External structure of single chip microcomputer

If you get a chip and want to use it, you must first know how to connect it. The chip we use is called 89C51. Let's take a look at how to connect it.

1. Power supply: This is of course essential. The microcontroller uses a 5V power supply, with the positive pole connected to pin 40 and the negative pole (ground) connected to pin 20. 2. Oscillator circuit: The microcontroller is a timing circuit that must provide a pulse signal to work properly. The oscillator has been integrated inside the microcontroller, using a crystal oscillator, connected to pins 18 and 19. Just buy a crystal oscillator and a capacitor and connect them. Just connect them according to Figure 1. 3. Reset pin: Connect according to the method shown in Figure 1. As for what reset means and why it is necessary to reset, it is introduced in the microcontroller function. 4. EA pin: The EA pin is connected to the positive power supply terminal. At this point, a microcontroller is connected, and the microcontroller starts working after power is turned on.

Figure 1

2. Task Analysis

Our first task is to use the microcontroller to light up a light-emitting diode (LED). Obviously, this LED must be connected to a pin of the microcontroller, otherwise the microcontroller cannot control it. So which pin should it be connected to? In addition to the 5 pins just used, there are 35 pins on the microcontroller. We connect this LED to pin 1. (See Figure 1, where R1 is the current limiting resistor)

According to the connection method in this diagram, when pin 1 is at a high level, the LED will not light up, and only when pin 1 is at a low level, the LED will light up. Therefore, we need to be able to control pin 1, that is, we need to be able to make pin 1 change to a high or low level as required. Since we want to control pin 1, we have to give it a name. We can't just call it pin 1, right? What name should we call it? INTEL, the company that designed the 51 chip, has already named it P1.0. This is a rule and cannot be changed by us.

Now that we have a name, how do we make it 'high' or 'low'? To tell someone to do something, we just need to say it, which is called issuing a command. To tell a computer to do something, we also need to send a command to the computer. The command that the computer can understand is called a computer instruction. The instruction to make a pin output a high level is SETB, and the instruction to make a pin output a low level is CLR. Therefore, if we want P1.0 to output a high level, we just need to write SETB P1.0, and if we want P1.0 to output a low level, we just need to write CLR P1.0.

Now we have a way to make the computer output P10 high or low, but how can we make the computer execute this instruction? We can't just tell the computer and leave it at that. To solve this problem, there are still several steps to go. First, the computer cannot understand instructions such as SETB CLR. We have to translate the instructions into a way that the computer can understand, and then let the computer read it. What can the computer understand? It only understands one thing - numbers. Therefore, we have to change SETB P1.0 to (D2H, 90H) and CLR P1.0 to (C2H, 90H). As for why these two numbers are used, this is also stipulated by the designer of the 51 chip - INTEL, and we will not study it. The second step is, after obtaining these two numbers, how to let these two numbers enter the internal of the microcontroller? This requires the help of a hardware tool "programmer".

Figure 2

We connect the programmer to the computer, run the programmer software, and then write (D2H, 90H) in the editing area (see Figure 2). Write... OK, take the chip off, insert the chip into the finished circuit board, and turn on the power... What? The light is not on? That's right, because the instruction we wrote in is to make P10 output a high level, so of course the light is not on. If it is on, it is wrong. Now we unplug this chip again, put it back on the programmer, and change the content of the editing area to (C2H, 90H), that is, CLR P1.0, write the chip, take the chip off, insert the chip into the circuit board, and connect the power. OK, the light is on. Because the () we wrote is the instruction to make P10 output a low level. In this way, we can see that the connection of the hardware circuit has not been changed. As long as the content written into the microcontroller is changed, the output effect of the circuit can be changed.