I'm designing a circuit comprised of two boards separated 23 inches away and connected through a ribbon cable which carries an I2C bus (400Khz) and various interrupt digital signals, plus VCC and GND.
Since it's a battery powered device I initially chose a supply of 1.8v (which is the minimum common voltage between the various ICs), but after reading some white papers I realised that such low supply could be way too prone to pick noise for such a long bus. A possible solution is to increase the supply to 2.5v or even 3v, but I would like to be sure since I would loose around a 25% of battery life.
So the question is, is a 1.8v supply too low considering the length of the bus? And if so, would 2.5v be enough?
Best Answer
In I2C, it is typically the "1" that is noise prone due to the RC/pull-up. You can make the channel more robust by increasing the pull-up strength (i.e., reduce R value). You could also place a pull-up on both ends of the line. Another idea might be to run your logic at 1.8V, but have your pull-ups referenced to a higher voltage to give you better noise immunity. Do you need to run at 400kHz? Could you accomplish the same task with a lower clock rate? If you are using ribbon cable, interleaving GND will go a long way to alleviating noise in low-speed digital. It also gives you better impedance control.
What aggressor circuits are nearby? What environment is this going into?