Техническая Спецификация для Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

Atmel-8271H-AVR- ATmega-Datasheet_08/2014

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After a STOP or REPEATED START has been received while still addressed as a Slave.

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When a bus error has occurred due to an illegal START or STOP condition.

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.

 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 description is quite abstract, a more detailed 
explanation follows later in this section. A simple code example implementing the desired behavior is also 
presented.

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

START

SLA+W

A

Data

A

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

Application

Action

TWI

Hardw

are

Action