3/9/2015 Keil Embedded C Tutorial http://www.8051projects.net/wiki/Keil_Embedded_C_Tutorial#Functions_in_Keil_C 1/13 Home (/) Rikipedia (/wiki/) Keil Embedded C Tutorial / / Keil Embedded C Tutorial From Rikipedia Embedded Wiki Introduction to Keil C The use of C language to program microcontrollers is becoming too common. And most of the time its not easy to buld an application in assembly which instead you can make easily in C. So Its important that you know C language for microcontroller which is commonly known as Embedded C. As we are going to use Keil C51 Compiler, hence we also call it Keil C. Keywords Keil C51 compiler adds few more keywords to the scope C Language: _at_ far sbit alien idata sfr bdata interrupt sfr16 Discussion (/mediawiki/index.php?title=Talk:Keil_Embedded_C_Tutorial&action=edit&redlink=1) Edit Nav Introduction to Keil C Keywords Memory Models Pointers in Keil C Generic Pointers Memory-Specific Pointers Functions in Keil C Function Declaration Writing First C program in Keil Basic of a C program Writing Hardware specific code Writing C and Assembly together Interfacing C program to Assembler Segment naming Advanced C programming Function Parameters Fucntion Return Values Table of Content Page (/wiki/Keil_Embedded_C_Tutorial)
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Introduction to Keil CThe use of C language to program microcontrollers is becoming too common. And most of the time its not easy to buld an application inassembly which instead you can make easily in C. So Its important that you know C language for microcontroller which is commonly known asEmbedded C. As we are going to use Keil C51 Compiler, hence we also call it Keil C.
Keywords
Keil C51 compiler adds few more keywords to the scope C Language:
_at_ far sbit
alien idata sfr
bdata interrupt sfr16
Discussion (/mediawiki/index.php?title=Talk:Keil_Embedded_C_Tutorial&action=edit&redlink=1) Edit Nav
Description: The variable will be stored in internal data memory of controller. example:
1. unsigned char data x;2. //or3. unsigned char idata y;
bdata
Description: The variable will be stored in bit addressable memory of controller. example:
1. unsigned char bdata x;2. //each bit of the variable x can be accessed as follows3. x ^ 1 = 1; //1st bit of variable x is set4. x ^ 0 = 0; //0th bit of variable x is cleared
xdata
Description: The variable will be stored in external RAM memory of controller. example:
1. unsigned char xdata x;
code
Description: This keyword is used to store a constant variable in code memory. Lets say you have a big string which is not going to changeanywhere in program. Wasting ram for such string will be foolish thing. So instead we will make use of the keyword "code" as shown inexample below. example:
1. unsigned char code str="this is a constant string";
pdata
Description: This keyword will store the variable in paged data memory. This keyword is used occasionally. example:
1. unsigned char pdata x;
_at_
Description: This keyword is used to store a variable on a defined location in ram. example:
1. unsigned char idata x _at_ 0x30;2. // variable x will be stored at location 0x303. // in internal data memory
Description: This keyword is used to define a special bit from SFR (special function register) memory. example:
1. sbit Port0_0 = 0x80;2. // Special bit with name Port0_0 is defined at address 0x80
sfr
Description: sfr is used to define an 8-bit special function register from sfr memory. example:
1. sfr Port1 = 0x90;2. // Special function register with name Port1 defined at addrress 0x90
sfr16
Description: This keyword is used to define a two sequential 8-bit registers in SFR memory. example:
1. sfr16 DPTR = 0x82;2. // 16‐bit special function register starting at 0x823. // DPL at 0x82, DPH at 0x83
using
Description: This keyword is used to define register bank for a function. User can specify register bank 0 to 3. example:
1. void function () using 22. {3. // code4. }5. // Funtion named "function" uses register bank 2 while executing its code
interrupt
Description: This keyword will tells the compiler that function described is an interrupt service routine. C51 compiler supports interruptfunctions for 32 interrupts (0-31). Use the interrupt vector address in the following table to determine the interrupt number.
There are three kind of memory models available for the user:
SmallAll variables in internal data memory.CompactVariables in one page, maximum 256 variables (limited due to addressing scheme, memory accessed indirectly using r0 and r1 registers);largeAll variables in external ram. variables are accessed using DPTR.
