C Programming for Embedded Systems

Embedded Software is a key element in every embedded project that is used to run the micro-controller to perform the desired operations. In our daily life, we frequently use many electronic devices such as washing machines, refrigerators, mobile phones, security system, digital camera so on which will be controlled using embedded C program. If you press a button to take a photo with your digital camera, then micro-controller will perform the functions essential to capture the image and store it. This article presents basics of embedded systemsmicro-controller consists of many ports toconstruct the embedded C programming tutorial.

7-Steps to Building Embedded C Programming Tutorial

The embedded C programming is a collection of one or more functions. Every function is a collection of statements that are used to perform some specific tasks. The embedded C programming tutorial is similar to a C language is constructed with some basic elements such as character set, variables, constants, data types, keywords, variable declaration, statements, expressions etc. that are used to write the program easily. However, we are providing 7-steps with embedded C programming tutorial to easily write the program such as:

1. Comments
2. Preprocessor directives
3. Port configuration
4. Global variables
5. main() function or core function
6. Variable declaration
7. Program Logic

7 Steps to Build Embedded C Programming Tutorial

Step1: Comments

The comments are important to the programming languages which describes function of program. Comments are non-executable code that is used to provide documentation to the program. The comments make an easy way to understand function of the program. There are two types of comments in embedded C programming tutorial such as:

• Single Line Comment
• Double Line Comment or Multi Line Comment

Single Line Comment

Generally single line comments are useful for the programming languages that can be used to explain a part of the code. The single line comments starts with double slash(//) which can be placed anywhere in the program. Single line comments are used to ignore complete line in a program.

Single Line Comment

Multi Line Comment

The multi line comments starts with single slash and an asterisk (/*) that can be used to explain a block of code. The multi line comments can be placed any where in the program. The multi line comments are used to ignore a complete block of code in a program.

Multi Line Comment

Step2: Processor Directives

Preprocessor directives are lines integrated in the code of programs which can be followed by a hash sign (#). These lines are not programmed statements, but directives for the preprocessor. The preprocessor inspects the code before actual compilation of code begins and resolves all these directives before any code is actually generated by regular statements. Even though there are many different preprocessor directives, but two directives are very useful in the embedded C programming tutorial such as

• #include

• #define

Which can be called as a header file, containing C declarations and macro definitions to be shared between several source files. The #include directive is normally used to include standard library such as study. h that can be used to access I/O functions from the C library. The #define directive normally used to define the string of variables and to assign the values by performing the operations in a single instruction it can be defined as macros.

Step 3:Port Configuration

In every micro-controller consists of many ports, each port contains many pins which can be used to control the interfacing devices. These pins are declared in a program using keywords. The embedded C has consist standard and predefined keywords such as bit, sbit, SFR which can be used to declare the single pin and bits in a program.

Port Configuration

sbit:

This data type is used in case of accessing a single bit of SFR register.

• Syntax: sbit variable name = SFR bit ;
• Ex: sbit a=P2^1;
• Explanation: If we assign p2.1 as ‘a’ variable, then we can use ‘a’ instead of p2.1 anywhere in the program, which reduces the complexity of the program.

Bit:

This data type is used for accessing the bit addressable memory of RAM (20h-2fh).

• Syntax: name of bit variable;
• Ex: bit c;
• Explanation: It is a bit sequence setting in a small data area that is used by a program to remember something.

SFR:

This data type is used to get the SFR register peripheral pots by another name. All the SFR registers must be declared with capital letters.

• Syntax: SFR variable name = SFR address of SFR register;
• Ex: SFR port0=0×80;
• Explanation: If we assign 0×80 as ‘port0’, then we can use 0×80 instead of port0 anywhere in the program, which reduces the complexity of the program.

SFR Register:

‘Special Function Register’ is represented as SFR register. Microcontroller 8051 has 256 bytes of RAM memory, which is separated into two parts: the first part of 128 bytes is used for data storage, and the other of 128 bytes is used to SFR registers. All peripheral devices like timers and counters, I/O ports are stored in the SFR register, and each element has a unique address.

Step4: Global Variables

The variable declared before the main function is called a global variable, that can be accessed on any function in the program. The life time of the global variable depends on the program until program comes to an end.

Step5: Main Function or Core Function

The main function is a core of every program execution, starts with main function only. Every program uses only one main function because if program contains more than one main function, then the compiler will get confused where to start the program execution.

Main Fucntion

Step6: Variable Declaration

The variable is a name that can be used to store the values. That variable must be declared before used in the program. The declaration of a variable specifies its name and data type. The storage representation of data is called data type. The embedded C programming uses four basic data types such as float, integer, character, etc. used to store the data in the memory. The size and range of data type defined based on compiler.

