From Waveshare Wiki
CM4-TO-CM3 Board adapter is an alternative solution for Raspberry Pi Module 3.
Onboard Resource Comparision
|GPIO28, GPIO29||GPIO44(SDA0), GPIO45（SCL0）|
|GPIO30, GPIO32, GPIO33~45||NC|
- CM4 connector: for connecting the CM4.
- CM4 working status indicator: used to judge whether CM4 is working.
- MP28164: DC buck-boost chip.
- Power supply selection: the resistor on the left - direct power supply; the resistor on the right - power supply after passing through the DC buck-boost chip (by default).
- IO level selection: select the IO level of CM4 (3.3V is selected by default).
- CM3 interface: used to connect to the CM3 baseboard.