Embedded Programming Concepts

EMBEDDED PROGRAMMING CONCEPTS

Programming in Assembly and HLL

• Processor and memory-sensitive instructions: Program codes maybe written in assembly

• Most of codes: Written in a high level language (HLL), 'C‘, 'C++‘or Java

Assembly Language Programming

Advantage

Assembly codes sensitive to the processor, memory, ports and devices hardware Gives a precise control of the processor internal devices

Enables full use of processor specific features in its instruction set and its addressing modes Machine codes are compact, processor and memory sensitive

System needs a smaller memory.

Memory needed does not depend on the programmer data type selection and rule declarations

Not the compiler specific and library functions specific

Device driver codes may need only a few assembly instructions. Bottom-up-design approach

Advantage of using high level language (HLL) for Programming

Short Development Cycle

• Code reusability─ A function or routine can be repeatedly used in a program

•  Standard library functions─ For examples, the mathematical functions and delay ( ), wait ( ), sleep ( ) functions

• Use of the modular building blocks

•  Sub-modules are designed first for specific and distinct set of actions, then the modules and finally integration into complete design.

•  First code the basic functional modules and then build a bigger module and then integrate into the final system

•  First design of main program (blueprint),then its modules and finallythe sub-modules are designed for specific and distinct set of actions.

• Top-down design Most favoured program design approach

Use of Data Type and Declarations

• Examples, char, int, unsigned short, long, float, double, Boolean.

•  Each data type provides an abstraction of the (i) methods to use, manipulate and represent, and (ii) set of permissible operations.

Use of Type Checking

• Type checking during compilation makes the program less prone to errors.

•  Example─ type checking on a char data type variable (a character) does not permit subtraction, multiplication and division.

Use of Control Structures, loops and Conditions

• Control Structures and loops

• Examples─ while, do-while, break and for

• Conditional Statements examples

if, if- else, else - if and switch - case)

• Makes tasks simple for the program flow Design

Use of Data Structures _ Data structure

- A way of organizing large amounts of data.

_ A data elements‘ collection

_ Data element in a structure identified and accessed with the help of a few pointers and/or indices and/or functions.

Standard Data structure

• Queue

• Stack

• Array – one dimensional as a vector

• Multidimensional

• List

• Tree

Use of Objects

_ Objects bind the data fields and methods to manipulate those fields

_ Objects reusability

_ Provide inheritance, method overloading, overriding and interfacing

_ Many other features for ease in programming

Advantage of using C for Programming

C

•Procedure oriented language (No objects)

•  Provision of inserting the assembly language codes in between (called inline assembly) to obtain a direct hardware control.

•  A large program in ‗C‘ splits into the declarations for variables, functions and data structure, simpler functional blocks and statements.

In-line assembly codes of C functions

•  Processor and memory sensitive part of the program within the inline assembly, and the complex part in the HLL codes.

• Example function ouportb (q, p)

• Example─ Mov al, p; out q, al

C Program Elements

Preprocessor include Directive

_ Header, configuration and other available source files are made the part of an embedded system program source file by this directive

Examples of Preprocessor include Directives

# include "VxWorks.h" /* Include VxWorks functions*/

# include "semLib.h" /* Include Semaphore functions Library */

# include "taskLib.h" /* Include multitasking functions Library */

# include "sysLib.c" /* Include system library for system functions */

# include "netDrvConfig.txt" /* Include a text file that provides the 'Network Driver Configuration'. */

include "prctlHandlers.c" /* Include file for the codes for handling and actions as per the protocols used for driving streams to the network. */

Preprocessor Directive for the Definitions

• Global Variables ─ # define volatile Boolean IntrEnable

Constants ─ # define false 0

• Strings─ # define welcomemsg"Welcome To ABC Telecom"

Preprocessor Macros

• Macro - A named collection of codes that is defined in a program as preprocessor directive.

•  Differs from a function in the sense that once a macro is defined by a name, the compiler puts the corresponding codes at the macro at every place where that macro-name appears.re used for short codes only.

Difference between Macro and Function

• The codes for a function compiled once only

• On calling that function, the processor has to save the context, and on return restore the context.

• Macros are used for short codes only.

•  When a function call is used instead of macro, the overheads (context saving and return) will take a time, T overheads that is the same order of magnitude as the time, Texec for execution of short codes within a function.

• Use the function when the Toverheads<<Texec and macro when Toverheads ~= or >

Texec.

Use of Modifiers

·        auto

·        unsigned

·        static

·        const

·        register

·        interrupt

·        extern

·        volatile

·        volatile static

Use of infinite loops

_ Infinite loops- Never desired inusual programming. Why? The program will never end and never exit or proceed further to the codes after the loop.

_ Infinite loop is a feature in embedded system programming! Example:

A telephone is never switching off.

The system software in the telephone has to be always in awaiting loop that finds the ring onthe line. An exit from the loop will make the system hardware redundant.

# define false 0

# define true 1

# void main (void) {

/* Call RTOS run here */

rtos.run ( );

/* Infinite while loops follows in each task. So never there is return from the RTOS. */

}

void task1 (....) {

/* Declarations */.

while (true) {

/* Run Codes that repeatedly execute */ /* Run Codes that execute on an event*/

 if (flag1) {....;}; flag1 =0;

/* Codes that execute for message to the kernel */

message1 ( ); } }

Use of typedef

_ Example─ A compiler version may not process the declaration as an unsigned byte _ The 'unsigned character' can then be used as a data type.

