Summary of How to use I2C / TWI (Two Wire Interface) in AVR ATmega32
This article demonstrates TWI (I2C) interfacing between two ATmega32 controllers where the master sends a byte, the slave returns its complement, and the master shifts the original left—cycling from 0x01 up to 0x80—displaying values on PORTB. It outlines TWI modes, master initialization (TWBR, TWSR), sending START (TWCR, TWINT, TWSTA, TWEN, TWSR checks), and sending slave address/read-bit with ACK checking via TWDR and TWCR.
Parts used in the ATmega32 TWI Interfacing Project:
- Two ATmega32 microcontrollers
- Pull-up resistors for SDA and SCL lines
- Connecting wires/jumper wires
- Power supply (Vcc and GND)
- Two 8-bit output ports connected to PORTB LEDs or display
- Clock source for ATmega32 (internal or external)
- Breadboard or PCB for circuit assembly
This article explores the TWI interfacing between two ATmega32 controllers. Readers are advised to go through TWI Communication and TWI registers of ATmega32 before going further.
TWI works in four modes:
1. MASTER as a transmitter.
2. MASTER as a receiver.
3. SLAVE as a receiver.
4. SLAVE as a transmitter.
Generally modes 1 & 3 and modes 2 & 4 are used together. This article explains the use of these four modes by an experiment.
Objective: To establish the communication between two ATmega32 using TWI interface. First the Master starts by sending data then the slave transmits complement of the received data to the master. When the Master receives the complemented data, it shifts the original data to left. This process of transmitting and receiving continues. As the data value reaches 0x80 the whole process is repeated. At the starting, value of the original data is 0x01. The received value is displayed on PORTB at both the ends.
Circuit description:
Make the connections as shown in the circuit diagram.
Code explanation for MASTER Controller:
Step 1: Initialization of master.
Initialization of MASTER means to set the TWI clock frequency (SCL). It is done by setting bit rate in TWBR and pre scaler bits in TWSR.
void TWI_init_master(void) // Function to initialize master
{
TWBR=0x01; // Bit rate
TWSR=(0<<TWPS1)|(0<<TWPS0); // Setting prescalar bits
// SCL freq= F_CPU/(16+2(TWBR).4^TWPS)
}
Step 2: Send start condition
The start condition in TWI is explained before. The AVR microcontroller has in built registers which makes this job much easier.
1. Clear TWINT flag by writing a logic one to it.
2. Set TWSTA bit to send start condition.
3. Set TWEN bit to initialize the TWI.
4. Monitor the status of TWINT flag.
5. Check the ACK byte (using while condition since the SCL frequency is very small as compared to micro controller clock frequency). The ACK byte can be compared by monitoring the status of TWSR.
void TWI_start(void)
{
// Clear TWI interrupt flag, Put start condition on SDA, Enable TWI
TWCR= (1<<TWINT)|(1<<TWSTA)|(1<<TWEN);
while(!(TWCR & (1<<TWINT))); // Wait till start condition is transmitted
while((TWSR & 0xF8)!= 0x08); // Check for the acknowledgement
}
Step 3: Send the slave address, data direction bit (write) and wait for the ACK signal
These three processes are controlled by AVR’s TWI registers.
1. Put the seven bit slave address and the direction control bit in TWDR.
2. Clear TWINT flag.
3. Enable TWI by writing logic one to TWEN bit.
4. Monitor the status of TWINT flag, the TWINT flag will get cleared when the data in TWDR is been transmitted.
5. Check for the correct acknowledgement.
void TWI_read_address(unsigned char data)
{
TWDR=data; // Address and read instruction
TWCR=(1<<TWINT)|(1<<TWEN); // Clear TWI interrupt flag,Enable TWI
while (!(TWCR & (1<<TWINT))); // Wait till complete TWDR byte received
while((TWSR & 0xF8)!= 0x40); // Check for the acknoledgement
}
For more detail: How to use I2C / TWI (Two Wire Interface) in AVR ATmega32
- What is the objective of this TWI experiment?
To establish TWI communication between two ATmega32s where the master sends data, the slave returns its complement, and the master shifts the original left, displaying values on PORTB. - Which TWI modes are described?
The article lists MASTER as transmitter, MASTER as receiver, SLAVE as receiver, and SLAVE as transmitter. - How is the TWI clock frequency set for the master?
By setting TWBR for bit rate and pre-scaler bits in TWSR, using the SCL freq formula given. - How do you send a START condition in TWI on ATmega32?
Set TWINT by writing one, set TWSTA and TWEN in TWCR, wait for TWINT, and check TWSR for acknowledgement. - How is the slave address and direction sent?
Load the address and read/write bit into TWDR, clear TWINT and set TWEN in TWCR, wait for TWINT, then check TWSR for ACK. - Where are transmitted and received values displayed?
The received value is displayed on PORTB at both master and slave ends. - What initial data value is used in the example?
The starting original data value is 0x01. - When does the transmit/receive process repeat?
When the data value reaches 0x80 the whole process is repeated.
