I2C programming with C and Arduino

How to use I2C with AVR

Sunday, April 28, 13

I2C abstract(1)

AVR1 AVR2SCL

SDA

Vdd

I2C is one of communication systems for Microcomputer.I2C uses 2 lines, SCL(clock), SDA(data).


I2C abstract(2)

AVR1 AVR2SCL

SDA

SCL(clock) is used to send clock for synchronization.SDA(data) is used for sending data.1 bit data is transferred every clock.

1 0 1 0


I2C abstract(3)

Master

You can connect more than 2 boards.One board must be Master and the others are Slave.Each slave has own address.Master can send data to each slave using the address.

Slave1 Slave2 Slave3 Slave4

SDA

address1 address2 address3 address4

SCL


I2C abstract(4)

Master

For example, if Master wants to send data to Slave2, Master sends address2 to SDA line. All Slaves receives the address.Only Slave2 replies.


SDA


SCL

address2

reply

The data is mine!I must send replay.

The data is not mine. I will ignore it.




I2C abstract(5)

Master

Next Master sends data to SDA line. Only Slave2 receives the data and reply.


SDA


SCL

data

reply

I’ll receive this data.I must send replay.

The data is not mine. I will discard it.



discard discard discard


Program I2C with AVR

In this instance, you use arduino uno.Arduino uno has Atmega168. I am going to explain how to program Atmega168 with C, without Arduino library.


ConnectionSDA pin and SCL pin are fixed. On Arduino Uno, analog4 = SDA, analog 5 = SCL.Connect A4 and A5 of each Arduino.

Vcc


C programming on Arduino IDE

Arduino language is based on C. You can use all construction of C. When you write C on Arduino IDE, you have to replace “main and while” with “setup() and loop()”. Initialize process must be inside setup() and loop process must be inside loop().

#include<avr.io.h>

void main(int){//initialize process

while(1){//loop process

}

}

#include<avr.io.h>

void setup(){//initialize process

}

void loop(){//loop process

}

Normal C C on Arduino IDE


I2C registers(Atmega168 version)

If you want to use I2C function on Microcomputer, you have to use registers for I2C.(Sometimes different microcomputers have different registers, you have to see data sheet.)These are I2C resisters(NOT all). TWAR is TWI(Slave) Address Register. Slave address is set in this register.TWBR is TWI Bit Rate Register. This is to decide SCL clock.TWCR is TWI Control Register. You can control I2C with this register.TWDR is TWI Data Register. The data to be transferred or received is set in this register.TWSR is TWI Status Register. The status of I2C communication is set in this register.

TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCETWAR

TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0

TWINT TWEA TWSTA TWSTO TWWC TWEN - TWIE

TWD7 TWD6 TWD5 TWD4 TWD3 TWD2 TWD1 TWD0

TWS7 TWS6 TWS5 TWS4 TWS3 - TWPS1 TWPS0

TWBR

TWCR

TWDR

TWSR


The simplest I2C sequence

This is the simplest sequence of I2C.If you want to send data more than 1 byte. You have to repeat (4)(5).

Master Slave

(1)Start

(2)Slave Adress

(3)Ack

(4)Data

(5)Ack

(6)Stop


How to use TWBR and TWSR

First, you have to decide the communication speed(SCL) with TWBR and TWSR.SCL = CPU clock/(16+2(TWBR)xPrescaler)

TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0

TWS7 TWS6 TWS5 TWS4 TWS3 - TWPS1 TWPS0

TWBR

TWSRPrescaler parameter

(ex)TWBR =0b00000001=1TWBR =0b00000011=3TWBR =0b11111111 = 255

(ex)CPU clock = 1MhzTWBR=255Prescaler=1SCL = 1000000/(16+2(255)x1)=2kHz

(ex)TWBR =(0<<TWPS1)|(0<<TWPS0) : prescaler is 1TWBR =(0<<TWPS1)|(1<<TWPS0) : prescaler is 4TWBR =(1<<TWPS1)|(1<<TWPS0) : prescaler is 64


How to use TWCR

You can control I2C message with TWCR.

TWINT TWEA TWSTA TWSTO TWWC TWEN - TWIETWCR

Send start condition


Clear TWINT flag after sending message


Send stop condition

TWCR = (1<<TWINT)|(1<<TWSTA)|(1<<TWEN); //Start conditionwhile(!(TWCR & 1<<TWINT)) {} //After TWCR, always you have to check whether TWINT is updatedTWCR = 0b10000100; //flag clearwhile(!(TWCR & 1<<TWINT)) {} TWCR = 0b10010100; //finish translationwhile(!(TWCR & 1<<TWINT)) {}

Source code sample


How to use TWSR

You can check status of I2C sequence from TWSR.

TWS7 TWS6 TWS5 TWS4 TWS3 - TWPS1 TWPS0TWSR

Source code sample

//check condition if((TWSR & 0xF8) == 0x08){ TWDR = 0b00000010; //set slave address TWCR = 0b10000100; //flag clear while(!(TWCR & 1<<TWINT)) {} _delay_ms(1); if((TWSR & 0xF8) == 0x18){ TWDR = 0b10101010; //write data(1byte) TWCR = 0b10000100; //send data while(!(TWCR & (1<<TWINT))); _delay_ms(1); } }


The simplest sample

(1)Connect A4 and A5 between Master and Slave(You need pull-up resisters)(2)Connect LED and resister to Slave digital 9(3)Connect Master V5 to Slave Vin(4)Connect Master GND and Slave GND(5)Connect Master to PC with USB cable(6)Install program from next page

Vcc

Master

Slave

LEDGND

This sample is to turn on LED connected to Slave using data from Master


Sample code(master)

#include <avr/io.h>#include <util/delay.h>

void setup(){ TWSR = (0<<TWPS1)|(0<<TWPS0); //Prescaler TWBR = 0xFF; //TWBR=255}

void loop(){ TWCR = (1<<TWINT)|(1<<TWSTA)|(1<<TWEN); //Start condition while(!(TWCR & 1<<TWINT)) {} //wait for flag update _delay_ms(1); if((TWSR & 0xF8) == 0x08){ //check condition TWDR = 0b00000010; //send slave address TWCR = 0b10000100; //flag clear while(!(TWCR & 1<<TWINT)) {} _delay_ms(1); if((TWSR & 0xF8) == 0x18){ //check condition TWDR = 0b10101010; //write data(1byte) TWCR = 0b10000100; //send data while(!(TWCR & (1<<TWINT))); _delay_ms(1); } } TWCR = 0b10010100; //finish translation _delay_ms(2);}


Sample code(slave)

#include <avr/io.h>#include <util/delay.h>

void setup(){ DDRB = 0xFF; //all PINB is output mode PORTB = 0x00; //all PINB is Low TWAR = 0b00000010; //salve address setting}

void loop(){ TWCR = 0b11000100; //start as slave(wait for address) if((TWSR & 0xF8) == 0x60){ //check status while(1){ TWCR = 0b11000100; //wait for data while(!(TWCR & (1<<TWINT))); if((TWSR & 0xF8) == 0x80){ //check status char data = TWDR; //receive data PORTB = data; //insert data to PORTB } if((TWSR & 0xF8) == 0xA0){ //check stop status _delay_ms(10); break; } } }}


I2C programming with C and Arduino

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