Because of the programming involved, learning ARM microcontroller system is somewhat difficult for designers engaged in electronic circuits. Learning knowledge is not difficult. It is difficult to sort out the development ideas and find a good starting point. This article will start from this step, to introduce to you the first contact with ARM development should be clear from the aspects of development ideas.
Be a minimal system board: If you have never done ARM development, it is recommended not to be greedy at first, and to do all the applications well, because ARM's startup method is different from dsp or microcontroller, and often encounters various kinds. Problem, so it is recommended to first install a small system board with only Flash, SRAM or SDRAM, CPU, JTAG, and reset signals, leaving the expansion interface. The minimum system can run normally, and the task is half completed. Fortunately, the peripheral interfaces of ARM are basically standard interfaces. It is better if you have the wiring experience of these hardware. 
Write startup code
Write a small code that can be started according to the hardware address, including the following parts: initializing the port, masking the interrupt, copying the program to the SRAM, completing the code re-mapping, configuring the interrupt handler, and connecting to the C language entry. Maybe there are a lot of things in the bootloader in some sample programs, but don't be bothered by these complicated programs, because this is the design that needs to be designed during the development board, and is not included in the ARM design.
Research chip data
Although ARM is compatible on the kernel, each chip has its own characteristics, and these issues must be considered when writing programs. Especially girls, don't rely on psychology here, always want to use other people's sample program to modify, but the more chaos.
Understand the operating system program
There are a lot of open source code programs in ARM. If you want to improve yourself, you should look at other people's programs. Linux, uc/os-II, etc. are all good original code.
hardware
If the designer makes the hardware himself, each manufacturer basically has a DEMO board schematic for the chip. If you first digest the schematic. In the future, when designing, you can know how to allocate resources. The DATSHEET of the device must be well digested.
Minimum system board
Many people will ask, is the minimum system board 2 or 4 layers good? A: Only AT91 can use two layers, the other minimum of 4 layers; 44b0 ground and power processing can also be used two layers; talk about four layers And 33 ohm resistor: the choice of four-layer board is not only a problem of power supply and ground, high-speed digital circuit has the requirement of the impedance of the trace, and the two-layer board does not control the impedance. The 33 ohm resistor is generally applied to the driver terminal, and also functions as an impedance matching; the data address line must be arranged first and the high speed line to be guaranteed when wiring; at high frequencies, the trace on the PCB board should be regarded as the transmission line.
The transmission line has its characteristic impedance. It is known that the transmission line theory knows that when there is a sudden change in impedance (mismatch) at a certain point on the transmission line, the signal will be reflected when it passes, and the reflection will cause interference to the original signal. If it is serious, it will affect the normality of the circuit. jobs. When a four-layer board is used, the outer layer usually has a signal line, and the middle two layers are respectively a power source and a ground plane, so that on the one hand, the two signal layers are isolated, and more importantly, the outer layer traces form a plane close to them. For the "microstrip" transmission line, its impedance is relatively fixed and can be calculated.
It is more difficult to do this for a two-layer board. This transmission line impedance is primarily related to the width of the trace, the distance to the reference plane, the thickness of the copper, and the properties of the dielectric material. There are many off-the-shelf formulas and procedures available for calculation. 33 ohm resistors are usually placed in series at the end of the drive (in fact, not necessarily 33 ohms, from a few ohms to five or sixty ohms, depending on the circuit), the role is to connect with the output impedance of the transmitter The impedance of the line is matched so that the signal reflected back (assuming the impedance of the unresolved end is not matched) will not be reflected back (absorbed), so that the signal at the receiving end will not be affected.
The receiving end can also be matched, for example, using resistors in parallel, but it is less used in digital systems, because it is more troublesome, and often it is a multi-receipt, such as an address bus, which is not as easy to match as the source. The high frequency mentioned here is not necessarily a circuit with a high clock frequency. Whether the high frequency is not only the frequency but also the rise and fall time of the signal. It is usually possible to estimate the frequency of the circuit with a rising (or falling) time, typically taking half of the reciprocal of the rise time. For example, if the rise time is 1 ns, then its reciprocal is 1000 MHz, which means that the design circuit is to be considered in the 500 MHz band. .
Sometimes the edge time is deliberately slowed down, and the output slope of many high-speed IC drivers is adjustable. Linux itself has a complete set of toolchains that make it easy to build embedded system development environments and cross-running environments, and can overcome the barriers of simulation tools (ICE) in embedded system development. The complete openness of the kernel allows one to design and develop a truly hard real-time system, and the soft real-time system is easily implemented in Linux. Powerful network support makes it possible to develop it into an embedded TCP/IP network protocol stack using Linux's network protocol stack. Linux provides a basic kernel for embedding and all the user interfaces needed, it is multifaceted. It handles embedded tasks and user interfaces.
A small embedded Linux system requires only three basic elements: * Boot tool * Linux microkernel, consisting of memory management, process management, and transaction processing * Initialization process If you want it to do something and continue to be miniaturized, Plus: *Hardware Drivers* One or more applications that provide the required functionality. Adding functionality, perhaps these are needed: * A file system (perhaps in ROM or RAM) * TCP/IP network stack.
We will explain the novices from the four aspects of startup code, chip data, operating system program, and minimum system version. In the development of ARM, we need to start from these four aspects, then we will start in the process of learning. I feel that I am suddenly more open than a confused one. I believe that there will be unexpected gains.
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