using nRF51822 with 5V I2C

[OOk]

Hello

I want to connect nRF51822 (on a readbearlab ble nano) to arduino micro which uses 5V logic on a 32U4 IC. I know I can use an external logic level shift, but I need my project to be small. I read in some posts that connecting the I2C lines to a pull-up of 3.3 is enough for the I2C to work.

is this OK or must I use the level shifter?

Thanks

[Ole Bauck]

Having pull-up resistors to 3.3V may work, but it depends on the other chip if 3.3V will be high enough for a logic 1 to be registered. If the logic high threshold is 0.7xVcc, 3.5V for Vcc=5V, then it will be just below, so you will be living on the edge. The best solution is probably to add that level shifter.

I advise you to read this.