Arduino for STM32

The internal resistance of common multi-meter is around 10M and can affect voltage measurement if you choose high values for the potential divider.

Please spend some time search and read properly before raising an unwise question.

https://github.com/stm32duino/Arduino_C ... ki/API#spi

You are not alone regarding the frustration to enter DFU mode with STM32F411. There is a workaround below: https://www.stm32duino.com/viewtopic.php?t=1680 Here is a bit explanation about the problem associated with HSI. https://www.st.com/resource/en/application_note/an2606-stm32-microcontroller-sys...

Yes your understanding is correct.
v2.1 has Tx/Rx pins which can be connected to the Bluepill serial port.

I always use STLink v2.1. It comes with a virtual com port. STLink v2 do not have the virtual com port. Some more info here: https://stm32world.com/wiki/DIY_STM32_Programmer_(ST-Link/V2-1) Nucleo boards also has a built-in STLink v2.1 and it can be "broken off" from the board: https://jeel...

So you have used Maple core.
If you use STM32 core, the JTAG pins PB3, PB4, PA15 will be released once you set them as output port:

Not sure if the core you used was up to date. You can install the up to date version below: Roger's original core: https://github.com/rogerclarkmelbourne/Arduino_STM32 or A fork of Roger's and is more frequently maintained: https://github.com/stevstrong/Arduino_STM32 When picking options in the u8g2...

Although the chips are not moisture-proof, I have not heard of any failures due to moisture trapped inside the chip. I do not keep my development boards within enclosures and I have not encountered failures after prolonged storage. Since your boards are partially functional, I suggest to investigate...

PB2 is Boot1 of the bluepill and generally a jumper is fitted. The Boot1 jumper is usually tied to LOW. You should either tie the jumper to HIGH or remove this jumper before running your code.

Why have you made it so complicated? No need to anything on SPI1. Just need to define the pins of SPI2 for your board. #define SD_PIN PB12 #define SetSDSpeed 48 SPIClass SPI_2 (PB15, PB14, PB13); // for blackpill STM32F401 or bluepill STM32F103 #define SD_CONFIG SdSpiConfig (SD_PIN, DEDICATED_SPI, S...