Content originally posted in LPCWare by bob_walker on Wed Jan 20 12:28:35 MST 2016
I believe the 'fully spec compliant' note refers to the 5V capable pins. Initially, the I2C spec was for 5V open drain buses. Nowadays, we are using lower voltage (2V5, 3V3) processors. The issue is with the protection diodes on the I/O port lines - they will be clamped to GND and the VCC of the chip to provide under/over-voltage limiting. That means that, for a chip running off of 3.3V and utilizing a 5V I2C bus, I/O's without the clamp diodes (fully spec compliant),will swing the full 5V. The non compliant drivers will clamp the I2C bus to 3.3V+Vfd, limiting the 5V swing. I'm *guessing* that the fully spec compliant I/Os are truly open drain devices with no protection diodes. So you're safe using I2C on the non-compliant buses. However, do not exceed your micro's VCC. I use 3.3V I2C buses all over the place....no problems.