Embedded C Programming - Neff Siteneff.site/courses/other/legacy/embedded_c/script.pdf ·...

Embedded C Programming

Networks and Embedded Software

Embedded Systems

by Wolfgang Neff

Programming C (1)

• Code file organization

Header File(Declarations)

Source Code(Implementation)

Program(Executable)

11.02.2016 Embedded C Programming, © W. Neff 2

Programming C (2)

• Header file

#ifndef ADD_H_#define ADD_H_

#define FIVE 5#define ADD(x,y) x+y

extern int result;void add5(int x);

#endif /* ADD_H_ */

Preprocessor directives

Definition of a constant value

Definition of a macro

Declaration of a global variable

Declaration of a function


Programming C (3)

• Source file

#include "add.h"

int result;

void add5(int x) {result = ADD(x,FIVE);

}

Include header file

Definition of the global variable

Implementation of the function


Programming C (4)

• Main file

#include <avr/io.h>#include "add.h"

int main(void) {

add5(4);

while (1) {/* do something */

}

}

Include a user header file

Main program

Call function declared in add.h

Infinite main loop(embedded systems, only)

(result now in global variable)

Include a system header file

No good idea!


Programming C (5)

• Data types

Standard C Alias Embedded Systems* Size

signed char int8_t 8 Bits

unsigend char char uint8_t 8 Bits

signed short int int16_t 16 Bits

unsigend short uint16_t 16 Bits

signed long int32_t 32 Bits

unsigned long uint32_t 32 Bits

float 32 Bits

double (* Defined in <stdint.h>) 64 Bits


Programming C (6)

• Indirect addressing with pointers

*p = 5;int x = 4;int y = 4;int *p = &y;

SRAM0108hex…0107hex4y:0106hex?0105hex?0104hex?0103hex107hexp:0102hex4x:0101hex…

SRAM0108hex…0107hex5y:0106hex?0105hex?0104hex?0103hex107hexp:0102hex4x:0101hex…

p points to y


Programming C (7)

• Input / Output

– Special drivers

• E. g. uart_put, uart_get

– Standard input / output

• #include <stdio.h>

• Simple input / output– Characters: getchar, putchar

– Strings: gets, puts

• Complex input / output– scanf / printf


Programming C (8)

• Formatted output

– int printf(const char *format, ...);

– Format Specifiers

• %d decimal signed integer

• %x hex integer

• %u unsigned integer

• %c character

• %s string

• %p pointer


Programming C (9)

• Example

– char s = 'A'; // define sensorint d = adc_read(s); // read sensorint v = d / 15; // convert adc to voltageprintf("Sensor %c read '%x'. This are %d V.",s,d,v);

– Result:

• Sensor A read 'A1'. This are 10 V.

• Drawback

– Very expensive – storage and performance


Programming C (10)

• Logical versus bitwise operators

– Logical operators: !, &&, ||

• No Boolean data type – false = 0, true ≠ 0

• Example: !(a<0 || a>9) = 0 if a ∊ {0 … 9}

– Bitwise operators: ~, &, |, ^, <<, >>

• Same name but different functionality

• Manipulate individual bits

• Do not mix

• Do not confuse


Programming C (11)

• Operator ~ - bitwise NOT

• Bitwise & - bitwise AND

a 0 0 1 1 0 1 1 0

~a 1 1 0 0 1 0 0 1

a 0 0 1 1 0 1 1 0

b 0 0 0 0 1 1 1 1

a&b 0 0 0 0 0 1 1 0


Programming C (12)

• Operator | - bitwise OR

• Bitwise ^ - bitwise XOR

a 0 0 1 1 0 1 1 0

b 0 0 0 0 1 1 1 1

a|b 0 0 1 1 1 1 1 1

a 0 0 1 1 0 1 1 0

b 0 0 0 0 1 1 1 1

a^b 0 0 1 1 1 0 0 1


Programming C (13)

