This is a short question but a seemingly important one.
Is it possible for a multi-master i2c bus to have two masters send the start byte at almost exactly the same time?
If so, what would happen?
Electronic – i2c multiple master collision
i2c
Related Solutions
I2C Master is decided by who owns the bus meaning who provides the clock, in the give figure below you can see that 2 i2c masters try to access the slave device for information. Typical lythe slave could be an eeprom.
When Master1 needs the bus it polls the bus whether it is free or not (if the line is low than the I2C bus is taken by someone) if the line is high it pulls down and sends across the address (7 bit and 8 bit being R/W) each of the I2C device connected to the bus samples the same and responds with an ack if it is being addressed by pulling it low. once after the required communication the master releases the bus. An App note from renesas Microcontroller explaining multi master mode I2C
The STOP is an indication to the slave device that communications are complete. It's ingrained in the protocol and spec-compliant slave devices expect to see it, so you simply can't wish it away.
For instance, if you're doing a page-write to a 24AA04 serial EEPROM (up to 16 bytes of data), the EEPROM will stop buffering data and start writing to flash upon receipt of the STOP from the master. You're not obligated to send a full page, so you need the STOP as a clear indication that there are no more bytes coming. (Some EEPROMs will throw away pending transactions if a STOP is not received at an expected/appropriate time, which can lead to some head-scratching debugging problems.)
Some devices also use the STOP to enter a reduced-power-consumption mode, drawing less power until the next START wakes them up.
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Best Answer
It is perfectly possible for two masters to send a start bit at exactly the same time, where with exactly I mean that the difference between the start bits falling fronts is smaller than the smaller measurable difference by the masters.
That's the point: when a master is writing on the i2c bus it is also monitoring it, thus it is possible to determine if a collision is occurring.
Let's say that two masters, A and B, connected to the same bus, want to address two different devices, one with address 0xAE (from A), the other with address 0xA6 (from B). The masters send the start bit at almost the same time, then begin sending the address. Since 0xA = 0b1010 they send the sequence 1010, so far so good, but 0xE = 0b1110, while 0x6 = 0b0110. The fifth bit differs, and A tries to send a 1, while B sends a 0.
Who wins? B of course, because i2c is an open collector bus, lines can only be tied low. To send a 1 you just turn off the output and an external pull up resistor ties the line high, while to send a 0 you turn on a pull down. Master A then is sending a 1, but measuring a 0. Big red flag, bus collision! Master A state machine should properly handle the situation, leaving the bus to B.
Please note that from B point of view nothing has happened, and it continues happily with the frame. That is very good because no time is lost and B can do whatever it needs to do.
While it seems that an open collector bus is the solution to all the problems, unfortunately it is not so. The bus can easily be blocked by any malfunctioning device that decides to hold SCL or SDA low. In such situations it is very difficult to reset the bus, sometimes devices have a reset pin, sometimes you have to power cycle them, sometimes you have to think of something even different. Had to do all the three in different applications.
As a side note be aware that not all masters implement the full spec, some masters do not implement bus collision detection and this can be very bad. In the previous example, if nor A nor B can detect loss of arbitration the address sent on the bus is 0b10100110, i.e. B slave address, but when the ACK comes A thinks it comes from its slave, and after that things can get very bad.
There also is the possibility that A and B wants the same slave. If this happens the ACK is received, and then many things can happen.
Usually i2c devices have many registers that can be written and/or read, the communication frame then looks something like:
(Omitting start and stop bits and WR bit).
The masters start sending the register address. If they want to access a different register it just happens what would happen for the slave address: at a certain point one of them detects the loss of arbitration and gives up.
If the masters want to access the same register, or the i2c device has a default behaviour like "if I detect my address I'll tell you the temperature", then... Everything is perfectly fine! Both masters wanted exactly the same thing, and they are getting it, and this is OK per the standard.
Two communications in only one frame, take that Nyquist!