Programming

Networks and Embedded Software

Introduction

by Wolfgang Neff

ATxmega128A1 (1)

• Atmel AVR 8-bit Microcontroller

– Introduced 2009

– Harvard RISC architecture

– 32 registers (8 bit)

– 138 machine code instructions

– 100 multiplexed and configurable pins

– Flash and SRAM integrated on chip

– Wide range of peripheral modules

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ATxmega128A1 (2)

• Block Diagram

– Power lines

– I/O Ports

• A, B, C, D, E, F, …

– Serial Ports

• USART, SPI, I²C

– Converter

• ADC, DAC

– Timer etc.

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Timer USART SPI I²CPort

sP

WR

AD

CD

AC

EEPROM

Flash SRAM

CPU

Inte

rru

pts

ATxmega128A1 (3)

• Alternate Port Functions

– Pins are multiplexed (consult datasheet for details)

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Mic

roco

ntr

olle

r

PC7

SYNC

TXD1

SCK

MU

X Pin 7

PC6

SYNC

RXD1

MISO

MU

X Pin 6

Port INT USARTC1 SPIC

PC0 SYNC

PC1 SYNC

PC2 SYNC

PC3 SYNC

PC4 SYNC SS

PC5 SYNC XCK1 MOSI

PC6 SYNC RXD1 MISO

PC7 SYNC TXD1 SCK

ATxmega128A1 (4)

• Peripheral Modules

– Peripherals are organised as modules

– Modules have several instances

• Eight timers, eight USARTs, four SPIs etc.

– Modules and instances are like classes and objects

– Example

• PORTA.DIR PORTE.DIRPORTA.OUT PORTE.OUT

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Two Instances of Module Port Two Registers of Ports

C Programming (1)

• Code file organization

Header File(Interface)

Source Code(Implementation)

Program(Executable)

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C Programming (2)

• Code generation

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Program(Executable)

IDE(compiler & linker)

Header Files#include <…>

Object Files

System files

Header Files#include "…"

Source Files

User files

C Programming (3)

• 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

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Programming C (4)

• 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

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Programming C (5)

• 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!

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C Programming (6)

• Data types

Standard C Alias Embedded Systems* Size

signed char int8_t 8 Bits

unsigned char char uint8_t 8 Bits

signed short int int16_t 16 Bits

unsigned 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

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C Programming (7)

• Indirect addressing with pointers

*p = 5; // indirect

int x = 4; // immediate

int y = x; // direct

int *p = &y;

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

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

p points to y

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C Programming (8)

• Input / Output

– Special drivers

• E. g. usart_put, usart_get

– Standard input / output

• #include <stdio.h>

• Simple input / output– Characters: getchar, putchar

– Strings: gets, puts

• Complex input / output– scanf / printf

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C Programming (9)

• 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

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C Programming (10)

• Example

– char s = 'A'; // define sensoruint16_t d = adc_read(s); // read sensorint v = d / 15; // convert to voltage

– printf("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

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C Programming (11)

• 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 USART.DATA changed?Dead code is optimized away if volatile is omitted.

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