I2C programming with C and Arduino

How to use I2C with AVR
Sunday, April 28, 13

I2C abstract(1)
AVR1 AVR2
SCL
SDA
Vdd
I2C is one of communication systems for Microcomputer.
I2C uses 2 lines, SCL(clock), SDA(data).

I2C abstract(2)
AVR1 AVR2
SCL
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.
I will ignore it.
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.
I will discard it.
I will discard it.
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.

Connection
SDA 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
TWBR
TWSR
Prescaler parameter
(ex)
TWBR =0b00000001=1
TWBR =0b00000011=3
TWBR =0b11111111 = 255
(ex)
CPU clock = 1Mhz
TWBR=255
Prescaler=1
SCL = 1000000/(16+2(255)x1)=2kHz
(ex)
TWBR =(0<<TWPS1)|(0<<TWPS0) : prescaler is 1

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 condition
while(!(TWCR & 1<<TWINT)) {} //After TWCR, always you have to check whether TWINT is updated
TWCR = 0b10000100; //flag clear
while(!(TWCR & 1<<TWINT)) {}
TWCR = 0b10010100; //finish translation
Source code sample

How to use TWSR
You can check status of I2C sequence from TWSR.
TWSR
Source code sample
//check condition
if((TWSR & 0xF8) == 0x08){
TWDR = 0b00000010; //set slave address
_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 MasterV5 to SlaveVin
(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
_delay_ms(1);
if((TWSR & 0xF8) == 0x18){ //check condition
TWDR = 0b10101010; //write data(1byte)
TWCR = 0b10000100; //send data
_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
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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