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Embedded C

Written By: 

Akshay Daga
Introduction to Embedded C
Looking around, we find ourselves to be surrounded by various types of embedded systems. Be it a digital camera or a mobile phone or a washing machine, all of them has some kind of processor functioning inside it. Associated with each processor is the embedded software. If hardware forms the body of an embedded system, embedded processor acts as the brain, and embedded software forms its soul. It is the embedded software which primarily governs the functioning of embedded systems.
 
During infancy years of microprocessor based systems, programs were developed using assemblers and fused into the EPROMs. There used to be no mechanism to find what the program was doing. LEDs, switches, etc. were used to check correct execution of the program. Some ‘very fortunate’ developers had In-circuit Simulators (ICEs), but they were too costly and were not quite reliable as well.
 
As time progressed, use of microprocessor-specific assembly-only as the programming language reduced and embedded systems moved onto C as the embedded programming language of choice. C is the most widely used programming language for embedded processors/controllers. Assembly is also used but mainly to implement those portions of the code where very high timing accuracy, code size efficiency, etc. are prime requirements.
 
Initially C was developed by Kernighan and Ritchie to fit into the space of 8K and to write (portable) operating systems. Originally it was implemented on UNIX operating systems. As it was intended for operating systems development, it can manipulate memory addresses. Also, it allowed programmers to write very compact codes. This has given it the reputation as the language of choice for hackers too.
 
As assembly language programs are specific to a processor, assembly language didn’t offer portability across systems. To overcome this disadvantage, several high level languages, including C, came up. Some other languages like PLM, Modula-2, Pascal, etc. also came but couldn’t find wide acceptance. Amongst those, C got wide acceptance for not only embedded systems, but also for desktop applications. Even though C might have lost its sheen as mainstream language for general purpose applications, it still is having a strong-hold in embedded programming. Due to the wide acceptance of C in the embedded systems, various kinds of support tools like compilers & cross-compilers, ICE, etc. came up and all this facilitated development of embedded systems using C.
 
Subsequent sections will discuss what is Embedded C, features of C language, similarities and difference between C and embedded C, and features of embedded C programming.
 
EMBEDDED SYSTEMS PROGRAMMING
Embedded systems programming is different from developing applications on a desktop computers. Key characteristics of an embedded system, when compared to PCs, are as follows:
·         Embedded devices have resource constraints(limited ROM, limited RAM, limited stack space, less processing power)
·         Components used in embedded system and PCs are different; embedded systems typically uses smaller, less power consuming components. ·         Embedded systems are more tied to the hardware.
 
Two salient features of Embedded Programming are code speed and code size. Code speed is governed by the processing power, timing constraints, whereas code size is governed by available program memory and use of programming language.  Goal of embedded system programming is to get maximum features in minimum space and minimum time.
 
Embedded systems are programmed using different type of languages:
·           Machine Code
·           Low level language, i.e., assembly
·           High level language like C, C++, Java, Ada, etc.
·           Application level language like Visual Basic, scripts, Access, etc.
 
Assembly language maps mnemonic words with the binary machine codes that the processor uses to code the instructions. Assembly language seems to be an obvious choice for programming embedded devices. However, use of assembly language is restricted to developing efficient codes in terms of size and speed. Also, assembly codes lead to higher software development costs and code portability is not there. Developing small codes are not much of a problem, but large programs/projects become increasingly difficult to manage in assembly language. Finding good assembly programmers has also become difficult nowadays. Hence high level languages are preferred for embedded systems programming.
 
Use of C in embedded systems is driven by following advantages
·      It is small and reasonably simpler to learn, understand, program and debug.
·      C Compilers are available for almost all embedded devices in use today, and there is a large pool of experienced C programmers.
·      Unlike assembly, C has advantage of processor-independence and is not specific to any particular microprocessor/ microcontroller or any system. This makes it convenient for a user to develop programs that can run on most of the systems.
·      As C combines functionality of assembly language and features of high level languages, C is treated as a ‘middle-level computer language’ or ‘high level assembly language’
·      It is fairly efficient
·      It supports access to I/O and provides ease of management of large embedded projects.
 
Many of these advantages are offered by other languages also, but what sets C apart from others like Pascal, FORTRAN, etc. is the fact that it is a middle level language; it provides direct hardware control without sacrificing benefits of high level languages.
 
Compared to other high level languages, C offers more flexibility because C is relatively small, structured language; it supports low-level bit-wise data manipulation.
 
Compared to assembly language, C Code written is more reliable and scalable, more portable between different platforms (with some changes). Moreover, programs developed in C are much easier to understand, maintain and debug. Also, as they can be developed more quickly, codes written in C offers better productivity. C is based on the philosophy ‘programmers know what they are doing’; only the intentions are to be stated explicitly. It is easier to write good code in C & convert it to an efficient assembly code (using high quality compilers) rather than writing an efficient code in assembly itself. Benefits of assembly language programming over C are negligible when we compare the ease with which C programs are developed by programmers.
 
Objected oriented language, C++ is not apt for developing efficient programs in resource constrained environments like embedded devices. Virtual functions & exception handling of C++ are some specific features that are not efficient in terms of space and speed in embedded systems. Sometimes C++ is used only with very few features, very much as C.
 
Ada, also an object-oriented language, is different than C++. Originally designed by the U.S. DOD, it didn’t gain popularity despite being accepted as an international standard twice (Ada83 and Ada95). However, Ada language has many features that would simplify embedded software development.
 
