I2C 3.3v to 5v

series resistors between I/O pins and bus lines) depend on worst case values of the supply voltages and logic levels, the length and load of the bus lines, and rise and fall time requirements. These have to be calculated for each bus system separately. Their values must be chosen in such a way that there is sufficient noise margin at both HIGH and LOW levels.

See original: Del.icio.us I2C 3.3v to 5v