• Operator << - bitwise left shift

• Bitwise >> - bitwise right shift

– Attention: implementation dependent

a 0 0 1 1 0 1 1 0

a<<2 1 1 0 1 1 0 0 0

a 0 0 1 1 0 1 1 0

a>>2 0 0 0 0 1 1 0 1


Programming C (14)

• Volatile values

– Not under the control of the C compiler

– Change asynchronously

– Optimization would be harmful

– Example

• volatile uint8_t i = USART.DATA;volatile uint8_t j = USART.DATA;if (i != j) { … } // Has DATA changed?// dead code is optimized away if volatile is omitted


Bit Manipulation (1)

• Individual bits

– Bit position (_bp)

• Example: USART. CTRLC

– USART_SBMODE_bp = 3

– Bit mask (_bm)

• Byte with one bit set at bit position

• Example: USART_SBMODE_bm = 00001000


Bit 7 6 5 4 3 2 1 0

Name CMODE PMODE SBMODE CHSIZE


• Bits groups

– Group position (_gp)

• Start position of a bit group

• Example: USART_PMODE_gp = 4

– Group mask (_gm)

• Byte with all bits of a bit group set

• Example: USART_PMODE_gm = 0011 0000



• Bits groups (continued)

– Group configuration (_gc)

• A meaningful configuration of group bits

• Example: PMODE configuration– USART_PMODE_DISABLED_gc = 00000000

– USART_PMODE_EVEN_gc = 00100000

– USART_PMODE_ODD_gc = 00110000

• Header file with definitions

– #include <avr/io.h>



• Testing bits

– Test if a specific bit is set

• if (REG & BIT_bm) { … }

– Test if a specific bit is cleared

• if (!(REG & BIT_bm)) { … }

– Test if a bit group has a specific configurationif ((REG & GROUP_gm) == GROUP_gc) { … }

logical bitwise


parenthesis


• Manipulation of bit groups

– Clearing a bit group

• REG = REG & ~GROUP_gm;

– Configuring a bit group

• REG = (REG & ~GROUP_gm) | GROUP_gc;

• First clear bit group then set configuration bits

– Assigning a specific value to a bit group

• REG = (REG & ~GROUP_gm) | (value << GROUP_gp);


Finite State Machines (1)

• They have

– States

– Transitions

– Conditions

– An initial state

• They …

– Do something subject to state

– Switch state if a transition condition is met


• State diagram

start statestates

transition

condition

start state


• Example

– A LED is controlled by three buttons

• If button 0 is pressed the LED is turned on.

• If button 1 is pressed the LED is turned off.

• While button 2 is pressed the LED is turned on.

– States

• Q0: LED is off

• Q1: LED is on, waiting for a press on button 1

• Q2: LED is on, waiting for the release of button 2



• Example (continued)

– Transitions

• t0: State Q0, button 0 pressed → state Q1



• t3: State Q2, button 2 released → state Q0




– State diagram




– Program• int state=0;

• while(1) {

• switch (state) {

• case 0:

• // Handle state

• led = off;

• // Handle transitions

• if (button0) state=1;


• break;




– Program (continued)

• case 1:

• led = on;


• break;

• case 2:

• led = on;

• if (!button2) state=0;

• break;

• } /* switch */

• } /* while */



• Splitting states into more states



• Example

– Program• int state=0;

• while(1) {

• switch (state) {

• case 0:

• doSomething(0);

• if (button) { state = STOP; break; }

• state = 1;

• break;

• case 1:

• doSomething(1);

• …




– Program (continued)

• void doSomething(int action)

• {

• switch (action) {

• case 0:

• action0();

• return;

• case 1:

• action1();

• …

• } /* switch */

• } /* function */



• Timer controlled transitions


Common Mistakes (1)

• Inconsistent indention

– Incorrect code

• while (!ready) {for (int i=0; i<10; i++) {if (i == 5) {}break;}


Common Mistakes (2)

• Inconsistent indention (continued)

– Correct code

• while (!ready) {for (int i=0; i<10; i++) {

if (i == 5) {} // Empty statement?break; // Unconditional break?

