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platformdev_blackpill_f4x1.md

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WeAct Blackpill (STM32F4x1)

This document applies to the F401- and F411-based Blackpills.

The WeAct Blackpill is a popular choice for handwired boards, as it offers a powerful micro controller, USB Type C, a good number of pins to use, and a large amount of firmware space. All for a ~$6 USD price tag.

Blackpill F411

Pin Usage Limitations

While the Blackpill is a great choice to use in your keyboard, there are a number of caveats in regards to using them. The first is that a number of exposed pins cannot be used, or have special considerations/hardware tweaks that are required for proper operation.

Unusable pins

  • Pins A11 and A12 are not usable because they're used for USB connection, and cannot be shared.
    • In theory, these pins can be used. However, doing so may disable USB connectivity, outright, if used for anything other than a USB port
  • Pin B2 is used by BOOT1 and cannot be used, without causing problems.
  • VBAT is not a usable pin.
  • NRST is not a usable pin.

Pins to be avoided

  • Pin A9 is meant for VBUS Sense and should not be used, if it can be avoided. It has an internal pull-down resistor, which may cause issues with usage. However, a pull-up resistor can work (~5.1k), but should be avoided.
  • Pin A10 can be used, but should be avoided. Any connection on this pin can prevent the bootloader from entering the proper mode for DFU flashing. A pull-up resistor (~22k) on this pin fixes the bootloader issue.

Shared Usage

  • Pin A0 is shared with the User Key (button) on the controller. It can be used.
  • Pin C13 is shared with the onboard LED indicator, and is connected to +3.3V. This can be used, but may cause the LED to blink intermittently, depending on activity on the pin.
  • Pins A4, A5, A6 and A7 are used by the SOI8 footprint on the back of the controller, that can be used for either an SPI Flash chip, or an SPI EEPROM chip. A4 is the Chip Select pin, and cannot be shared. However, A5, A6, and A7 are the SCK, MISO, and MOSI pins, respectively, and can be shared with other SPI devices.

Limited Usage

  • Pins C13, C14, and C15 have limits on output current. They should be used only as input, e.g., they should not be used for row pins in COL2ROW matrix configurations, but can be used as column pins.

    • This is because the column pins (in COL2ROW) are pulled up (the pull-up strength is independent of the current sourcing limitation) and the ROW is driven low and sinks current, then we check the state of the COLs to look for keypresses.

  • Pins A0 and B5 are not 5V tolerant, and should only be used with 3.3V compatible functionality.

Additional Information

Bootloader issues

Due to the use of a 25MHz crystal, the controller may have issues entering the bootloader. Heating up the controller can help with this issue.

Also, if pin A10 is connected to anything at all, it needs to have a pull-up resistor (see Pins to be avoided, above)

Tiny UF2 Support

There is tinyuf2 support for the WeAct Blackpill. Instructions on how to compile the bootloader can be found here. Setting BOOTLOADER = tinyuf2 will enable support for this user bootloader, and the correct configuration to prevent it from being overwritten when flashing firmware.