Rainbow Electronics ATmega128RFA1 - Manual (Page 381)

381
8266A-MCU Wireless-12/09
ATmega128RFA1
• 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.
• 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.
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.
Figure 25-7. SCL Synchronization Between Multiple Masters
TA
low
TA
high
SCL from
Master A
SCL from
Master B
SCL Bus
Line
TB
low
TB
high
Masters Start
Counting Low Period
Masters Start
Counting High Period
Arbitration is carried out by all masters continuously monitoring the SDA line after
outputting data. If the value read from the SDA line does not match the value the
Master had output, it has lost the arbitration. Note that a Master can only lose arbitration
when it outputs a high SDA value while another Master outputs a low value. The losing
Master should immediately go to Slave mode, checking if it is being addressed by the
winning Master. The SDA line should be left high, but losing masters are allowed to
generate a clock signal until the end of the current data or address packet. Arbitration
will continue until only one Master remains, and this may take many bits. If several
masters are trying to address the same Slave, arbitration will continue into the data
packet.
Note that arbitration is not allowed between:
• A REPEATED START condition and a data bit.
• A STOP condition and a data bit.
• A REPEATED START and a STOP condition.
It is the user software’s responsibility to ensure that these illegal arbitration conditions
never occur. This implies that in multi-master systems, all data transfers must use the
same composition of SLA+R/W and data packets. In other words: All transmissions