Depending on our hardware configuration we can specify the momory models as shown below:
1. //For Small Memory model2. #pragma small3. //For Compact memory model4. #pragma compact5. //For large memory model6. #pragma large
Pointers in Keil CPointers in keil C is are similar to that of standard C and can perform all the operations that are available in standard C. In addition, keil Cextends the operatability of pointers to match with the 8051 Controller architecture. Keil C provides two different types of pointers:
Generic Pointers
Memory-Specific Pointers
Generic Pointers
Generic Pointers are declared same as standard C Pointers as shown below:
1. char *ptr; //Character Pointer2. int *num; //Integer Pointer
Generic pointers are always stored using three bytes. The first byte is the memory type, the second byte is the high-order byte of the offset,and the third byte is the low-order byte of the offset. Generic pointers maybe used to access any variable regardless of its location.
Memory-Specific Pointers
Memory specific pointers are defined along with memory type to which the pointer refers to, for example:
1. char data *c;2. //Pointer to character stored in Data memory3. 4. char xdata *c1;5. //Pointer to character stored in External Data Memory.6. 7. char code *c2;8. //Pointer to character stored in Code memory
As Memory-Specific pointers are defined with a memory type at compile time, so memory type byte as required for generic pointers is not
As Memory-Specific pointers are defined with a memory type at compile time, so memory type byte as required for generic pointers is notneeded. Memory-Specific pointers can be stored using 1 byte (for idata, data, bdata and pdata pointers) or 2 bytes (for code and xdatapointers).
The Code generated by keil C compiler for memory-specific pointer executes mroe quickly than the equivalent code generated for a genericpointer. This is because the memory area accessed by the pointer is known at the compile time rather at run-time. The compiler can use thisinformation to optimize memory access. So If execution speed is your priority then it is recommended to use memory-specific pointers.Generic pointers and Memory-Specific pointers can be declared with memory area in which they are to be stored. For example:
1. //Generic Pointer2. char * idata ptr;3. //character pointer stored in data memory4. int * xdata ptr1;5. //Integer pointer stored in external data memory6. 7. //Memory Specific pointer8. char idata * xdata ptr2;9. //Pointer to character stored in Internal Data memory
10. //and pointer is going to be stored in External data memory11. int xdata * data ptr3;12. //Pointer to character stored in External Data memory13. //and pointer is going to be stored in data memory
Functions in Keil CKeil C compiler provides number of extensions for standarad C function declerations. These extensions allows you to:
Return_typeThe type of value returned from the function. If return type of a function is not specified, int is assumed by default.Function_nameName of functionArgumentsArguments passed to function
Optional Stuff
These are options that you can specify along with function declaration. Memory_model: explicit memory model (Large, Compact, Small) forthe function. Example:
1. int add_number (int a, int b) Large
reentrantTo indicate if the function is reentrant or recursive. This option is explained later in the tutorial.interruptIndicates that function is an interrupt service routine. This option is explained later in the tutorial.usingSpecify register bank to be used during function execution. We have three register banks in 8051 architecture. These register banks arespecified using number 0 for Bank 0 to 3 for Bank 3 as shown in example
1. void function_name () using 22. {3. //function uses Bank 24. //function code5. }
Interrupt Service Routines
A function can be specified as an interrupt service routine using the keyword interrupt and interrupt number. The interrupt number indicatesthe interrupt for which the function is declared as service routine.
Following table describes the default interrupts:
8051 Interrupt vector
As 8051 vendors create new parts, more interrupts are added. Keil C51 compiler supports interrupt functions for 32 interrupts (0-31). Use theinterrupt vector address in the following table to determine the interrupt number.
Interrupt vector
The interrupt function can be declared as follows:
Please make sure that interrupt service routines should not have any arguments or return type except void.
Reentrant Functions
In ANSI C we have recursive function, to meet the same requirement in embedded C, we have reentrant function. These functions can becalled recursively and can be called simultaneously by two or more processes. Now you might be thinking, why special definition for recursivefunctions?
