AT90USB646 datasheet(229/434 Pages) ATMEL

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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 20-11 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 imple-

menting the desired behavior is also presented.

Figure 20-11. 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

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 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 success-

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

packet or not.

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

AT90USB646 Datasheet(PDF) 229 Page - ATMEL Corporation

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