}// Missing }


Common Mistakes (3)

• Inconsistent indention (finished)

– Explanation

• Code blocks embraced witch braces ({, }) are very important in C. Such blocks must be seen at first sight. Correct indention is the best way to ensure this.

• Incorrect indention is error prone. Orientation in the code is lost. Missing braces are hard to find and the meaning of the code is hard to understand.

• Inconsistent indention is bad programming style and a typical beginner's mistake.


Common Mistakes (4)

• Ignoring warnings

– Incorrect code

• short i = 1000000;Warning: overflow in implicit constant conversion

– Correct code

• long i = 1000000; // Fix code

• short i = (short)1000000; // Make intention explicit


Common Mistakes (5)

• Ignoring warnings (continued)

– Explanation

• C has a the programmer knows what he doesphilosophy. Therefore many potentially dangerous things are possible in C. The compiler, however, often emits a warning. Warnings are important. Do not ignore them.


Common Mistakes (6)

• Ignoring warnings (finished)

– Explanation (continued)

• The compiler produces correct code even if warnings were emitted. They disappear if you simply build you again as nothing has to bee done. You have to rebuildyour code to see them again. This is how a build system works.

• Avoid warnings at all. Either correct your code or make your intentions explicit.


Common Mistakes (7)

• Missing prototypes

– Incorrect code

• _delay_ms(100);Warning: implicit declaration of function '_delay_ms'

• int main (void) { …; f(); … }void f(void) { … }Warning: implicit declaration of function 'f'


Common Mistakes (8)

• Missing prototypes (continued)

– Correct code

• #include <util/delay.h>…_delay_ms(100);

• void f(void);int main (void) { …; f(); … }void f(void) { … }

• void f(void) { … }int main (void) { …; f(); … }


Common Mistakes (9)

• Missing prototypes (finished)

– Explanation

• C has to know the prototype (signature) of a function. Otherwise it assumes one (implicit declaration). This assumption is in the majority of cases not correct.

• Always include the appropriate header files.

• Provide prototypes for your own functions or define them before they are used. This is often better as you do not need to provide a prototype.


Common Mistakes (10)

• Missing void in prototype

– Incorrect code

• void f() { … };f(7);

– Correct code

• void f(void) { … };f(7);Error: too many arguments to function 'f'



• Missing void in prototype (continued)

– Explanation

• In C you must explicitly declare that a function has no arguments. This is done by void.

• If you declare a function with empty parentheses it accepts any number of arguments.

• Here C++ differs from C. In C++ you need not and must not put a void between the parentheses



• Misplaced semicolons

– Incorrect code

• if (i > 10); i = 10;

– Correct code

• if (i > 10) i = 10;

• if (i > 10) {i = 10;

}



• Misplaced semicolons (continued)

– Explanation

• In C statements must be closed by a semicolon. A if statement i. g. has the following form: if (condition) statement;. for, while etc. are similar.

• Unfortunately there is a null (empty) statement which is simply a colon. So if (condition); is valid but means: if condition do nothing. This is usually not intended.

• If there is only one statement you may omit the braces but this is considered bad programming style.



• Missing break in switch statement

– Incorrect code

• switch(i) {case 1:n = n + 1;

case 2:n = n - 1;break;

}



• Missing break in switch statement (continued)

– Correct code

• switch(i) {case 1:n = n + 1;break;

case 2:n = n + 1;break;

}



• Missing break in switch statement (continued)

– Correct code

• switch(i) {case 1:case 2: // Possible but meaning is differentn = n + 1;break;

}



• Missing break in switch statement (finished)

– Explanation

• In C the cases of a switch statement fall through. This enables case 1: case 2: … sequences but apart from that is usually not what you want. So don’t forget to place a break before each case.

