Datasheet

Figure 23-13. Data Transfer in Master Receiver Mode
Device 1
MASTER
RECEIVER
Device 2
SLA VE
TRANSMITTER
Device 3
Device n
SD A
SCL
........
R1 R2
V
CC
A START condition is sent by writing to the TWI Control register (TWCRn) a value of the type
TWCRn=1x10x10x:
• TWCRn.TWEN must be written to '1' to enable the 2-wire Serial Interface
• TWCRn.TWSTA must be written to '1' to transmit a START condition
• TWCRn.TWINT must be cleared by writing a '1' to it.
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 TWSRn will be 0x08 (see Status Code table below). In order to enter MR mode, SLA
+R must be transmitted. This is done by writing SLA+R to TWDR. Thereafter, the TWINT flag should be
cleared (by writing '1' to it) to continue the transfer. This is accomplished by writing the a value to TWCRn
of the type TWCRn=1x00x10x.
When SLA+R have been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSRn 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 the table
below. 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 repeated START condition is
sent by writing to the TWI Control register (TWCRn) a value of the type TWCRn=1x10x10x again. A
STOP condition is generated by writing TWCRn=1x01x10x:
After a repeated START condition (status code 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.
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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