How to handle voltage mismatch on I2C/TWI bus as battery voltage drops?

[Stefan]

I need to connect an I2C/TWI device to the nRF51822. The I2C device is running at a fixed 3.3V (from a boost DC/DC), but the nRF51822 is powered directly from a CR2032 battery (starting at around 3.0V).

As the battery voltage drops to 2V, now the I2C bus is running at 3.3V but VDD for the nRF51822 is 2V. I know technically the input should never be more than VDD, but does the open collector of the I2C bus give a bit of leeway here? This is my first battery-powered design and it seems like a pretty typical situation - how are these I2C bus voltage mismatches typically handled?

I know I could run both devices off the same VDD but I really don't want to power the nRF51822 using the 3.3V DC/DC because I only need to turn it on occasionally.

[John Kelley]

If it's I2C, why don't you pull the lines high on the nRF51822 side instead of the I2C side? You can use the internal pull-ups of the TWI block for this (enable them only when needing to talk I2C) and 1.8v is still above logic-high for a 3v3 I2C part.

[Stefan]

I believe the pull-ups on the nRF51822 will actually pull the bus up to VDD (which can be from 3V to 2V when running from the battery), but I'll have to check the datasheet of the I2C part to see if it will accept the lower voltage. I think it should work, though, but it's equivalent to using external pullups and running the I2C bus off the battery instead of the regulated 3.3V, right? Saves some resistors, though :)