Well you must know how these functions work when they are called recursively. when a function is running there is some runtime data
Well you must know how these functions work when they are called recursively. when a function is running there is some runtime dataassociated with it, like local variables associated with it etc. when the same function called recursively or two process calls same function, CPUhas to maintain the state of function along with its local variables. Reentrant functions can be defined as follows:
Each reentrant function has reentrant stack associated with it, which is defined by startup.A51 file. Reentrant stack area is simulated internalor external memory depending upon the memory model used:
Small model reentrant functions simulate reentrant stack in idata memory.
Compant model reentrant functions simulate reentrant stack in pdata memory.
Large model reentrant functions simulate reentrant stack in xdata memory.
Real-time Function Tasks
Keil or C51 provides support for real-time operating system (RTOS) RTX51 Full and RTX51 Tiny. Real-time function task are declared using_task_ and _priority_ keywords. The _task_ defines a function as real-time task. The _priority_ keyword specify the priority of task.
Numberis task ID from 0 to 255 for RTX51 Full and 0 to 15 for RTX51 Tiny.Priorityis priority of task.
Real-time task functions must be declared with void return type and void argument list (say no arguments passed to task function).
Writing First C program in Keil
Basic of a C program
As we already discussed, Keil C is not much different from a normal C program. If you know assembly, writing a C program is not a problem,only thing you have to keep in mind is forget your controller has general purpose registers, accumulators or whatever. But do not forgetabout Ports and other on chip peripherals and related registers to them.
In basic C, all programs have atleast one function which is entry point for your application that function is named as \"main\" function.Similarly in keil, we will have a main function, in which all your application specific work will be defined. Lets move further deep into theworking of applications and programs.
When you run your C programs in your PC or computer, you run them as a child program or process to your Operating System so when youexit your programs (exits main function of program) you come back to operating system. Whereas in case of embedded C, you do not haveany operating system running in there. So you have to make sure that your program or main file should never exit. This can be done with thehelp of simple while(1) or for(;;) loop as they are going to run infinitely. Following layout provides a skeleton of Basic C program.
1. void main()2. {3. //Your one time initialization code will come here4. while (1) {5. //while 1 loop6. //This loop will have all your application code7. //which will run infinitely8. }9. }
When we are working on controller specific code, then we need to add header file for that controller. I am considering you have already gone
When we are working on controller specific code, then we need to add header file for that controller. I am considering you have already gonethrough "Keil Microvision" tutorial. After project is created, add the C file to project. Now first thing you have to do is adding the header file.All you have to do is right click in editor window, it will show you correct header file for your project.
Figure below shows the windows context for adding header file to your c file.
Include Header file in Keil
Writing Hardware specific code
In harware specific code, we use hardware peripherals like ports, timers and uart etc. Do not forget to add header file for controller you areusing, otherwise you will not be able to access registers related to peripherals. Lets write a simple code to Blink LED on Port1, Pin1.
1. #include <REGx51.h>2. //header file for 89C513. void main()4. {5. //main function starts6. unsigned int i;7. 8. //Initializing Port1 pin19. P1_1 = 0; //Make Pin1 o/p
10. 11. while (1) {12. //Infinite loop main application13. //comes here14. for(i=0;i<1000;i++)15. ; //delay loop16. 17. P1_1 = ~P1_1;18. //complement Port1.119. //this will blink LED connected on Port1.120. }21. }
You can now try out more programs. "Practice makes a man perfect".
You can easily interface your programs to routines written in 8051 Assembler. All you need to do is follow few programming rules, you cancall assembly routines from C and vice-versa. Public variables declared in assembly modules are available to your C program.
There maybe several reasons to call an assembly routine like faster execution of program, accessing SFRs directly using assembly etc. In thispart of tutorial we will discuss how to write assembly progarms that can be directly interfaced with C programs.
For any assembly routine to be called from C program, you must know how to pass parameters or arguements to fucntion and get returnvalues from a function.
Segment naming
C51 compiler generates objects for every program like program code, program data and constant data. These objects are stored in segmentswhich are units of code or data memory. Segment naming is standard for C51 compiler, so every assembly program need to follow thisconvention.