Java is another language used for embedded systems programming. It primarily finds usage in high-end mobile phones as it offers portability across systems and is also useful for browsing applications. Java programs require Java Virtual Machine (JVM), which consume lot of resources. Hence it is not used for smaller embedded devices.
 
Dynamic C and B# are some proprietary languages which are also being used in embedded applications.
 
Efficient embedded C programs must be kept small and efficient; they must be optimized for code speed and code size. Good understanding of processor architecture embedded C programming and debugging tools facilitate this.
 
DIFFERENCE BETWEEN C AND EMBEDDED C
Though C and embedded C appear different and are used in different contexts, they have more similarities than the differences. Most of the constructs are same; the difference lies in their applications.
 
C is used for desktop computers, while embedded C is for microcontroller based applications. Accordingly, C has the luxury to use resources of a desktop PC like memory, OS, etc. While programming on desktop systems, we need not bother about memory. However, embedded C has to use with the limited resources (RAM, ROM, I/Os) on an embedded processor. Thus, program code must fit into the available program memory. If code exceeds the limit, the system is likely to crash.
 
Compilers for C (ANSI C) typically generate OS dependant executables. Embedded C requires compilers to create files to be downloaded to the microcontrollers/microprocessors where it needs to run. Embedded compilers give access to all resources which is not provided in compilers for desktop computer applications.
 
Embedded systems often have the real-time constraints, which is usually not there with desktop computer applications.
 
Embedded systems often do not have a console, which is available in case of desktop applications.
 
So, what basically is different while programming with embedded C is the mindset; for embedded applications, we need to optimally use the resources, make the program code efficient, and satisfy real time constraints, if any. All this is done using the basic constructs, syntaxes, and function libraries of ‘C’.
 

Programming using Embedded C

 
PROGRAMMING USING EMBEDDED C
Embedded C use most of the syntax and semantics of standard C, e.g., main() function, variable definition, datatype declaration, conditional statements (if, switch. case), loops (while, for), functions, arrays and strings, structures and union, bit operations, macros, etc. In addition, there are some specifics to embedded C which are mentioned below: 
1.      Low Level Codes
Embedded programming requires access to underlying hardware, i.e., timers, memory, ports, etc. In addition, it is often needed to handle interrupts, manage job queues, etc. As C offers pointers and bit manipulation features, they are extensively used for direct hardware access.
 
2.      In-line Assembly Code
For a particular embedded device, there may be instructions for which no equivalent C code is available. In such cases, inline assembly code, i.e., assembly code embedded within C programs is used; the syntax depends upon the compiler. An example for ‘gcc’ is shown here.
int a=10, b;
asm (“movl %1, %%eax;
movl %%eax, %0;”
:”=r”(b) /* output */
:”r”(a) /* input */
:”%eax” /* clobbered register */
);
Assembly code is written in C program itself.  Above code assigns ‘a’ to ‘b’. Writing inline assembly code is much easier than writing full fledged assembly code.
 
3.      Features like Heap, recursion
Embedded devices have no or limited heap area (where dynamic memory allocation takes place). Hence, embedded programs do not use standard C functions like malloc. Structures like linked lists/trees are implemented using static allocation only.
Similarly, recursion is not supported by most embedded devices because of its inefficiency in terms of space and time.
Such other costly features of standard C which consume space and execution time are either not available or not recommended
 
4.      I/O Registers
Microcontrollers typically have I/Os, ADCs, serial interfaces and other peripherals in-built into the chips. These are accessed as IO Registers, i.e., to perform any operation on these peripherals, bits in these registers are read/written.
Special function registers (SFRs) are accessed as shown below:
             SFR portb = 0x8B;
It is used to declare portB at location 0x8B.
Some embedded processors have separate IO space for such registers. Since there are no such concepts in C, compilers provide special mechanisms to access them
             unsigned char portB @portB 0x8B;
In this example, ‘@portB <address>’ declares portB at location 0x8B by the variable portB.
Such extensions are not a part of standard C, syntax and semantics differ in various embedded C compilers.
 
5.      Memory Pointers
Some CPU architectures allow us to access IO registers as memory addresses. This allows treating them just like any other memory pointers.
 
6.      Bit Access
Embedded controllers frequently need bit operations as individual bits of IO registers corresponds to the output pin of an I/O port. Standard C has quite powerful tools to do bitwise operations. However, care must be taken while using them in structures because C standard doesn’t define the bitfield allocation order and C compilers may allocate bitfields either from left to right or from right to left.
 
7.      Use of Variable data type
In C, datatypes can be simply declared, and compiler takes care of the storage allocation as well as that of code generation. But, datatypes usage should be carefully done to generate optimised code. For most 8-bit C compilers, ‘char’ is 8-bits, ‘short’ and ‘int’ are 16-bits, long is ’32-bits’.
Some embedded processors favour use of unsigned type. Use of ‘long’ and floating variable should be avoided unless it is very necessary. Using long data types increase code size and execution time. Use of floating point variables is not advised due to intrinsic imprecise nature of floating point operations, alongside speed and code penalty.
 
8.      Use of Const and Volatile
Volatile is quite useful for embedded programming. It means that the value can change without the program touching it.  Consequently, the compiler cannot make any assumptions about its value. The optimizer must reload the variable every time it is used instead of holding a copy in a register.
Const is useful where something is not going to change, for e.g., function declarations, etc.