Datasheet

23.7.1. Master Transmitter Mode
In the Master Transmitter (MT) mode, a number of data bytes are transmitted to a Slave Receiver, see
figure below. In order to enter a Master mode, a START condition must be transmitted. The format of the
following address packet determines whether MT or Master Receiver (MR) 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 23-11. Data Transfer in Master Transmitter Mode
Device 1
MASTER
TRANSMITTER
Device 2
SLA VE
RECEIVER
Device 3
Device n
SD A
SCL
........
R1 R2
V
CC
A START condition is sent by writing a value to the TWI Control Register n (TWCRn) of the type
TWCRn=1x10x10x:
• The TWI Enable bit (TWCRn.TWEN) must be written to '1' to enable the 2-wire Serial Interface
• The TWI Start Condition bit (TWCRn.TWSTA) must be written to '1' to transmit a START condition
• The TWI Interrupt Flag (TWCRn.TWINT) must be written to '1' to clear the flag.
The TWI n 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 MT mode, SLA
+W must be transmitted. This is done by writing SLA+W to the TWI Data Register (TWDRn). Thereafter,
the TWCRn.TWINT Flag should be cleared (by writing a '1' to it) to continue the transfer. This is
accomplished by writing a value to TWRC of the type TWCR=1x00x10x.
When SLA+W have been transmitted and an acknowledgment 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 the Status
Code table below.
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 TWCRn Register. After updating TWDRn,
the TWINT bit should be cleared (by writing '1' to it) to continue the transfer. This is accomplished by
writing again a value to TWCRn of the type TWCRn=1x00x10x.
This scheme is repeated until the last byte has been sent and the transfer is ended, either by generating
a STOP condition or a by a repeated START condition. A repeated START condition is accomplished by
writing a regular START value TWCRn=1x10x10x. A STOP condition is generated by writing a value of
the type 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
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
270