_ Declared as follows: typedef unsigned character port A data _ Used as follows: #define PbyteportAdata0xF1

Use of Pointers

Pointers are powerful tools when used correctly and according to certain basic principles.

# define COM ((structsio near*) 0x2F8);

This statement with a single masterstroke assigns the addresses to all 8variables

Byte at the sio Addresses

0x2F8: Byte at RBR/THR /DLATCH-LByte

0x2F9: Byte at DLATCH-HByte

0x2FA: Byte at IER;

0x2FB: Byte at LCR;

0x2FC: Byte at MCR;

0x2FD: Byte at LSR;

0x2FE: Byte at MSR

0x2FF: Byte Dummy Character

Example

Free the memory spaces allotted to a datastructure.

#define NULL (void*) 0x0000

•  Now statement & COM ((structsionear*) = NULL;assigns the COM to Null and make freethe memory between 0x2F8 and 0x2FFfor other uses.

Data structure

• Example─ structure sio

•  Eight characters─ Seven for thebytes in BR/THR/DLATCHLByte,IER, IIR, LCR, MCR, LSR, MSR registers of serial line device and one dummy variablere consisting of 8 character variables structure for the COM port 2 in the UART serial line device at an IBMPC.

Example of Data structure declaration

• Assume structured variable COM at the addresses beginning 0x2F8.

# define COM ((structsio near*) 0x2F8)

•  COM is at 8 addresses 0x2F8-0x2FF and is a structure consisting of 8 character variables structure for the COM port 2 inthe UART serial line device at an IBMPC.

# define COM1 ((structsio near*) 0x3F8);

It will give another structured variableCOM1 at addresses beginning 0x3F8using the data structure declared earlier as sio

Use of functions

(i) Passing the Values (elements):

The values are copied into the arguments of the functions. When the function is executed in this way, it does not change a variable's value at the function, which calls new function.

(ii) Passing the References

When an argument value to a function passes through a pointer, the called function can change this value. On returning from this function, the new value may be available in the calling program or another function called by this function.

Use of Reentrant Function

•  Reentrant function- A function usable by the several tasks and routines synchronously (at the same time). This is because all the values of its argument are retrievable from the stack.

Three conditions for a function called as reentrant function

1. All the arguments pass the value sand none of the argument is a pointer (address) whenever a calling function calls that function.

2. When an operation is not atomic, that function should not operate on any variable, which is declared outside the function or which an interrupt servicer outine uses or which is a global variable but passed by reference and not passed by value as an argumentnto the function. [The value of such a variable or variables, which is not local, does not save on the stack when there is call to another program.]

3. That function does not call any other function that is not itself Reentrant.

Data Structures: Arrays

• Array: A structure with a series of data items sequentially placed in memory

(i) Each element accessible by anidentifier name (which points tothe array) and an index, i (which define offset from the first element)

(ii)             istarts from 0 and is +ve integer

One dimensional array (vector)

Example 1:

Unsigned int salary [11];

 salary[0] – 1st month salary. salary[11] – 12th month salary Each integer is of 32-bit (4 bytes);

salary assigned 48 bytes addressspace

Example 2: sioCOM [1];

COM [0]– COM1 port data record with structure equivalent to sio

COM [1]– COM2 port data record with structure equivalent to sio

COM assigned 2*8 characters = 16 bytes address space

Two dimensional array

Example 3:

Unsigned int salary [11, 9];

salary[3, 5]– 4th month 6th year salary

salary[11, 4] – 12th month 5th year

salary salary assigned 12*10*4 = 480 bytes address space

Multi-dimensional array

Example 4:

char pixel [143,175, 23];

pixel [0, 2, 5] – 1st horizontal line index x,3rd vertical line index y, 6th color c.pixel assigned 144*176*24 = 608256bytes address space in a colored picture of resolution 144x 176 and 24colors.

Programming using functions and function queues

_ Use of multiple function calls in the main ( )

_ Use of multiple function calls in cyclic order

_ Use of pointer to a function

_ Use of function queues and

_ Use of the queues of the function pointers built by the ISRs.

It reduces significantly the ISR latency periods. Each device ISR is therefore able to execute within its stipulated deadline

1.Multiple function calls

2. Multiple function calls in cyclic order Use

• One of the most common methods is the use of multiple function-calls in a cyclic order in an infinite loop of the main ( ).

3. Use of function pointers

* sign when placed before the function name then it refers to all the compiled form of the statements in the memory that are specified within the curly braces when declaring the function.

•  A returning data type specification (for example, void) followed by'(*function Name) (function Arguments)'calls the statements of the function Name using the function Arguments, and on a return, itreturns the specified data object. We can thus use the function pointer for invoking a call to the function.

4. Queue of Function-pointers

Application

_ Makes possible the designing of ISRs with short codes and by running the functions of the ISRsat later stageso all pending ISRsfinishes

Multiple ISRs insertion of Function pointers into a Queue

• The ISRs insert the function pointers

• The pointed functions in the queue execute at later stages by deleting from the queue

• These queued functions execute after the service to all pending ISRs finishes

Priority Function Queue of Multiple ISRs

•  When there are multiple ISRs, a high priority interrupt service routine is executed first and the lowest priority.

•  The ISRs insert the function pointers into a priority queue of function pointers[ISR can now be designed short enough sothat other source don‘t miss a deadline forservice]