Developing for the Atmel AVR Microcontroller on Linux
Whether you are creating a small Internet appliance, some hardware instrumentation, data loggers or an army of autonomous robots to do your bidding, in numerous situations you need the flexibility of a programmable computer. In many cases, a general-purpose system, such as the workhorse sitting under your desk, doesn't meet size, cost or power-consumption constraints and is simply overkill. What you need is a microcontroller.
This article provides step-by-step instructions for setting up a complete development system for the Atmel AVR series of microcontrollers, using free software and Linux. The detailed instructions provided here will allow you to transform your Linux system into a complete AVR development environment. This article walks you through all the steps of building, debugging and installing a simple program.
When all the electronic components required to make a central processing unit (CPU)—instruction decoder, arithmetic/logic unit, registers and so on—are integrated into a single chip, you have a microprocessor. When, in turn, you bundle this CPU with supporting components, memory and I/O peripherals, you've got a microcomputer. Extending the integration and miniaturization even further, you can combine all the elements of a microcomputer onto a single integrated circuit—behold the microcontroller.
The semiconductor industry evolves rapidly, making it difficult to provide an accurate and complete definition of the term microcontroller. Consider this: some microcontroller chips have capacities and clock speeds that surpass the 74KB of program memory and 4KB of RAM available to the 30kg Apollo Lunar Module computer. You can expect today's screamer PCs to be running tomorrow's embedded applications, with the definition of microcontroller shifting accordingly.
Microcontrollers all have a microprocessor core, memory and I/O interfaces, and many have additional peripherals onboard. The specific configuration of a particular chip influences its physical packaging, number of pins and cost. If you are accustomed to working with microcomputers, you may feel that microcontrollers are tight spots. They have a handful of kilobytes of program ROM and in the area of 256 bytes of RAM. Don't fret though; a lot can be done in this space, as the MIT Instrumentation Lab demonstrated when developing the Apollo Lunar Module software that controls its moon landing, return from the surface and rendezvous in orbit.
The AVRs are 8-bit RISC platforms with a Harvard architecture (program and data memory are separate). Figure 1 details the ATtiny26 AVR chip internal organization. Like each member of a family, it has its own particular combination of I/O and peripherals, but it shares a basic architecture and instruction set with all the other AVRs. The ATtiny26 has 2KB of program Flash memory, 128 bytes of onboard SRAM and EEPROM, two 8-bit counters and pulse-width modulators, 11 interrupts, 16 I/O pins spread over two 8-bit ports, an 11-channel 10-bit analog-to-digital converter and more—all on a single tiny 20-pin DIP.
A number of factors make the AVR microcontrollers a good choice, especially for beginners. AVRs are:
Easy to code for: AVRs were designed from the ground up to allow easy and efficient programming in high-level languages, with a particular focus on C.
Easy to program: the combination of onboard reprogrammable Flash program memory and the in-system programming interface keeps the process of transferring software to the microcontroller simple and cheap.
Powerful and inexpensive: AVRs pack a lot of power (1 MIPS/MHz, clocks up to 16MHz) and space (up to 128K of Flash program memory and 4K of EEPROM and SRAM) at low prices. Most AVRs even include additional peripherals, such as UARTs and analog-to-digital converters.
Hobbyist-friendly: most of the chips in the AVR family come in easy-to-use 8-, 20-, 28- or 40-pin dual in-line packages (DIPs) and can be ordered in unit quantities from a number of distributors.
The processor core, composed of the components in the upper-left portion of Figure 1, includes elements to read the program memory and to decode and execute the instructions within that memory. The CPU also can fetch and store data to and from the EEPROM, SRAM and the 32 registers. The registers act as extremely efficient storage for 8-bit values (1 byte), and the ALU (arithmetic/logic unit) can operate directly on each of the 32 registers. This AVR features a RAM-based stack. In a few other AVRs, which don't have any SRAM, the stack is hardware-based, limiting the stack depth to three.
Most instructions take only a single clock cycle to execute, and there is no internal clock division on AVRs. The CPU fetches and decodes the next instruction as it is executing the current instruction. These combined facts mean that AVRs can reach performances of nearly 1 MIPS (million instructions per second) per MHz. With clock rates of up to 16MHz, you can choose the right balance of speed, power consumption and electromagnetic noise for your particular application.