1. when stm32f030c8t6 is powered by 3.3V and AD7705 is powered by 3.3V, they can communicate with each other.
2. ST is powered by 3.3V, and AD7705 is powered by 5V, so they cannot communicate with each other.
3. ST is always powered by 3.3V. I checked the corresponding pins of AD7705, and when it is powered by 5V, it is compatible with 3.3v level, and ST is also compatible with 5V level.