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ATmega8(L)

This scheme is repeated until the last byte has been sent and the transfer is ended by generating a STOP condition or a repeated START condition. A STOP condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

1

X

0

1

X

1

0

X

A REPEATED START condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

1

X

1

0

X

1

0

X

After a repeated START condition (state 0x10) the Two-wire Serial Interface can access the same Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control of the bus..

Table 66. Status codes for Master Transmitter Mode

Status Code

Application Software Response

(TWSR)

Status of the Two-wire Serial

To/from TWDR

To TWCR

Prescaler Bits

Bus and Two-wire Serial Inter-

are 0

face Hardware

STA

STO

TWINT

TWEA

Next Action Taken by TWI Hardware

0x08

A START condition has been

Load SLA+W

0

0

1

X

SLA+W will be transmitted;

transmitted

ACK or NOT ACK will be received

0x10

A repeated START condition

Load SLA+W or

0

0

1

X

SLA+W will be transmitted;

has been transmitted

ACK or NOT ACK will be received

Load SLA+R

0

0

1

X

SLA+R will be transmitted;

Logic will switch to Master Receiver mode

0x18

SLA+W has been transmitted;

Load data byte or

0

0

1

X

Data byte will be transmitted and ACK or NOT ACK will

ACK has been received

No TWDR action or

1

0

1

X

be received

Repeated START will be transmitted

No TWDR action or

0

1

1

X

STOP condition will be transmitted and

No TWDR action

1

1

1

X

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x20

SLA+W has been transmitted;

Load data byte or

0

0

1

X

Data byte will be transmitted and ACK or NOT ACK will

NOT ACK has been received

No TWDR action or

1

0

1

X

be received

Repeated START will be transmitted

No TWDR action or

0

1

1

X

STOP condition will be transmitted and

No TWDR action

1

1

1

X

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x28

Data byte has been transmitted;

Load data byte or

0

0

1

X

Data byte will be transmitted and ACK or NOT ACK will

ACK has been received

X

be received

No TWDR action or

1

0

1

Repeated START will be transmitted

No TWDR action or

0

1

1

X

STOP condition will be transmitted and

1

X

TWSTO Flag will be reset

No TWDR action

1

1

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x30

Data byte has been transmitted;

Load data byte or

0

0

1

X

Data byte will be transmitted and ACK or NOT ACK will

NOT ACK has been received

No TWDR action or

1

0

1

X

be received

Repeated START will be transmitted

No TWDR action or

0

1

1

X

STOP condition will be transmitted and

No TWDR action

1

1

1

X

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x38

Arbitration lost in SLA+W or data

No TWDR action or

0

0

1

X

Two-wire Serial Bus will be released and not addressed

bytes

No TWDR action

1

0

1

X

Slave mode entered

A START condition will be transmitted when the bus be-

comes free

177

2486O–AVR–10/04

Figure 79. Formats and States in the Master Transmitter Mode

MT

Successfull

S

SLA

W

A

DATA

A

P

transmission

to a slave

receiver

$08

$18

$28

Next transfer

RS

SLA

W

started with a

repeated start

condition

$10

Not acknowledge

R

P

received after the

A

slave address

$20

MR

Not acknowledge

received after a data A P byte

$30

Arbitration lost in slave

Other master

Other master

address or data byte

A or A

continues

A or A

continues

$38

$38

Arbitration lost and

A

Other master

addressed as slave

continues

To corresponding

$68

$78

$B0

states in slave mode

From master to slave

DATA

A

Any number of data bytes

and their associated acknowledge bits

From slave to master

n

This number (contained in TWSR) corresponds

to a defined state of the Two-Wire Serial Bus. The

prescaler bits are zero or masked to zero

178 ATmega8(L)

2486O–AVR–10/04

ATmega8(L)

Master Receiver Mode

In the Master Receiver mode, a number of data bytes are received from a Slave Trans-

mitter (see Figure 80). In order to enter a Master mode, a START condition must be

transmitted. The format of the following address packet determines whether Master

Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT

mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes

mentioned in this section assume that the prescaler bits are zero or are masked to zero.

