Hardware selection for ARM development

Introduction: If you have experience in hardware and microcontrollers, it is recommended that you make a minimum system board yourself. If you have never done ARM development, it is recommended that you do not try to be too ambitious at the beginning and make all the applications complete, because the startup method of ARM is different from that of DSP or microcontroller, and you will often encounter various problems.

1. If you have experience in hardware and single-chip microcomputers, it is recommended that you make a minimum system board yourself: If you have never done ARM development, it is recommended that you do not be greedy for perfection at the beginning, and make all the applications well, because the startup method of ARM is different from that of DSP or single-chip microcomputers, and you often encounter various problems. Therefore, it is recommended to first lay out a small system board with only Flash, SRAM or SDRAM, CPU, JTAG, and reset signals, leaving an expansion interface. If the minimum system can run normally, your task is half completed. Fortunately, the peripheral interfaces of ARM are basically standard interfaces. If you have experience in wiring these hardware, it is a very easy thing for you.

2. Write the startup code. According to the hardware address, write a small code that can be started, including the following parts:

Initialize the port, shield the interrupt, copy the program to SRAM; complete the code remapping; configure the interrupt handle and connect it to the C language entry. You may see that the bootloader has a lot of things in some sample programs given to you, but don't be bothered by these complicated programs, because you are not making a development board, your task is to make a small program to make your application run.

3. If you are working on hardware, each manufacturer basically has a DEMO board schematic for the chip. Digest the schematic first. This way, when you design in the future, you will have a clear idea of ​​how to allocate resources. You must digest the device's DATSHEET well.

4. Carefully study the information of the chip you are using. Although ARM is compatible in terms of the kernel, each chip has its own characteristics, and these issues must be considered when writing programs. Especially for girls, don't be dependent here. You always want to modify other people's sample programs, but the more you modify them, the more messy they become.

5. Read more operating system programs. There are many open source programs for ARM applications. If you want to improve yourself, you should read more programs from others. Linux, UC/OS-II, etc. are all very good source codes.

6. If you are doing software, it is best to have some understanding of the operating system mechanism. Of course, this is a piece of cake for software engineers. But if you are a hardware engineer, it will be a bit difficult.

Q: Is it better to make a minimum system board with 2 layers or 4 layers?

Answer: Only AT91 can use two-layer boards, and the others can use at least 4 layers. If the ground and power supply of 44b0 are well processed, two-layer boards can also be used.

Talk about four-layer board and 33 ohm resistor:

The selection of a four-layer board is not only a matter of power supply and ground. High-speed digital circuits have requirements for the impedance of the wiring, and a two-layer board is not easy to control the impedance. A 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 traces on the PCB board should be considered as transmission lines. 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. On the one hand, this isolates the two signal layers, and more importantly, the outer layer of the traces and the planes they are close to form a transmission line called a "microstrip". 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 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.

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 frequency of the circuit can be estimated by the rise (or fall) time, 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 the circuit should be designed based on a 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!

The 33 ohm resistor is usually connected in series at one end of the driver (not necessarily 33 ohms, ranging from a few ohms to 50 or 60 ohms, depending on the specific circuit). 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 reflected signal (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, such as using resistors in parallel, but it is rarely used in digital systems because it is more troublesome, and in many cases, one send and multiple receive, such as the address bus, it is not as easy to do as the source end matching!