A must-read for beginners: some difficult-to-grasp concepts for 51 single-chip microcomputers

With the rapid development of electronic technology, computers have deeply penetrated into our lives. Many electronics enthusiasts have begun to learn about single-chip microcomputers, but the content of single-chip microcomputers is relatively abstract. Compared with the analog circuits and digital circuits that electronics enthusiasts are already familiar with, there are some new concepts in single-chip microcomputers. These concepts are so basic that most authors are unwilling to talk about them, and textbooks naturally will not explain these concepts in depth. However, these contents must be understood in learning. Here, based on my own learning and teaching experience, I will explain these most basic concepts, and I hope it will be helpful to self-learners.

1. Bus: We know that a circuit is always made up of components connected by wires. In analog circuits, wiring is not a problem because the devices are generally in serial relationship and there are not many wires between the devices. However, computer circuits are different. They are based on microprocessors. All devices must be connected to the microprocessor and the work between the devices must be coordinated with each other, so a lot of wires are needed. If the microprocessor and the device are connected separately as in analog circuits, the number of wires will be amazing. Therefore, the concept of bus is introduced in microprocessors. All devices share the wires. All 8 data lines of all devices are connected to 8 common lines, which is equivalent to connecting all devices in parallel. However, this is not enough. If two devices send data at the same time, one is 0 and the other is 1, then what does the receiver receive? This situation is not allowed, so it is necessary to control through control lines to make the devices work in time-sharing mode. Only one device can send data at any time (multiple devices can receive data at the same time). The data line of the device is called the data bus, and all the control lines of the device are called the control bus. There are storage units inside the microcontroller or in external memory and other devices. These storage units must be assigned addresses before they can be used. The assigned addresses are of course given in the form of electrical signals. Since there are many storage units, there are also many lines used for address allocation. These lines are called address buses.

2. Data, address, and instruction: The reason why these three are put together is that their essence is the same - numbers, or a sequence of '0' and '1'. In other words, addresses and instructions are also data. Instruction: A number specified by the designer of the microcontroller chip. It has a strict one-to-one correspondence with the commonly used instruction mnemonics and cannot be changed by the developer of the microcontroller. Address: It is the basis for finding the internal and external storage units and input and output ports of the microcontroller. The address value of the internal unit has been specified by the chip designer and cannot be changed. The external unit can be determined by the microcontroller developer, but some address units must be there (see the execution process of the program for details). Data: This is the object processed by the microprocessor. It is different in various application circuits. Generally speaking, the data being processed may have the following situations:
1. Address (such as MOV DPTR, #1000H), that is, address 1000H is sent to DPTR.
2. Mode word or control word (such as MOV TMOD, #3), 3 is the control word.
3. Constants (such as MOV TH0, #10H) 10H is the timing constant.
4. Actual output value (such as P1 port connected to colored lights, if you want all the lights to be bright, execute the instruction: MOV P1, #0FFH, if you want all the lights to be dark, execute the instruction: MOV P1, #00H) Here 0FFH and 00H are both actual output values. Another example is the font code used for LED, which is also the actual output value.
After understanding the nature of addresses and instructions, it is not difficult to understand why the program will run away during operation and why the data will be executed as instructions.

3. The second function usage of P0 port, P2 port and P3 port. When you first learn, you are often confused about the second function usage of P0 port, P2 port and P3 port. You think that there must be a switching process between the second function and the original function, or there must be an instruction. In fact, the second function of each port is completely automatic and does not need to be converted by instructions. For example, P3.6 and P3.7 are WR and RD signals respectively. When the microprocessor is connected to an external RAM or has an external I/O port, they are used as the second function and cannot be used as a general I/O port. As long as the microprocessor executes the MOVX instruction, the corresponding signal will be sent from P3.6 or P3.7, without the need for prior instruction instructions. In fact, "cannot be used as a general I/O port" does not mean "cannot" but (users) "will not" use it as a general I/O port. You can arrange a SETB P3.7 instruction in the instruction, and when the microcontroller executes this instruction, P3.7 will also become a high level, but users will not do this because it usually causes the system to crash (i.e. freeze).

