedance state of the P0 port, we can write a simple program:
```c
#include
sbit TOUCH = P0^0;
sbit LED = P1^0;
void main() {
TOUCH = 1; // Set as high-impedance input
while(1) {
LED = TOUCH;
}
}
```
In this setup, the LED is connected to P1.0, and P0.0 is left unconnected. On many development boards, P0.0 is pulled up externally, making this experiment difficult. It's best to use a breadboard for this test.
After programming, the TOUCH pin should be in a high-impedance state, meaning its level is undefined. As a result, the LED's state should be unpredictable. In practice, however, the LED might stay lit due to residual effects or minor leakage currents.
By placing a finger or a large resistor (like 100kΩ) between P0.0 and VCC, you may notice the LED turning off or dimming. This happens because the high-impedance state allows even a small current to influence the voltage level.
Note: Skin resistance varies among individuals, so results may differ. If the LED doesn’t change, try using a larger resistor instead of your finger. Also, if the LED is initially off, try connecting two fingers to P0.0 and GND to see a change. If nothing works, double-check your program or the microcontroller’s minimum system.