Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Ficha De Dados
Códigos do produto
MEGA328P-XMINI
211
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
Figure 22-6.
Typical Data Transmission
22.4
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:
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:
An algorithm must be implemented allowing only one of the masters to complete the transmission. All
other masters should cease transmission when they discover that they have lost the selection process.
This selection process is called arbitration. When a contending master discovers that it has lost the
arbitration process, it should immediately switch to Slave mode to check whether it is being addressed by
the winning master. The fact that multiple masters have started transmission at the same time should not
be detectable to the slaves, i.e. the data being transferred on the bus must not be corrupted.
other masters should cease transmission when they discover that they have lost the selection process.
This selection process is called arbitration. When a contending master discovers that it has lost the
arbitration process, it should immediately switch to Slave mode to check whether it is being addressed by
the winning master. The fact that multiple masters have started transmission at the same time should not
be detectable to the slaves, i.e. the data being transferred on the bus must not be corrupted.
Different masters may use different SCL frequencies. A scheme must be devised to synchronize the serial
clocks from all masters, in order to let the transmission proceed in a lockstep fashion. This will facilitate
the arbitration process.
clocks from all masters, in order to let the transmission proceed in a lockstep fashion. This will facilitate
the arbitration process.
The wired-ANDing of the bus lines is used to solve both these problems. The serial clocks from all masters will
be wired-ANDed, yielding a combined clock with a high period equal to the one from the Master with the
shortest high period. The low period of the combined clock is equal to the low period of the Master with the
longest low period. Note that all masters listen to the SCL line, effectively starting to count their SCL high and
low time-out periods when the combined SCL line goes high or low, respectively.
be wired-ANDed, yielding a combined clock with a high period equal to the one from the Master with the
shortest high period. The low period of the combined clock is equal to the low period of the Master with the
longest low period. Note that all masters listen to the SCL line, effectively starting to count their SCL high and
low time-out periods when the combined SCL line goes high or low, respectively.
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