Question: I have the PSoC operating as an I2C Slave (or Master) with a VDD of 3.3V. It is connected to an external I2C Master (or Slave) operating at 5V. Which voltage should the Pull Up resistors on the SCL and SDA lines be connected?
Answer: Preferably, the pull up resistors should be connected to 3.3V. This will ensure that the PSoC's GPIOs operate at the same voltage as that of VDD.
The pull up resistors can also be connected to the 5V. Under this condition, the internal clamping diodes (refer GPIO cell diagram below) in the GPIO of the PSoC will limit the voltage on the SDA and SCL lines to (3.3V + Diode Voltage). The pull up resistors will also act as current limits which will limit the current through the clamping diodes. Design the Pull up resistors in such a way that the current through the internal clamping diodes is less than 5mA. This is applicable only when the SDA and SCL lines of the other I2C device are also configured as Open Drain mode.