An Undocumented iPod Chip, 256 Register Values, and a Working Driver

All three buttons on Apple’s wired earbud remote now work on an iPod Classic running Rockbox. The driver is under review upstream as Gerrit change 7677, and a prebuilt image, ipod6g-mikey-v1, exists for people who want buttons today rather than after the merge. Rockbox on this iPod exists because volunteers reverse engineered Apple hardware for free since 2011, and the earbud remote never made the top of anyone’s list.

The chip is Mikey: I2C bus 0, address 0x72, eight registers, no public documentation anywhere. Rockbox has known about it for years but used it for exactly one job, raising the mic bias voltage so jack recording works. The TODO in the audio driver, added in 2017, listed jack accessory detection and remote buttons, and nobody picked it up until now. Searches for prior art came up empty.

The method: sweep the mode register through all 256 values on the device itself; 0x2f is the one that turns on button reporting. The hardware lies. Its event engine falls asleep after about five seconds and reports button releases 1.8 seconds late. The only prior art was a 16-year-old writeup on a blog that no longer exists, kept alive by a wiki mirror.

What I like is the shape of the work: the scaffold and the write-up stay public, the driver goes upstream, and nobody needs a private fork. Skip it if you don’t already run Rockbox; none of this touches a stock iPod. If you do, unzip ipod6g-mikey-v1 onto the drive root in disk mode. If it misbehaves, delete .rockbox and restore an official build.