Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet

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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

Atmel-8271H-AVR- ATmega-Datasheet_08/2014

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 

). 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 
following 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 

. 

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 Register. 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 generating 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:

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 without losing control of the bus. 

 

value

1

X

0

0

X

1

0

X

value

1

X

0

0

X

1

0

X

value

1

X

0

1

X

1

0

X

value

1

X

1

0

X

1

0

X

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

T

TWE

A

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

0

0

0

0

1

1

X

X

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 or

No TWDR action

0

1
0

1

0

0
1

1

1

1
1

1

X

X
X

X

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 or

No TWDR action

0

1
0

1

0

0
1

1

1

1
1

1

X

X
X

X

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