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ATmega128
Master Receiver Mode
In the Master Receiver Mode, a number of data bytes are received from a slave transmitter (see
Figure 98 ). 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 98. Data Transfer in Master Receiver Mode
V CC
Device 1
MASTER
Device 2
SLAVE
Device 3
........
Device n
R1
R2
RECEIVER
TRANSMITTER
SDA
SCL
A START condition is sent by writing the following value to T W CR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
1
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
T W EN must be written to one to enable the Two-wire Serial Interface, T W STA must be written to
one to transmit a START condition and T W INT must be set to clear the T W INT flag. The T W I 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 T W INT flag is set by hardware, and the
status code in T W SR will be $08 (See Table 88 ). In order to enter MR mode, SLA+R must be
transmitted. This is done by writing SLA+R to T W DR. Thereafter the T W INT bit should be
cleared (by writing it to one) to continue the transfer. This is accomplished by writing the follow-
ing value to T W CR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
W hen SLA+R have been transmitted and an acknowledgment bit has been received, T W INT is
set again and a number of status codes in T W SR are possible. Possible status codes in Master
mode are $38, $40, or $48. The appropriate action to be taken for each of these status codes is
detailed in Table 97 . Received data can be read from the T W DR Register when the T W INT 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 T W CR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
0
TWSTO
1
TWWC
X
TWEN
1
–
0
TWIE
X
A REPEATED START condition is generated by writing the following value to T W CR:
TWCR
value
TWINT
1
TWEA
X
TWSTA
1
TWSTO
0
TWWC
X
TWEN
1
–
0
TWIE
X
After a repeated START condition (state $10) the Two-wire Serial Interface can access the
same slave again, or a new slave without transmitting a STOP condition. Repeated START
216
2467X–AVR–06/11
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