Variable Declaration

Step7: Program Logic

The plan of path is called a program logic that presents the theory behind and expected outcomes of a program’s actions. It describes the assumption or theory about why the program will work, showing the acknowledged effects of activities or resources.

Program Logic

LED flash light Program

LED flash light Program

Hope this article gives basic information to the beginners of embedded C programming. Good understanding of the Embedded C programming is most essential for designing embedded based projects. We encourage and welcome queries, suggestions and comments from our readers. Therefore, you can post your queries and feedback about this article in the comments section given below.

C Programming for Embedded System

Introduction

Now for embedded system development people are using operating system to add more features and at the same time reduce the development time of a complex system. This article gives a simple & understandable overview of scheduling technique of embedded system programming for beginners and intermediate programmers. I am considering “C” as the programming language and an application running in a dedicated hardware/device without operating system. The binary output (*.bin file) can be directly running from the device after Power on. In this case, the time scheduling is an important part of system development. We should ensure that the right task should execute at the right time.

What is Embedded System?

An embedded system is some combination of computer hardware and software, either fixed in capability or programmable - it is specifically designed for a particular kind of application device. Or in short we can say, an embedded system is a special-purpose computer system designed to perform one or a few dedicated functions.

All embedded systems need not be a real time system. Real Time systems are those in which timeliness is as important as the correctness of the outputs. Performance estimation and reduction are crucial in real time system. By definition, we can say a real time system is a system that must satisfy explicit (bounded) response time constraints or risk severe consequences, including failure.

Embedded system plays an important part in our daily lives. Most of the people around the globe are highly dependent on different types of gadgets like mobile phones, iPods and many more. The embedded systems used in industrial machines, automobiles, medical equipment, airplanes, and vending machines have to be real time.

I thought of sharing my experience in Embedded systems. This is my first ever article on CodeProject, so I am expecting your valuable feedback and suggestions for betterment.

Background

People are using operating system (RTOS) for complex devices to make it more flexible, add more features and minimize the development time. But it will increase the cost of the device for a small application. So for small application firmware development without operating system is very much popular.

Time scheduling is an important aspect of real-time system. Real time software are executed in response to external events. This event may be periodic, in which case an appropriate scheduling of events and related task is required to guarantee performance. The scheduling strategy also depends on scheduling facilities that the chosen RTOS offers. This article emphasis on the event based technique when there was no operating system running in the device.

Scheduling

Two kinds of scheduling techniques are used in Real-Time system:

1. Static Scheduling
2. Dynamic Scheduling

Static Scheduling

This involves analyzing the tasks statically and determining their timing properties. This timing property can be used to create a fixed scheduling table, according to which tasks will be dispatched for execution at run time. Thus the order of execution of the task is fixed, and it is assumed that their execution time is also fixed.

Round-Robin Scheduling

Round Robin scheduling by Time Slicing is one of the ways to achieve static scheduling. Round robin is one of the simplest and most widely used scheduling algorithms; in which a small unit of time known as time slice is defined. Schedulers go around the queue of ready-to-run processes and allocate a time slice to each such process.

Scheduling with Priority

Priority indicates the urgency or importance assigned to a task. There are two approaches of scheduling based on priority based execution – when the processor is idle, the ready task with the highest priority is chosen for execution; once chosen, the task is run to completion.

Pre-Emptive Scheduling

Preemptive Priority based execution is when the processor is idle, the ready task with highest priority is chosen for execution; at any time, the execution of a task can be preempted if a task of higher priority becomes ready. Thus, at all times, the processor is idle or executing the ready task with the highest priority.

Dynamic Scheduling

Another kind of scheduling mechanism is known as Dynamic Scheduling – In this case, a real-time program requires a sequence of decisions to be taken during execution of the assignment of resource to transactions. Here each decision must be taken without prior knowledge of the needs of future tasks. Dynamic scheduling is not in the scope of this article, so I am not discussing it in detail here. Perhaps we can discuss it in another article.

Code Snippet

Let us take a example of a Master-Slave communication system. Master system is connected to n number of slave systems over serial port (RS 485 network) in multi-drop architecture. Figure 1 shows the typical configuration of this system. Here only one system can talk at a time and others are in listen mode. The Master controls the communication.

Main Routine

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void main(void)
{
    /* Initialise all register of processor and the peripheral devices */
    InitMain();

    /* Register the event handler */
    RegisterTask(MainEventHandler);

    RegisterTask(CheckDataIntegrity);
    ..............

