Bonsai C4: An ARM-based ProMicro drop-in replacement

If you're soldering a custom mechanical keyboard designed to use a ProMicro, but want more processing power and storage space than a ProMicro has to offer, then you may be interested in Bonsai C4.

Inspired by the Proton C microcontroller board (which has been sold out since 2020 due to silicon shortages), Bonsai C4 offers even more processing power, more storage, and is open source. Designed by customMK, Bonsai C4 uses an STM32F411 microcontroller to achieve significantly higher performance than a ProMicro while being a drop-in replacement for most keyboards. With OLEDs and LCDs becoming more common on ProMicro-based custom mechanical keyboards, Bonsai C4 gives you the storage space and speed to take your animations to the next level.

• USB-C
• ARM microcontroller running at 96MHz with 512kB flash and 128kB RAM
• 8kB of FRAM on-board
• 16MB of flash on-board
• Reset circuit on-board
• Compatible with QMK
• $25

Bonsai C4 on our GitHub. Our Github repo also includes the open source Bonsai C which is functionally identical to Proton C, but Bonsai C is not manufacturable right now due to the lack of STM32F303 microcontrollers.

This Interest Check is to determine viability of a Group Buy for Bonsai C4. The microcontroller is readily available; the time between the group buy and shipment to participants is estimated to be approximately two months.