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:
- 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.
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 must contain the same number of data packets, otherwise the result of the arbitration is undefined.

Ale jak ktoś lubi sobie dodawać roboty to można nawet zrobić przekazywanie sobie czasu między procesorami po WiFi

Najbardziej optymalnym rozwiązaniem w tym przypadku jest multimaster na I2C.

No chyba że faktycznie te procki za sobą gadają bo coś tam... np. po SPI. No to jeden drugiemu mógłby podrzucić od czasu do czasu również aktualny czas.

A kol. sauron80 rzucił hasło i zniknął

2. Na jednym procesorze ustawię przerwanie na zbocze narastające, a na drugim opadające.

Dwa komputery w jednej obudowie będą ponieważ pomiar temperatury za pomocą 18B20 wymaga długich przestojów

W czasie konwersji temperatury możesz wykonywać inne operacje i odczytać temperaturę dopiero jak będzie na to pora.

Nie mówcie teraz, że da się podłączyć dwa wyświetlacze alfanumeryczne do jednego procka

Dwa komputery w jednej obudowie będą ponieważ pomiar temperatury za pomocą 18B20 wymaga długich przestojów

5. sterownie PWM diodami 4 kolorowymi RGB + biały

6. wyświetlenie godziny na 4 wyświetlaczach 7-segmentowych z multipleksowaniem

6. wyświetlenie godziny na 4 wyświetlaczach 7-segmentowych z multipleksowaniem (opcja, jeżeli uda się kupić wyświetlacz z maskownicą (zielony).
7. Dwa LCD 2x16 lub 1 LCD 2x16 + wyświetlacz 7-segmentowy