ATmega328P datasheet(224/448 Pages) ATMEL

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224

8161D–AVR–10/09

ATmega48PA/88PA/168PA/328P

Figure 21-10. 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 success-

fully 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 otherwise, the applica-

tion 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 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.

When the address packet has been transmitted, the TWINT Flag in TWCR is set, and

TWSR is updated with a status code indicating that the address packet has successfully

been sent. The status code will also reflect whether a Slave acknowledged the packet or

not.

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

address packet was successfully transmitted, and that the value of the ACK bit was as

expected. 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 must load a data packet into TWDR. Subsequently, a specific value must be

written to TWCR, instructing the TWI hardware to transmit the data packet 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

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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