Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
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8266A-MCU Wireless-12/09
ATmega128RFA1
pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the SDA line
high, a NACK is signaled. When the Receiver has received the last byte, or for some
reason cannot receive any more bytes, it should inform the Transmitter by sending a
NACK after the final byte. The MSB of the data byte is transmitted first.
Figure 25-5. Data Packet Format
1
2
7
8
9
Data MSB
Data LSB
ACK
Aggregate
SDA
SDA from
Transmitter
SDA from
Receiver
SCL from
Master
SLA+R/W
Data Byte
STOP, REPEATED
START or Next
Data Byte
25.3.5 Combining Address and Data Packets into a Transmission
A transmission basically consists of a START condition, a SLA+R/W, one or more data
packets and a STOP condition. An empty message, consisting of a START followed by
a STOP condition, is illegal. Note that the Wired-ANDing of the SCL line can be used to
implement handshaking between the Master and the Slave. The Slave can extend the
SCL low period by pulling the SCL line low. This is useful if the clock speed set up by
the Master is too fast for the Slave, or the Slave needs extra time for processing
between the data transmissions. The Slave extending the SCL low period will not affect
the SCL high period, which is determined by the Master. As a consequence, the Slave
can reduce the TWI data transfer speed by prolonging the SCL duty cycle.
shows a typical data transmission. Note that several data bytes can
be transmitted between the SLA+R/W and the STOP condition, depending on the
software protocol implemented by the application software.
Figure 25-6. Typical Data Transmission
1
2
7
8
9
Data Byte
Data MSB
Data LSB
ACK
SDA
SCL
START
1
2
7
8
9
Addr MSB
Addr LSB
R/W
ACK
SLA+R/W
STOP
25.4 Multi-master Bus Systems, Arbitration and Synchronization
The TWI protocol allows bus systems with several masters. Special concerns have
been taken in order to ensure that transmissions will proceed as normal, even if two or
more masters initiate a transmission at the same time. Two problems arise in multi-
master systems: