Datasheet

213
2490R–AVR–02/2013
ATmega64(L)
Master Transmitter
Mode
In the Master Transmitter mode, a number of data bytes are transmitted to a slave receiver (see
Figure 96). 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.
Figure 96. 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 Two-wire Serial Interface, TWSTA must be written to one to
transmit a START condition and TWINT must be written to one to clear the TWINT flag. The TWI
will then test the Two-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 hard-
ware, and the status code in TWSR will be 0x08 (see Table 88). 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 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 Table 88.
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:
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
Device 1
MASTER
TRANSMITTER
Device 2
SLAVE
RECEIVER
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC