If you're new to ARM development, it's wise not to overcomplicate things at first. Focus on mastering the basics and ensuring that all applications function properly. Unlike DSPs or microcontrollers, ARM has a different startup mechanism, and you may encounter various issues along the way. To simplify your learning process, start with a minimal system board that includes only essential components such as Flash, SRAM or SDRAM, the CPU, JTAG interface, and reset signals. Leave the expansion interfaces for later. A minimal system can run smoothly, which is half the battle won. Fortunately, most ARM peripheral interfaces are standardized, so having some hardware wiring experience will be very helpful.
Writing startup code is another critical step. Create a small piece of code that can boot according to the hardware address. This should include port initialization, interrupt masking, copying the program to SRAM, completing code remapping, configuring the interrupt handler, and linking to the C language entry point. While some sample programs might have complex bootloader features, don't get discouraged. These are part of the board design and aren't directly related to the ARM architecture itself.
Understanding the chip’s datasheet is crucial. Even though ARM cores are compatible, each chip has its own unique characteristics that must be considered when writing software. Don’t rely solely on others’ sample code—this often leads to confusion. Instead, take time to thoroughly study the data sheets and understand how the chip operates.
Studying open-source operating systems like Linux or uC/OS-II can greatly enhance your knowledge. These provide excellent examples of real-world embedded programming and can help you improve your skills.
When designing hardware yourself, most manufacturers provide a demo board schematic for their chips. Study these schematics carefully. Understanding them will help you allocate resources effectively during future designs. Always make sure to fully digest the device's datasheet.
A common question is whether a two-layer or four-layer PCB is better for a minimal system. For AT91, two layers may work, but for other chips, a four-layer board is generally recommended. For chips like 44B0, even two layers can work if ground and power planes are handled correctly. However, for high-speed digital circuits, four-layer boards are preferred because they allow for controlled impedance traces. A 33-ohm resistor is often used at the driver end for impedance matching. High-speed signal lines need to be carefully routed, and at high frequencies, PCB traces should be treated as transmission lines.
Transmission lines have characteristic impedance. When there's an impedance mismatch, signal reflections occur, causing interference. On a four-layer board, the outer layer typically carries signal lines, while the middle layers are power and ground planes. This helps isolate signals and allows for more predictable impedance values. Calculating trace impedance involves factors like trace width, distance from the reference plane, copper thickness, and dielectric properties. While formulas exist, it's often easier to use existing tools.
At the receiving end, parallel resistors can also be used for matching, but this is less common in digital systems due to complexity. High-frequency design isn't always about clock speed—it also depends on signal rise and fall times. A 1 ns rise time corresponds to a frequency of around 500 MHz, which should be considered in the design.
Sometimes, edge rates are intentionally slowed down using adjustable output slopes in high-speed ICs. Linux offers a complete toolchain, making it easy to build embedded development environments and cross-compilation setups. It eliminates the need for expensive simulation tools like ICE. The open nature of the Linux kernel allows for the creation of true hard real-time systems, and soft real-time systems are easily implemented as well. Its powerful networking support makes it ideal for building embedded TCP/IP stacks.
To create a small embedded Linux system, you need three basic elements: a boot tool, a Linux microkernel (with memory, process, and transaction management), and an initialization process. If you want to expand functionality, add hardware drivers, applications, a file system (in ROM or RAM), and possibly a TCP/IP stack. With these components, you can build a fully functional embedded system tailored to your needs.
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