AT89LP6440 datasheet(113/194 Pages) ATMEL

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113

3706A–MICRO–9/09

AT89LP6440 - Preliminary

Figure 18-10. 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

TWIF bit is cleared in the value written. The TWI will not start any operation as long as

the TWIF bit in TWCR is set. Immediately after the application has cleared TWIF, the

TWI will initiate transmission of the START condition.

When the START condition has been transmitted, the TWIF 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 appli-

cation 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 TWIF bit is cleared in the value writ-

ten. The TWI will not start any operation as long as the TWIF bit in TWCR is set.

Immediately after the application has cleared TWIF, the TWI will initiate transmission of

the address packet.

When the address packet has been transmitted, the TWIF flag in TWCR is set, and

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

fully 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 spe-

cial 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 TWIF bit is cleared in the value written. The TWI will not start any operation as

START

SLA+W

Data

STOP

1. Application writes

to TWCR to initiate

transmission of

START

2. TWIF set.

Status code indicates

START condition sent

4. TWIF set.

Status code indicates

SLA+W sent, ACK

received

6. TWIF 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

TWIF is written to zero and

STA 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

TWIF is written to zero.

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

TWIF is written to zero.

TWI bus

Indicates

TWIF set

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