From AsteroidOS

Ray and firefish are based on the Fossil Gen 4 platform and are available under several names and brands. Models were released by Fossil, Skagen, Diesel, Michael Kors (and possibly others). The ports are nearly complete, apart from microphone and GPS. Flashing AsteroidOS requires the back cover to be removed.


The difference between the two models is the display size - otherwise the two install images are completely interchangeable, and work fine but with a badly sized display. The round watches have OLED displays, a rotating crown with 2 additional buttons, and come with NFC, a stepcounter and both GPS and heartrate monitoring, which are both nice to have for sports purposes - unfortunately the GPS is rather power hungry and limits battery life to less than a day. The watches are also, frustratingly, missing a compass, which makes GPS navigation a bit less user-friendly. Most firefish variants take easily swappable 22mm straps and most ray variants take 18mm straps.

Getting USB

USB pins on ray and firefish are inside the watch (see #location on the board). Getting access to these requires the watch to be opened, breaking the glue seal and compromising water resistance. It seems that a lot of watches were produced with faulty glue, you can buy many on ebay with the backs already removed. For more info see #Repair.

Available docks

If you are fine with not having USB access most of the time, you can 3d print a dock that will dock to the watch when the back cover is removed.

Read before reassembly

Before securing the watch, make sure to connect your watch to a wifi network using connmanctl. Otheriwise, you won't have shell access which will lock you out of a lot of features. When reassembling the watch, make sure not to use adhesive that is very strong. You will eventually need physical SSH access when something inevitably breaks - at this point you will need to pull the back off again and use the dock.

Replacement backplates

A more permanent but more complicated solution is the replacement backplate. These will expose USB permanently so that you can access it whenever you want without opening the watch. All the following designs are 3d printed.

Hotdog design

This design, courtesy of hotdog18882, makes use of a fairly common magnetic connector that is commonly available on aliexpress. The main disadvantage is that you lose the heartrate sensor. However, the design is easy to print on any 3d printer and the cables are very common and so you can buy several chargers for very cheap. Assembly instructions: TODO

Dodoradio design

This design, courtesy of dodoradio, makes use of two sets of pogo pins and is completely solderless. This probably cannot be printed on an FDM 3d printer as it takes advantage of tight geometry and clarity. While this design attempts to keep the functionality of the heartrate sensor, some tuning might be needed to get that working.

Location on the board

USB is not available on these watches by default. It is exposed in two places once you remove the back casing of the watch. TODO: add images of the contact pads on the watch The outer charging rings are 5v (inner,1) and ground (outer,2). These are picked up on the motherboard with a pair of spring contacts, 5v (3) and ground (4). A set of 4 pads is present around the connector for the heartrate flex (5). On the left, the bottom pad is 5v (6) and the top pad is ground (7), these directly duplicate the charging rings. The data pads are on the right of the connector. The top pad is data+ (8) and the bottom pad is data- (9). These are also directly duplicated on the heartrate flex cable, and can be accessed by removing a sticker (10) or folding up the extended section. The ring closest to the center is d+ (11) and the outer one is d- (12).


Opening any ray or firefish requires removing the back which is adhered in place. This compromises water resistance. There are not many, but replacement batteries seem to have appeared on the market, and it is easy to buy other faulty watches to fix your own due to their popularity. It is also common to see these watches being sold with either

  1. broken charging rings (they generally just fall out of the plastic back) or
  2. backs that have fallen off. For AsteroidOS purposes it might be worth buying one where the back has already fallen off, it seems that the glue was revised later in production to fix this defect and on these revised models it is very difficult to remove the back.

The motherboard is shared between the two models and can be swapped over, but most of the other internals are specific to ray or firefish. It is possible to swap all the internals (everything apart from the display is on a small plastic frame which sits inside the watch case) of a fossil gen 4 of one model into the casing of another (ray to ray or firefish to firefish).

Miscellaneous hardware docs


The central key, being also the rotating crown, is the power key. The top and bottom extra keys map to volume up and down respectively.

Manually getting to fastboot

After the watch stops vibrating on startup, immediately touch the top left & bottom right edges of the screen. Tapping repeatedly after pressing the central power key may be useful.