There are open-source projects that get attention because they’re new, flashy, or disruptive. And then there are projects that earn their place by simply working quietly in the background until they become part of the embedded landscape itself.
Elm-Chan’s FatFs falls squarely into the second category.
If you’ve ever logged data to an SD card, saved configuration files to flash, recorded audio, captured images, exported CSVs, or loaded firmware from removable storage on a microcontroller, there’s a very good chance FatFs was involved somewhere along the way. Often it’s hidden behind a vendor SDK, an RTOS abstraction layer, or a middleware checkbox. It doesn’t announce itself. It just does its job.
This post is an appreciation of that work (and the engineer behind it). I’m writing it while porting FatFs to a Renesas microcontroller, and I’m continually grateful for ChaN’s work.
Who Is ChaN?
ChaN is an embedded engineer from Japan who maintains a long-running personal site called ELM by ChaN. The site hosts a collection of embedded projects and technical documentation, written in a style that is clearly by engineers for engineers.
We don’t know much about ChaN other than their handle and what’s found on the website. The author modestly describes their work as that of a “mediocre embedded system engineer.” Anyone who has spent time with FatFs knows how misleading that description is. If “mediocre” means creating one of the most popular open source firmware libraries, then I have a long way to go in my engineering journey.
What FatFs Does
At its core, FatFs is a portable FAT and exFAT filesystem implementation written in C, designed specifically for small embedded systems. That sounds simple, but it addresses a tricky problem.
Embedded developers frequently need to store data in a way that can be read by a PC. Despite its age, FAT remains the de facto standard for removable media. Cameras, data loggers, test equipment, and consumer devices all rely on it because it’s universally understood by Windows, macOS, and Linux.
The challenge is that implementing FAT correctly is not trivial.
FatFs solves this by separating filesystem logic from low-level disk I/O. You provide a small set of functions that read and write fixed-size blocks to whatever storage medium you’re using (e.g. SPI SD cards, SDIO, QSPI flash, NAND, NOR, RAM disks). FatFs takes care of everything above that: directory structures, allocation tables, cluster chains, filenames, timestamps, buffering, and all the edge cases that come with them.
This separation is the key to FatFs’ success. It allows the same filesystem code to run on wildly different hardware, from tiny 8-bit microcontrollers to modern Cortex-M systems running an RTOS. It also allows embedded engineers to focus on their application logic instead of spending days diving into filesystem implementations.
If you’ve tried to write even a basic FAT implementation (nightmares from my OS class in college return…), you know how quickly the complexity piles up: fragmentation, long filenames, power loss during writes, media formatted by different operating systems, and so on. All of these issues show up in projects and products. FatFs exists so most of us never have to rediscover those problems the hard way.
Next year will mark FatFs’s 20th year. That’s a long time for a software library. It didn’t chase trends, balloon in size, or turn into a massive framework (I’m looking at you, Qt). It stayed focused on one job, and it did that job well.
FatFs as Embedded Infrastructure
One reason FatFs is easy to overlook is that it’s been absorbed into so many embedded ecosystems.
If you’ve used STM32Cube, you’ve likely used FatFs via ST’s official middleware. If you’ve worked with Zephyr RTOS, FatFs is available as a filesystem option. If you’ve used or taught MicroPython on microcontrollers, you’ve almost certainly interacted with a FatFs-derived implementation under the hood.
In many cases, engineers don’t consciously “choose” FatFs. It’s simply there, because it’s proven, well-understood, and trusted. It’s become a de facto standard for many open source embedded projects that require filesystem access.
Why FatFs Is Important
FatFs matters because it makes a hard problem boring.
It allows embedded systems to interact with the outside world using familiar file semantics. It enables data logging, diagnostics, firmware updates, media capture, and configuration storage without forcing every team to reinvent the same fragile code. It lowers the barrier to entry for students and professionals alike, making embedded development more approachable and more productive.
Perhaps most importantly, FatFs demonstrates what good embedded open source looks like. It’s small, focused, well-documented, and written with a clear understanding of constraints. It doesn’t assume infinite memory or perfect hardware, and it’s straightforward to port to a different architecture.
That’s why it has survived for so long.
Thank You, ChaN
FatFs doesn’t generate headlines. It doesn’t trend on social media. It doesn’t ask for attention. Yet it has quietly shipped inside countless devices across industries, often without explicit credit.
ChaN built something that thousands of embedded developers have relied on for nearly two decades and did so without fanfare. They solved a problem once, solved it well, and kept maintaining it long after most of us would have moved on (myself included).
So this is a simple thank you. To ChaN, and to FatFs, for making embedded filesystems small, reliable, and portable.