You are welcome. If I can I gladly help.
About STDIO
- How does the global printf() know where to send the output? - Mbed OS - Arm Mbed OS support forum
- Hitchhiker’s Guide to Printf in Mbed 6 - Mbed OS - Arm Mbed OS support forum
About Custom targets
- Mbed doc - Porting custom boards
- good example mbed-os-bluepill - Bluepill STM32F103C8 support for Mbed OS 6 | Mbed
BR, Jan