Segment names include module_name which is the name of the source file in which the object is declared. Each segment has a prefix thatcorresponds to memory type used for the segment. Prefix is enclosed in question marks (?). The following is the list of the standard segmentname prefixes:
C51 module prefix
Data Objects
Data objects are the variables and constants you declare in your C programs. The C51 compiler generates a saperate segment for eachmemory type for which variable is declared. The following table lists the segment names generated for different variable data objects. Dataobjects segment prefix
Data Segment Prefix
Program Objects
Program onjects includes code generated for C programs functions by C51 compiler. Each function in a source module is assigned a separate
Program onjects includes code generated for C programs functions by C51 compiler. Each function in a source module is assigned a separatecode segment using the ?PR?function_name?module_name naming convention. For example, for a function name send_char in file nameuart.c will have a segment name of ?PR?SEND_CHAR?UART.
C51 compiler creates saperate segments for local variables that are declared within the body of a function. Segment naming conventions fordifferent memory models are given in following tables:
Small model segment naming convention
Compact model segment naming convention
Large model segment naming convention
Function names are modified slightly depending on type of function (functions without arguments, functions with arguments and reentrantfunctions). Following tables explains the segment names:
function segment naming convention
Advanced C programming
Function Parameters
C51 make use of registers and memory locations for passing parameters. By default C function pass up to three parameters in registers andfurther parameters are passed in fixed memory locations. You can disable parameter passing in register using NOREGPARMS keyword.Parameters are passed in fixed memory location if parameter passing in register is disabled or if there are too many parameters to fit inregisters.
Parameter passing in registers
C functions may pass parameter in registers and fixed memory locations. Following table gives an idea how registers are user for parameter
C functions may pass parameter in registers and fixed memory locations. Following table gives an idea how registers are user for parameterpassing.
parameter passing to functions
Following example explains a little more clearly the parameter passing technique:
example parameter passing to functions
Parameter passing in Fixed Memory Locations
Parameters passed to assembly routines in fixed memory lcoation use segments named
?function_name?BYTEAll except bit parameters are defined in this segment.
?function_name?BITBit parameters are defined in this segment.
All parameters are assigned in this space even if they are passed using registers. Parameters are stored in the order in which they aredeclared in each respective segment.
The fixed memory locations used for parameters passing may be in internal data memory or external data memory depending upon thememory model used. The SMALL memory model is the most efficient and uses internal data memory for parameter segment. The COMPACTand LARGE models use external data memory for the parameter passing segments.
Fucntion Return Values
Function return values are always passed using CPU registers. The following table lists the possible return types and the registers used foreach.
Following example shows how these segment and function decleration is done in assembler.
1. ;Assembly program example which is compatible2. ;and called from any C program3. ;lets say asm_test.asm is file name4. name asm_test5. 6. ;We are going to write a function7. ;add which can be used in c programs as8. ; unsigned long add(unsigned long, unsigned long);9. ; as we are passing arguments to function
10. ;so function name is prefixed with '_' (underscore)11. 12. ;code segment for function "add"13. ?PR?_add?asm_test segment code14. ;data segment for function "add"15. ?DT?_add?asm_test segment data16. 17. ;let other function use this data space for passing variables18. public ?_add?BYTE19. ;make function public or accessible to everyone20. public _add21. 22. ;define the data segment for function add23. rseg ?DT?_add?asm_test24. ?_add?BYTE:25. parm1: DS 4 ;First Parameter26. parm2: ds 4 ;Second Parameter27. 28. ;either you can use parm1 for reading passed value as shown below29. ;or directly use registers used to pass the value.30. rseg ?PR?_add?asm_test31. _add:32. ;reading first argument33. mov parm1+3,r734. mov parm1+2,r635. mov parm1+1,r536. mov parm1,r437. ;param2 is stored in fixed location given by param238. 39. ;now adding two variables40. mov a,parm2+341. add a,parm1+342. ;after addition of LSB, move it to r7(LSB return register for Long)
43. mov r7,a44. mov a,parm2+245. addc a,parm1+246. ;store second LSB47. mov r6,a48. mov a,parm2+149. addc a,parm1+150. ;store second MSB51. mov r5,a52. mov a,parm253. addc a,parm154. ;store MSB of result and return55. 56. ;keil will automatically store it to57. ;varable reading the resturn value58. mov r4,a59. ret60. 61. end
Now calling this above function from a C program is very simple. We make function call as normal function as shown below:
1. extern unsigned long add(unsigned long, unsigned long);2. 3. void main()4. {5. unsigned long a;6. a = add(10,30);7. //a will have 40 after execution8. while(1);9. }
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