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

In the Master Receiver mode, a number of data bytes are received from a Slave Transmitter (Slave see 

). 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.

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

TWEN must be written to one to enable the 2-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 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 

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:

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 

. 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:

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 over the bus. 

value

1

X

1

0

X

1

0

X

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0

0

X

1

0

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1

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0

1

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1

0

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1

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Device 1

MASTER

RECEIVER

Device 2

SLAVE

TRANSMITTER

Device 3

Device n

SDA

SCL

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R1

R2

V

CC