Figure 80. Data Transfer in Master Receiver Mode

VCC

Device 1

Device 2

Device 3

........

Device n

R1

R2

MASTER

SLAVE

RECEIVER

TRANSMITTER

SDA

SCL

A START condition is sent by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

1

X

1

0

X

1

0

X

TWEN must be written to one to enable the Two-wire Serial Interface, TWSTA must be written to one to transmit a START condition and TWINT must be set to clear the TWINT Flag. The TWI will then test the Two-wire Serial Bus and generate a START condition as soon as the bus becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (See Table 66). In order to enter MR mode, SLA+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter the TWINT bit should be cleared (by writing it to one) to continue the transfer. This is accomplished by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

1

X

0

0

X

1

0

X

When SLA+R have been transmitted and an acknowledgement bit has been received, TWINT is set again and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x38, 0x40, or 0x48. The appropriate action to be taken for each of these status codes is detailed in Table 67. Received data can be read from the TWDR Register when the TWINT Flag is set high by hardware. This scheme is repeated until the last byte has been received. After the last byte has been received, the MR should inform the ST by sending a NACK after the last received data byte. The transfer is ended by generating a STOP condition or a repeated START condition. A STOP condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

1

X

0

1

X

1

0

X

A REPEATED START condition is generated by writing the following value to TWCR:

TWCR

TWINT

TWEA

TWSTA

TWSTO

TWWC

TWEN

–

TWIE

value

1

X

1

0

X

1

0

X

179

2486O–AVR–10/04

After a repeated START condition (state 0x10) the Two-wire Serial Interface can access

the same Slave again, or a new Slave without transmitting a STOP condition. Repeated

START enables the Master to switch between Slaves, Master Transmitter mode and

Master Receiver mode without losing control over the bus.

Table 67. Status codes for Master Receiver Mode

Status Code

Application Software Response

(TWSR)

Status of the Two-wire Serial

To TWCR

Prescaler Bits

Bus and Two-wire Serial Inter-

To/from TWDR

are 0

face Hardware

STA

STO

TWINT

TWEA

Next Action Taken by TWI Hardware

0x08

A START condition has been

Load SLA+R

0

0

1

X

SLA+R will be transmitted

transmitted

ACK or NOT ACK will be received

0x10

A repeated START condition

Load SLA+R or

0

0

1

X

SLA+R will be transmitted

has been transmitted

Load SLA+W

0

0

1

X

ACK or NOT ACK will be received

SLA+W will be transmitted

Logic will switch to Master Transmitter mode

0x38

Arbitration lost in SLA+R or NOT

No TWDR action or

0

0

1

X

Two-wire Serial Bus will be released and not addressed

ACK bit

No TWDR action

1

0

1

X

Slave mode will be entered

A START condition will be transmitted when the bus

becomes free

0x40

SLA+R has been transmitted;

No TWDR action or

0

0

1

0

Data byte will be received and NOT ACK will be

ACK has been received

No TWDR action

0

0

1

1

returned

Data byte will be received and ACK will be returned

0x48

SLA+R has been transmitted;

No TWDR action or

1

0

1

X

Repeated START will be transmitted

NOT ACK has been received

No TWDR action or

0

1

1

X

STOP condition will be transmitted and TWSTO Flag will

No TWDR action

1

1

1

X

be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x50

Data byte has been received;

Read data byte or

0

0

1

0

Data byte will be received and NOT ACK will be

ACK has been returned

Read data byte

0

0

1

1

returned

Data byte will be received and ACK will be returned

0x58

Data byte has been received;

Read data byte or

1

0

1

X

Repeated START will be transmitted

NOT ACK has been returned

Read data byte or

0

1

1

X

STOP condition will be transmitted and TWSTO Flag will

1

1

1

X

be reset

Read data byte

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

180 ATmega8(L)

2486O–AVR–10/04

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