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21.7 Transmission Modes

The TWI can operate in one of four major modes. These are named Master Transmitter (MT),

Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these

modes can be used in the same application. As an example, the TWI can use MT mode to write

data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters

are present in the system, some of these might transmit data to the TWI, and then SR mode

would be used. It is the application software that decides which modes are legal.

The following sections describe each of these modes. Possible status codes are described

along with figures detailing data transmission in each of the modes. These figures contain the

following abbreviations:

S: START condition

Rs: REPEATED START condition

R: Read bit (high level at SDA)

W: Write bit (low level at SDA)

A: Acknowledge bit (low level at SDA)

: Not acknowledge bit (high level at SDA)

Data: 8-bit data byte

P: STOP condition

SLA: Slave Address

In Figure 21-12 to Figure 21-18, circles are used to indicate that the TWINT Flag is set. The

numbers in the circles show the status code held in TWSR, with the prescaler bits masked to

zero. At these points, actions must be taken by the application to continue or complete the TWI

transfer. The TWI transfer is suspended until the TWINT Flag is cleared by software.

When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate soft-

ware action. For each status code, the required software action and details of the following serial

transfer are given in Table 21-2 to Table 21-5. Note that the prescaler bits are masked to zero in

these tables.

21.7.1 Master Transmitter Mode

In the Master Transmitter mode, a number of data bytes are transmitted to a Slave Receiver

(see Figure 21-11). 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 trans-

mitted, MR mode is entered. All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

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Figure 21-11. Data Transfer in Master Transmitter Mode

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

TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to trans-

mit a START condition and TWINT must be written to one to clear the TWINT Flag. The TWI will

then test the 2-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 21-2). In order to enter MT mode, SLA+W must be

transmitted. This is done by writing SLA+W to TWDR. Thereafter the TWINT bit should be

cleared (by writing it to one) to continue the transfer. This is accomplished by writing the follow-

ing value to TWCR:

When SLA+W 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 0x18, 0x20, or 0x38. The appropriate action to be taken for each of these status codes

is detailed in Table 21-2.

When SLA+W has been successfully transmitted, a data packet should be transmitted. This is

done by writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not,

the access will be discarded, and the Write Collision bit (TWWC) will be set in the TWCR Regis-

ter. After updating TWDR, 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:

This scheme is repeated until the last byte has been sent and the transfer is ended by generat-

ing a STOP condition or a repeated START condition. A STOP condition is generated by writing

the following value to TWCR:

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

TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE

value 1

X10X10 X

TWCR TWINT

TWEA TWSTA TWSTO TWWC TWEN – TWIE

value 1

X00X10 X

TWCR TWINT

TWEA TWSTA TWSTO TWWC TWEN – TWIE

value 1

X00X10 X

TWCR TWINT

TWEA TWSTA TWSTO TWWC TWEN – TWIE

value 1

X01X10 X

TWCR TWINT

TWEA TWSTA TWSTO TWWC TWEN – TWIE

value 1

X10X10 X

Device 1

MASTER

TRANSMITTER

Device 2

SLAVE

RECEIVER

Device 3

Device n

SDA

SCL

........

R1 R2

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After a repeated START condition (state 0x10) the 2-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 with-

out losing control of the bus.

Table 21-2. Status codes for Master Transmitter Mode

Status Code

(TWSR)

Prescaler Bits

are 0

Status of the 2-wire Serial Bus

and 2-wire Serial Interface

Hardware

Application Software Response

Next Action Taken by TWI Hardware

To/from TWDR To TWCR

STA STO TWIN

TWE

0x08 A START condition has been

transmitted

Load SLA+W 0 0 1 X SLA+W will be transmitted;

ACK or NOT ACK will be received

0x10 A repeated START condition

has been transmitted

Load SLA+W or

Load SLA+R

SLA+W will be transmitted;

ACK or NOT ACK will be received

SLA+R will be transmitted;

Logic will switch to Master Receiver mode

0x18 SLA+W has been transmitted;

ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

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;

NOT ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

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 transmit-

ted;

ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

TWSTO Flag will be reset

STOP condition followed by a START condition will be

transmitted and TWSTO Flag will be reset

0x30 Data byte has been transmit-

ted;

NOT ACK has been received

Load data byte or

No TWDR action or

No TWDR action

Data byte will be transmitted and ACK or NOT ACK will

be received

Repeated START will be transmitted

STOP condition will be transmitted and

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 bytes

No TWDR action or

No TWDR action

2-wire Serial Bus will be released and not addressed

Slave mode entered

A START condition will be transmitted when the bus

becomes free

Part #	ATMEGA48-20AU
Description	MCU 8BIT ATMEGA RISC 4KB FLASH 3.3V/5V 32TQFP - Trays
Category	IC
Availability	Out of Stock
Qty	0

ATMEGA48-20AU

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