overall plan
The tasks to be completed by the software have been specified in the overall design. In the specific software design, the hardware structure should be combined to further clarify the details of the tasks undertaken by the software, determine the specific implementation methods, and allocate resources reasonably.
1 programming technology
A reasonable software structure is the basis for designing a high-performance single-chip application system software. In the program design, the structured programming style should be cultivated, and each functional program is modularized and subprogrammed. There are generally two design methods.
a. Module programming
Module programming is a programming technique commonly used in microcontroller applications. It is to decompose a long program into a number of smaller program modules with relatively independent functions. Each program module is designed, programmed and debugged. Finally, each debugged module constitutes a large program.
The advantage is that the design and debugging of a single function-specific program module is convenient and easy to complete, and one module can be shared by multiple programs. The disadvantage is that the connection of each module is sometimes difficult.
b. Top-down programming
In top-down programming, design begins with the main program, and subordinate programs or subroutines are replaced with symbols. After the main program is compiled, each subordinate program and subroutine are compiled, and finally the design of the entire system software is completed.
The advantage is that it is more in line with people's daily thinking, design, debugging and connection at the same time according to a clue, program errors can be found earlier. The disadvantage is that the upper level of the program error will have an impact on the entire program, a modification may cause a comprehensive modification of the entire program.
2 programming
After selecting the software structure and the programming techniques used, you can proceed with the programming and turn the design tasks into specific programs.
a. Establish a mathematical model
According to the design task, the mathematical relationship between each input variable and each output variable is described. This process is to establish a mathematical model. Mathematical models vary with system tasks, and their accuracy is one of the decisive factors for system performance.
b. drawing program flow chart
Program flow diagrams are typically drawn prior to writing the program to improve the overall efficiency of the software design. The program flow chart describes the task in a concise and intuitive way, and it is easy to write the program from this, so it is especially suitable for beginners.
In the design process, first draw a simple functional flow chart (rough block diagram), then refine and concreteize the function flow chart, and make specific assignments and descriptions of memory, registers, flag bits, etc. The operation of each thick frame in the flowchart is converted into the operation of a specific memory unit, working register or I/O port, thereby giving a detailed program flow chart (detailed block diagram).
c. Programming of the program
After completing the program flow chart design, you can write the program. Programming languages ​​have a greater impact on programming. Assembly language is the most commonly used single-chip programming language. The programming code is simplified in assembly language and designed directly for hardware circuits. The speed is fast, but when a large number of data operations are performed, the writing difficulty will be greatly increased, and it is difficult to read and debug. C language (such as C51 of MCS-51) or PL/M language can be used when there are a large number of data operations.
When writing a program, you should pay attention to the reasonable allocation and use of the system hardware resources, and the setting and delivery of the entry/exit parameters of the subroutine. Use reasonable data structure and control algorithms to meet the accuracy requirements of the system.
In the storage space allocation, the most frequently used data buffer should be set in the internal RAM; the flag should be set in the on-chip RAM bit operation area (20H ~ 2FH); specify the user stack area, the size of the stack area should be left Quantity; the remaining part acts as a data buffer.
In the process of writing a program, according to the flow chart, the symbolic instructions are used to describe one by one, that is, the assembly language source program is obtained. It should be written according to the standard symbols and formats of the MCS-51 assembly language. At the same time as completing the system functions, attention should be paid to ensuring the reliability of the design, such as digital filtering, software traps, and protection. A number of functional notes can be made as necessary to improve the readability of the program.
3 program design
After editing each program module, it needs to be assembled or compiled and debugged. When the design requirements are met, each program module is connected according to the requirements of the software structure design, which is the software assembly, thus completing the software design. Pay attention to the software interface when assembling the software.
What problems should be considered in the hardware design of the microcontroller application system?
(1) Memory expansion: The capacity requirement is to consider the internal memory resources of the microcontroller when selecting the microcontroller. If it can meet the requirements, it does not need to be extended. Pay attention to the type, capacity and interface of the memory when it is necessary to expand. Generally, leave room for it. And minimize the number of chips. Choose the appropriate method, ROM and RAM form, RAM to perform power-down protection.
