i2cmulti-master
On a multimaster i2c bus, how to determine which device started the transfer?
update (from comments made below by the O.P.) :
I am trying to make sense of http://www.i2c-bus.org/serial-resistances-and-debugging/ where it says:
it is possible to recognize which I2C device is currently active by analyzing the low level on the I2C lines
What does that mean?
Three. Electrically the master controls the bus in exactly the same way as the slaves.
I'm not sure where your formula comes from. The spec states that pull-up resistors are to be chosen in function of bus capacitance and speed(page 55):
Start by hooking up the scope to your MCU's 5V power. First look in DC coupling, then look in AC coupling for the real fun view. For the mostestest (tm) fun, also look at the incoming "12V" at least once.
Motorvehicles are extremely noisy things, the power rail can contain noise many times more than a simple linear regulator and a few caps will solve. In fact the spikes in a motorcycle or car can go so high (or low) that they may break a simple 7805 if it's not protected by a MOV, a filter and reverse protection diode. (The diode will also act as a sort of filter with a 100uF or higher cap).
I2C is very sensitive because of its pull up resistors and a reasonably high capacity in the breadboard as well that couples in noise too.
Also: Vibrating contacts on a running motorcycle. Solderless + vibrations = not good.
Best Answer
I2C spec itself doesn't have provisions for "from" address, AFAIK.
You would have program the masters to transmit their IDs in the payload part of the I2C packet.