ATMEGA640 datasheet(244/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

24.6 Using the TWI

The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like reception of a

byte or transmission of a START condition. Because the TWI is interrupt-based, the application software is free to

carry on other operations during a TWI byte transfer. Note that the TWI Interrupt Enable (TWIE) bit in TWCR

together with the Global Interrupt Enable bit in SREG allow the application to decide whether or not assertion of the

TWINT Flag should generate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT

Flag in order to detect actions on the TWI bus.

When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In this

case, the TWI Status Register (TWSR) contains a value indicating the current state of the TWI bus. The application

software can then decide how the TWI should behave in the next TWI bus cycle by manipulating the TWCR and

TWDR Registers.

Figure 24-10 is a simple example of how the application can interface to the TWI hardware. In this example, a Mas-

ter wishes to transmit a single data byte to a Slave. This description is quite abstract, a more detailed explanation

follows later in this section. A simple code example implementing the desired behavior is also presented.

Figure 24-10. Interfacing the Application to the TWI in a Typical Transmission

1. The first step in a TWI transmission is to transmit a START condition. This is done by writing a specific

value into TWCR, instructing the TWI hardware to transmit a START condition. Which value to write is

described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to

TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Imme-

diately after the application has cleared TWINT, the TWI will initiate transmission of the START condition.

2. When the START condition has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated

with a status code indicating that the START condition has successfully been sent.

3. The application software should now examine the value of TWSR, to make sure that the START condition

was successfully transmitted. If TWSR indicates otherwise, the application software might take some spe-

cial action, like calling an error routine. Assuming that the status code is as expected, the application must

load SLA+W into TWDR. Remember that TWDR is used both for address and data. After TWDR has been

loaded with the desired SLA+W, a specific value must be written to TWCR, instructing the TWI hardware to

transmit the SLA+W present in TWDR. Which value to write is described later on. However, it is important

that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start

any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared

TWINT, the TWI will initiate transmission of the address packet.

START

SLA+W

Data

STOP

1. Application

writes to TWCR to

initiate

transmission of

START

2. TWINT set.

Status code indicates

START condition sent

4. TWINT set.

Status code indicates

SLA+W sent, ACK

received

6. TWINT set.

Status code indicates

data sent, ACK received

3. Check TWSR to see if START was

sent. Application loads SLA+W into

TWDR, and loads appropriate control

signals into TWCR, makin sure that

TWINT is written to one,

and TWSTA is written to zero.

5. Check TWSR to see if SLA+W was

sent and ACK received.

Application loads data into TWDR, and

loads appropriate control signals into

TWCR, making sure that TWINT is

written to one

7. Check TWSR to see if data was sent

and ACK received.

Application loads appropriate control

signals to send STOP into TWCR,

making sure that TWINT is written to one

TWI bus

Indicates

TWINT set

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