ATmega16 datasheet(177/315 Pages) ATMEL

177 / 315 page

177

ATmega16(L)

2466E–AVR–10/02

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 applica-

tion 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 85 is a simple example of how the application can interface to the TWI hardware.

In this example, a master wishes to transmit a single data byte to a slave. This descrip-

tion is quite abstract, a more detailed explanation follows later in this section. A simple

code example implementing the desired behaviour is also presented.

Figure 85. Interfacing the Application to the TWI in a Typical Transmission

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. Immediately after the application has cleared TWINT, the TWI

will initiate transmission of the START condition.

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.

The application software should now examine the value of TWSR, to make sure

that the START condition was successfully transmitted. If TWSR indicates other-

wise, the application software might take some special 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

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, making sure that TWINT is

written to one

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

Application

Action

TWI

Hardware

Action

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