251
XMEGA B [DATASHEET]
8291B–AVR–01/2013
[2]
The BAUD register should be written only while the master is disabled.
19.9.6 ADDR
–
Address register
When the address (ADDR) register is written with a slave address and the R/W bit while the bus is idle, a START
condition is issued and the 7-bit slave address and the R/W bit are transmitted on the bus. If the bus is already owned
when ADDR is written, a repeated START is issued. If the previous transaction was a master read and no acknowledge
is sent yet, the acknowledge action is sent before the repeated START condition.
After completing the operation and the acknowledge bit from the slave is received, the SCL line is forced low if arbitration
was not lost. WIF is set.
If the bus state is unknown when ADDR is written, WIF is set and BUSERR is set.
All TWI master flags are automatically cleared when ADDR is written. This includes BUSERR, ARBLOST, RIF, and WIF.
The master ADDR can be read at any time without interfering with ongoing bus activity.
19.9.7 DATA
–
Data register
The data (DATA) register is used when transmitting and receiving data. During data transfer, data are shifted from/to the
DATA register and to/from the bus. This implies that the DATA register cannot be accessed during byte transfers, and
this is prevented by hardware. The DATA register can only be accessed when the SCL line is held low by the master; i.e.,
when CLKHOLD is set.
In master write mode, writing the DATA register will trigger a data byte transfer followed by the master receiving the
acknowledge bit from the slave. WIF and CLKHOLD are set.
In master read mode, RIF and CLKHOLD are set when one byte is received in the DATA register. If smart mode is
enabled, reading the DATA register will trigger the bus operation as set by the ACKACT bit. If a bus error occurs during
reception, WIF and BUSERR are set instead of RIF.
Accessing the DATA register will clear the master interrupt flags and CLKHOLD.
BAUD
f
sys
2
f
TWI
----------------
5
–
=
Bit
7
6
5
4
3
2
1
0
ADDR[7:0]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
DATA[7:0]
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Содержание XMEGA B
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