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I’m open sourcing TinySD, a dependency-free C library for SD cards and SDIO on embedded and bare-metal systems - github.com/loopj/tinysd

TinySD supports SD memory cards (SDSC, SDHC and SDXC) and SDIO devices, both with high speed support. No dynamic allocation, and no threads, callbacks or interrupts required.

I built this after writing "just enough SD" for three separate bare-metal projects, and deciding it was time to do it properly. Everything else either forced a filesystem on me or was framework-specific.

It ships with an SDHCI host driver for SD Host Controller versions 1 to 3, plus a simple interface for adding your own hosts. If you’re bit-banging the bus or driving it with PIO, CRC and framing helpers are included too.