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XMEGA B [DATASHEET]
8291B–AVR–01/2013
19.5.1.4 Case M4: Address packet transmit complete - Direction bit set
If the master receives an ACK from the slave, the master proceeds to receive the next byte of data from the slave. When
the first data byte is received, the master read interrupt flag is set and the master received acknowledge flag is cleared.
The clock hold is active at this point, preventing further activity on the bus.
19.5.2 Transmitting Data Packets
Assuming case M3 above, the master can start transmitting data by writing to the master data register. If the transfer was
successful, the slave will signal with ACK. The master write interrupt flag is set, the master received acknowledge flag is
cleared, and the master can prepare new data to send. During data transfer, the master is continuously monitoring the
bus for collisions.
The received acknowledge flag must be checked by software for each data packet transmitted before the next data
packet can be transferred. The master is not allowed to continue transmitting data if the slave signals a NACK.
If a collision is detected and the master loses arbitration during transfer, the arbitration lost flag is set.
19.5.3 Receiving Data Packets
Assuming case M4 above, the master has already received one byte from the slave. The master read interrupt flag is set,
and the master must prepare to receive new data. The master must respond to each byte with ACK or NACK. Indicating
a NACK might not be successfully executed, as arbitration can be lost during the transmission. If a collision is detected,
the master loses arbitration and the arbitration lost flag is set.
19.6
TWI Slave Operation
The TWI slave is byte-oriented with optional interrupts after each byte. There are separate slave data and address/stop
interrupts. Interrupt flags can also be used for polled operation. There are dedicated status flags for indicating
ACK/NACK received, clock hold, collision, bus error, and read/write direction.
When an interrupt flag is set, the SCL line is forced low. This will give the slave time to respond or handle data, and will in
most cases require software interaction.
. shows the TWI slave operation. The diamond shapes symbols
(SW) indicate where software interaction is required.
Figure 19-13.TWI slave operation.
The number of interrupts generated is kept to a minimum by automatic handling of most conditions. Quick command can
be enabled to auto-trigger operations and reduce software complexity.
Promiscuous mode can be enabled to allow the slave to respond to all received addresses.
S
S3
ADDRESS
S2
A
S1
R
W
DATA
A/A
DATA
P
S2
Sr
S3
P
S2
Sr
S3
SLAVE ADDRESS INTERRUPT
SLAVE DATA INTERRUPT
A
Collision
(SMBus)
SW
SW
SW
SW
A/A
A/A
SW
Release
Hold
S1
A
S1
SW
Interrupt on STOP
Condition Enabled
S1
SW
Driver software
The master provides data
on the bus
Slave provides data on
the bus
Sn
Diagram connections
Summary of Contents for XMEGA B
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Page 322: ...322 XMEGA B DATASHEET 8291B AVR 01 2013 Table 25 14 16 segments Character Table...
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