(2) Extension of I/O interface: The single-chip application system should consider the volume, price, load capacity, function and other aspects when expanding the I/O interface. The appropriate address decoding method should be selected according to the number of externally extended circuits and the internal resources of the selected microcontroller (the number of free address lines).
(3) Input channel design: The input channel design includes the design of the switching quantity and the analog input channel. The switch quantity should consider the interface form, voltage level, isolation mode, expansion interface, and so on. The design of the analog channel should be combined with the signal detection link (sensor, signal processing circuit, etc.). It should be selected according to the system's requirements for speed, accuracy and price. It also needs to match the performance of the sensor and other devices. Type, type of transmission signal (current or voltage), linearization, compensation, optical isolation, signal processing, etc., should also consider the choice of A / D converter (conversion accuracy, conversion speed, structure, power consumption, etc.) and related Circuits, expansion interfaces, and sometimes software design. High-precision analog-to-digital converters are very expensive, so the requirements for A/D converters should be minimized. The functions that can be implemented in software are implemented in software as much as possible.
(4) Design of the output channel: The output channel design includes the design of the switching quantity and the analog output channel. The amount of switching should take into account the power and control methods (relays, thyristors, triodes, etc.). The analog output should consider the choice of D/A converter (conversion accuracy, conversion speed, structure, power consumption, etc.), the form of the output signal (current or voltage), isolation mode, expansion interface, and so on.
(5) Human-machine interface design: The design of the human-machine interface includes input keyboard, switch, dial, start/stop operation, reset, display, printing, indication, alarm, etc. Input keyboard, switch, dial should consider the type, number, parameters and related processing (such as button debounce processing). Start/stop, reset operations should consider the mode (automatic, manual) and its switching. The display should consider the type (LED, LCD), the type of display information, multiples, and so on. In addition, consider the expansion interface of various human-machine interfaces.
(6) Design of communication circuit: Single-chip application system is often used as on-site measurement and control equipment. It often forms a measurement and control network with the upper computer or the same position machine. It needs the ability of data communication. It is usually designed as RS-232C, RS-485, infrared transceiver, etc. Communication standard.
(7) Design and production of printed circuit boards: Circuit schematics and printed circuit board designs are often designed using professional design software such as Protel, OrCAD, etc. Designing printed circuit boards requires a lot of skill and experience. After designing the printed circuit board diagram, it should be sent to a professional manufacturer for production. After installing the components on the printed circuit board, the hardware design and production are completed.
(8) Load tolerance considerations: The load capacity of the MCU bus is limited. For example, the load capacity of the P0 port of the MCS-51 is 4 mA, and up to 8 TTL circuits are driven. The load capacity of the P1 to P3 ports is 2 mA, and up to four TTL circuits are driven. If there are more external loads, the bus drive method should be adopted to increase the load tolerance of the system. Commonly used drivers are: unidirectional driver 74LS244, bidirectional driver 74LS245 and so on.
(9) Signal logic level compatibility considerations: In the designed circuit, there may be both TTL and CMOS devices, as well as non-standard signal levels, and corresponding level compatibility and conversion circuits should be designed. When there are RS-232, RS-485 interfaces, level compatibility and conversion are also required. Commonly used integrated circuits are MAX232, MAX485 and so on.
(10) Configuration of the power supply system: The application system of the single-chip microcomputer must require the power supply, and the number of power supply groups, output power, and anti-interference should be considered. Be familiar with the application of commonly used three-terminal regulators (78Ñ…Ñ… series, 79Ñ…Ñ… series), precision power supplies (AD580, MC1403, CJ313/336/385, W431).
(11) Implementation of anti-interference: Taking necessary anti-interference measures is an important part of ensuring the normal operation of the single-chip system. It includes chips, device selection, decoupling filtering, printed circuit board routing, channel isolation, and more.
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