• In functions return might be in some cases an appropriate alternative.



• Confusing characters and strings

– Incorrect code

• char c = "H";Warning: initialization makes integer from pointer without a cast

– Correct code

• char c = 'H';



• Confusing characters and strings (continued)

– Explanation

• There is no string data type in C. Strings are arrays of char. 'H' is one character whereas "H" are two.

• Characters are like numbers. char c = 'H' actually assigns 72 which is the ASCII code of 'H'. Since characters are numbers c = c+1 is quite okay in C.

• "H" is the shorthand for { 'H', '\0' } as strings are zero terminated in C. { 'H', '\0' } is an array and char c = "H" assigns a pointer to the first element of this array.



• Confusing characters and strings (finished)



* Overflow not considered

char c = 'H';

RAM

0101h72c:0102h.

0103h.

0104h.0105h.0106h.

char c = "H";

RAM

0101h0x102*c:0102h72.

0103h0.

0104h.0105h.0106h.

Terminating Zero


• Wrong array bounds

– Incorrect code

• int a[5];for (int i<0; i<=5; i++) a[i]=i;

• char s[5] = "Hello";for (int i<0; strlen(s); i++) s[i]='a';



• Wrong array bounds (continued)

– Incorrect code

• int a[5];for (int i<0; i<5; i++) a[i]=i;

• char s[] = "Hello";for (int i<0; strlen(s); i++) s[i]='a';



• Wrong array bounds (continued)

– Explanation

• In an array declaration the number of elements is specified not the upper bound. So an array[5] ranges from 0 to 4. Unfortunately indices are not checked. Therefore values outside may be overwritten.

• Declare strings with implicit size. C does not add a trailing zero if no space is left. So strlen does not work with strings declared such as char s[5] = "Hello"; as they are not zero terminated.



• Wrong array bounds (finished)



char s[3] = "abc";int i = 7;

RAM

0101h0x102s:0102h'a'.

0103h'b'.

0104h'c'.0105h7i:0106h.

int a[4] = {0,1,2,3};int i = 7;a[4] = 4; // Wrong!

RAM

0101h0x102a:0102h0.

0103h1.

0104h2.0105h3.0106h4i:


• Assignment in conditions (continued)

– Explanation

• = is the assignment operator. x = 3 means that from now on x has a value of 3. The result of this operation is the value assigned. Therefore if (x = 3) is always true since 3 is not zero. In C only zero is regarded to be false.

• == checks for equality. x == 3 is true only if x has a value of 3.



• Assignment in conditions

– Incorrect code

• if (x = 3) { … }Warning: suggest parentheses around assignment used as truth value

– Correct code

• if (x == 3) { … }



• Assignment in conditions (continued)

– Explanation

• = is the assignment operator. x = 3 means that from now on x has a value of 3. The result of this operation is the value assigned. Therefore if (x = 3) is always true since 3 is not zero. In C only zero is regarded to be false.

• == checks for equality. x == 3 is true only if x has a value of 3.



• String processing without loops

– Incorrect code

• putchar("Hallo");Warning: passing argument 1 makes integer from pointer without a cast

– Correct code

• char s = "Hallo";for (int i=0; i<strlen(s); i++) putchar(s[i]);



• String processing without loops (continued)

– Explanation

• You must not pass an array of characters to a function which expects a single character. You have to pass the characters one after another in a for loop.



• Calling functions without parentheses

– Incorrect Codeint f(void) { return 3; }int x = f;Warning: assignment makes integer from pointer without a cast

– Correct Codeint f(void) { return 3; }int x = f();



• Calling functions without parentheses (continued)

– Explanation

• () is the function call operator. So it parentheses are needed also if a function has no arguments. If they are missing the name is considered to be a function pointer. Function pointers are an advanced feature.


Embedded C Programming - Neff Siteneff.site/courses/other/legacy/embedded_c/script.pdf ·...

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