4. Program execution process After the microcontroller is powered on and reset, the value of the program counter (PC) in 8051 is '0000', so the program always starts execution from the '0000' unit, that is to say: the unit '0000' must exist in the system's ROM, and there must be an instruction stored in the '0000' unit.

5. Stack The stack is an area used to store data. This area itself has nothing special. It is just a part of the internal RAM. What is special is the way it stores and retrieves data, which is the so-called "first in, last out, last in, first out". The stack has special data transmission instructions, namely "PUSH" and "POP", and there is a special unit dedicated to it, namely the stack pointer SP. Whenever a PUSH instruction is executed, SP automatically increases by 1 (based on the original value), and whenever a POP instruction is executed, SP automatically decreases by 1 (based on the original value). Since the value in SP can be changed by instructions, as long as the value of SP is changed at the beginning of the program, the stack can be set in the specified memory unit. For example, at the beginning of the program, a MOV SP, #5FH instruction is used to set the stack in the unit starting from memory unit 60H. Generally, there is always an instruction to set the stack pointer at the beginning of a program. Because when the computer is turned on, the initial value of SP is 07H, so the stack starts from unit 08H and goes backwards. The area from 08H to 1FH is the second, third, and fourth working register area of ​​8031, which is often used, which will cause data confusion. When different authors write programs, the initialization instructions for the stack are not exactly the same. This is a problem of the author's habit. When the stack area is set, it does not mean that the area becomes a special memory. It can still be used like a normal memory area, but programmers generally do not use it as normal memory.

6. The development process of the single-chip microcomputer The development process mentioned here is not the one that starts with task analysis as described in general books. We assume that the hardware has been designed and made, and the next step is to write the software. Before writing the software, we must first determine some constants and addresses. In fact, these constants and addresses have been determined directly or indirectly during the design stage. For example, when the connection of a device is designed, its address is also determined. When the function of the device is determined, its control word is also determined. Then use a text editor (such as EDIT, CCED, etc.) to write the software. After writing, use a compiler to compile the source program file and check for errors until there are no syntax errors. Except for very simple programs, the software is generally debugged using an emulator until the program runs correctly. After running correctly, you can write the chip (solidify the program in the EPROM). After the source program is compiled, a target file with the extension HEX is generated. Generally, the programmer can recognize files in this format. As long as this file is loaded, the chip can be written. Here, to give you an understanding of the whole process

Table 1 is the source program, Table 2 is the HEX file obtained after assembly, and Table 3 is the target file converted from the HEX file, that is, the file finally written into the EPROM, which is converted by the programmer or by programs such as HEXBIN. Those who have learned manual assembly should not have difficulty in finding the one-to-one correspondence between Table 3 and Table 1. It is worth noting that there is a long string of 'FF' starting from 02 00 40 to 75 81, which is the result of the pseudo instruction: ORG 040H.

7. Simulation and Simulator Simulation is a very important part in the development of single-chip microcomputers. Except for some very simple tasks, simulation is generally required in the process of product development. The main purpose of simulation is to debug software. Of course, with the help of the simulator, some hardware debugging can also be performed. A single-chip microcomputer application circuit board includes the single-chip microcomputer part and the application circuit designed to achieve the purpose of use. Simulation is to use the simulator to replace the single-chip microcomputer part of the application circuit board (called the target machine) to test and debug the application circuit part. There are two types of simulation: CPU simulation and ROM simulation. The so-called CPU simulation refers to the method of using the simulator to replace the CPU of the target machine, and the simulator provides various signals and data to the application circuit part of the target machine for debugging. This kind of simulation can run the program through multiple methods such as single-step running and continuous running, and can observe the changes inside the single-chip microcomputer, which is convenient for correcting errors in the program. The so-called ROM simulation is to use the simulator to replace the ROM of the target machine. When the CPU of the target machine is working, the program is read from the simulator and executed. This kind of simulation actually treats the simulator as an EPROM, but it saves the trouble of erasing and writing, and there are not many debugging methods. Usually these are two different types of emulators, that is, one emulator cannot do both CPU emulation and ROM emulation. If possible, CPU emulation is preferred.