    /* Turn on all the leds for 1 sec as lamp test */
    TurnOnLed(LED, 1000);

    /* Call the application event manager - no return */
    EventManager();
}

In the above case, the RegisterTast() and EventManager() are two important functions. For any application, we have number of tasks and a function represents the entry point of a task, like 'CheckDataIntegrity' . When a device receives a complete data packet, it goes for data checking. RegisterTask() function creates a link-list of function pointers where each node represents a single task. Here I have passed the function pointerMainEventHandler or CheckDataIntegrity as an argument.

Main.h should have the following lines:

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/* Application event handler function pointer */
typedef void (*tEventHandler)(unsigned short *);

/* Link-list definition */
typedef struct TaskRecord
{
    tEventHandler EventHandler;
    struct TaskRecord *pNext;
}tTaskRecord;

static tTaskRecord *mpTaskList = NULL;

Here mpTaskList represents a link-list of function pointers. Considering each node of link list as a entry point of a task, this will execute one by one in EventManager() function. Below is the definition of RegisterTask()function which adds the function pointer into the link-list.

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void RegisterTask(tEventHandler EventHandlerFunc)
{
    tTaskRecord *pNewTask;

    /* Create a new task record */
    pNewTask = malloc(sizeof(tTaskRecord));
    if(pNewTask != NULL)
    {
        /* Assign the event handler function to the task */
        pNewTask->EventHandler = EventHandlerFunc;
        pNewTask->pNext = NULL;

        if(mpTaskList == NULL)
        {
            /* Store the address of the first task in the task list */
            mpTaskList = pNewTask;
        }
        else
        {
            /* Move to the last task in the list */
            mpActiveTask = mpTaskList;
            while(mpActiveTask->pNext != NULL)
            {
                mpActiveTask = mpActiveTask->pNext;
            }

            /* Add the new task to the end of the list */
            mpActiveTask->pNext = pNewTask;
        }
    }
}

For this type of application, after initialization there should be an infinite loop for continuous execution. The function EventManager() at the end of the main which is nothing but a infinite loop always checks for active tasks or events. If any event occurs, then it passes that event flag as an argument of the function which is already added into the mpTaskList. So EventManager() function calls MainEventHandler() function with eventIDas an argument. MainEventHandler will check the eventId and do the necessary action or execute the corresponding code. Here the event should be unique for each event-handler function, i.e. two event-handler functions should not check the same eventID.

Definition of EventManager Function

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void EventManager(void)
{
    unsigned short AllEvents;
    tTaskRecord pActiveTask

    /* No return */
    while(1)
    {
        /* Read application events */
        AllEvents = mEventID;

        /* Process any application events */
        pActiveTask = mpTaskList;
        while((AllEvents != 0) && (pActiveTask != NULL))
        {
            if(pActiveTask->EventHandler != NULL)
            {
                /* Call the task's event handler function */
                (mpActiveTask->EventHandler)(&AllEvents);

                /* Read application events */
                AllEvents = mEventID;
            }

            /* Move to the next event handler */
            pActiveTask = pActiveTask->pNext;
        }
    }
}

Event can be generated from interrupt service routine or by checking the status of an input pin in polling mode.SerialReceiveISR function generates an event after receiving the complete packet. Since the variablemEventID is modified in the interrupt service routine, it is recommended to disable interrupt while reading.

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#pragma interrupt_level 0
void interrupt IService(void)
{
    /* Receive bit is set when a byte is received */
    if(Receivebit == 1)
    {
        SerialReceiveISR()
    }
    ...........
    /* code for other interrupt */
}

SerialReceiveISR Function

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#pragma inline SerialReceiveISR
void SerialReceiveISR(void)
{    
    static char RxMsgDataCount;
    
    /* If a framing or overrun error occurs then clear the error */
    if(Error == 1)
    {
        /* Indicate a problem was seen */
        mEventID = mEventID | ATTENTION_REQ_FLG;
    }
    else if( RxMsgCount == DataLength)
    {
        /* Packet receive complete */
        mEventID = mEventID | DATA_RECEIVE_COMPLETE;
    }
    else
    {
        /* Store data in memory */
        Store(RxByte);
        RxMsgCount++;
    }
}

Here ATTENTION_REQ_FLAG & DATA_RECEIVE_COMPLETE flags are two bits of a Global variable mEventID, which is 16 bits and each bit triggers the corresponding event when set.

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#define ATTENTION_REQ_FLAG  0x0008
#define DATA_RECEIVE_COMPLETE  0x0001

When the flag is set, the EventManager will call all the registered functions with the eventID as argument.

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void MainEventHandler(unsigned short *Event)
{
    if(*Event & DATA_RECEIVE_COMPLETE)
    {
        /* Do the corresponding action */
        .........
        /* Reset the flag */
        *Event &= ~DATA_RECEIVE_COMPLETE; 
    }
}

Now we can change the variable type to increase the number of flags. If you want to generate multiple number of events, then use a structure rather than a single variable.