ARM development ideas that need to be understood when first contacting ARM

Make a minimum system board: If you have never done ARM development, it is recommended not to be greedy and complete at the beginning, and make all the applications well, because the startup method of ARM is different from that of DSP or MCU, and various problems will often be encountered. Therefore, it is recommended to first lay out a small system board with only Flash, SRAM or SDRAM, CPU, JTAG, and reset signal, leaving an expansion interface. If the minimum system can run normally, the task is half completed. Fortunately, the peripheral interfaces of ARM are basically standard interfaces. It would be even better if you have experience in wiring these hardware.

Write startup code

Write a small bootable code based on the hardware address, including the following parts: initializing the port, shielding interrupts, copying the program to SRAM, completing code remapping, configuring interrupt handles, and connecting to the C language entry. Maybe some sample programs have a lot of things in the bootloader, but don't be bothered by these complicated programs, because they need to be designed in the process of designing the development board and are not included in the scope of ARM design.

Research chip information

Although ARM is compatible in terms of core, each chip has its own characteristics, and these issues must be considered when writing programs. Especially girls, don't be dependent here, always want to modify other people's sample programs, but the more you modify, the more messy it becomes.

Understand operating system programs

There are many open source programs for ARM applications. If you want to improve yourself, you should read more other people's programs. Linux, uc/os-II, etc. are all very good source codes.

hardware

If the designer makes the hardware by himself, each manufacturer basically has a DEMO board schematic for the chip. If you digest the schematic first, you will have a clear idea of ​​the resource allocation when you design in the future. The device's DATSHEET must be digested well.

Minimum system board

Many people will ask, is it better to make a minimum system board with 2 layers or 4 layers? Answer: Only AT91 can use a two-layer board, and the others can use at least 4 layers; 44b0 ground and power supply can also be used with a two-layer board; Talk about four-layer boards and 33 ohm resistors: The selection of four-layer boards is not only a matter of power and ground, but also the impedance of the high-speed digital circuit has requirements for the impedance of the wiring, and the impedance of the two-layer board is not easy to control. The 33 ohm resistor is generally added to the driver end, which also plays an impedance matching role; when wiring, the data address line and the high-speed line that needs to be guaranteed should be laid first; at high frequencies, the wiring on the PCB board should be regarded as a transmission line.

Transmission lines have their characteristic impedance. Those who have studied transmission line theory know that when there is a sudden impedance change (mismatch) somewhere on the transmission line, the signal will be reflected, and the reflection will interfere with the original signal. In serious cases, it will affect the normal operation of the circuit. When using a four-layer board, the outer layer usually has signal lines, and the middle two layers are power and ground planes. This isolates the two signal layers on the one hand, and more importantly, the outer layer of the lines and the planes they are close to form a transmission line called a "microstrip", whose 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 mainly related to the width of the trace, the distance to the reference plane, the thickness of the copper plating, and the characteristics of the dielectric material. There are many ready-made formulas and programs for calculation. A 33-ohm resistor is usually connected in series at one end of the driver (not necessarily 33 ohms, ranging from a few ohms to fifty or sixty ohms, depending on the specific circuit situation). Its function is to match the impedance of the trace after being connected in series with the output impedance of the transmitter, so that the signal reflected back (assuming that the impedance of the receiving end is not matched) will not be reflected back again (absorbed), so that the signal at the receiving end will not be affected.

The receiving end can also be matched, for example, by using resistors in parallel, but it is rarely used in digital systems because it is more troublesome, and in many cases, multiple signals are sent at once, such as the address bus, which is not as easy to do as the source end matching. The high frequency mentioned here does not necessarily refer to a circuit with a very high clock frequency. Whether it is a high frequency depends not only on the frequency, but more importantly on the rise and fall time of the signal. Usually, the rise (or fall) time can be used to estimate the frequency of the circuit, generally taking half of the reciprocal of the rise time. For example, if the rise time is 1ns, then its reciprocal is 1000MHz, which means that when designing the circuit, it is necessary to consider the frequency band of 500MHz.

Sometimes it is necessary to deliberately slow down the edge time. The output slope of the driver of many high-speed ICs is adjustable. Linux itself has a complete set of tool chains, which makes it easy to establish the development environment and cross-operation environment of embedded systems by yourself, and can overcome the barriers of simulation tools (ICE) in embedded system development. The complete openness of the kernel allows people to design and develop real hard real-time systems by themselves, and soft real-time systems are also easy to implement in Linux. Powerful network support makes it possible to use Linux's network protocol stack to develop it into an embedded TCP/IP network protocol stack. Linux provides the basic kernel for completing embedded functions and all the required user interfaces. It is multifaceted. It can handle embedded tasks and user interfaces.

A small embedded Linux system only needs the following three basic elements: * Boot tools * Linux microkernel, which consists of memory management, process management and transaction processing * Initialization process If you want it to do something and continue to keep it small, you have to add: * Hardware drivers * One or more applications that provide the required functions. To add more functions, you may need these: * A file system (perhaps in ROM or RAM) * TCP/IP network stack.