Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Data Sheet
Product codes
MEGA328P-XMINI
210
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
Figure 22-4.
Address Packet Format
22.3.4 Data Packet Format
All data packets transmitted on the TWI bus are nine bits long, consisting of one data byte and an acknowledge
bit. During a data transfer, the Master generates the clock and the START and STOP conditions, while the
Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is signalled by the Receiver
pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the SDA line high, a NACK is
signalled. 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.
bit. During a data transfer, the Master generates the clock and the START and STOP conditions, while the
Receiver is responsible for acknowledging the reception. An Acknowledge (ACK) is signalled by the Receiver
pulling the SDA line low during the ninth SCL cycle. If the Receiver leaves the SDA line high, a NACK is
signalled. 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 22-5.
Data Packet Format
22.3.5 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.
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.
software.
SDA
SCL
START
1
2
7
8
9
Addr MSB
Addr LSB
R/W
ACK
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