The **FunKey S** is the world's smallest foldable retro-gaming console.

Holding a world's record is great, but we wanted a device that was
_actually_ playable and that still could be attached to a keychain.

Reaching this goal required more than just the usual job to shrink
everything you can: it required a radical change in the design itself.

## Foldable Design

The idea to use a foldable clamshell design came up rapidly as the
only way to get _both_ a small form factor when closed, and offer the
largest screen and keypads while playing.

But this solution also brings a lot of challenges, as the required
hinge make connections between the 2 halfs of the shell more
difficult.

However, this has been done before in the classic GameBoy Advanced SP,
 or in many clamshell cellphones from some years ago, so why can't we?

## Design Goals

Passed this first design decision, we fixed ourselves some additional
goals in order to provide the user a good gaming experience:

 - have a CPU that is powerful enough to be able to emulate most
   retro-gaming consoles, up to the PS1

 - have a good quality screen with a resolution large enough to
   display even the games featuring small texts

 - have a set of keys and pads large enough and complete enough to be
   able to play comfortably

 - have a built-in audio capability (who can play without sound?)

 - be able for a 6-year old child to add games and emulators

 - as an extremely nomad device, be able to stop playing just by
   closing the console, and restart when opening it just where you
   left (we call this feature "**_Instant Play_**")

## Design Constraints

Besides the constraint brought by the hinge listed above, the main
difficulty is due to the maximum device dimensions that should not
exceed those of a (large) car key.

And because of the foldable design, the most constrained dimension is
thickness: having a folding screen also means doubling the plastic
enclosure around the screen and the other internal parts.

In order to meet these constraints, we had to take more
(heart-breaking) decisions:

 - a CPU with external DRAM chips would take too much real-estate on
   the PCB, so we had to find a CPU with integrated DRAM

 - an audio Jack would be very difficult to integrate, so we decided
   to use an internal speaker instead

 - an USB C connector is too thick and large, so we used a Micro USB
   connector instead

 - a Micro SD connector with external access would be too large, as
   the required "push/push" mechanism, plus the mechanical design to
   make sure the card does not get loose make the solution much larger

 - because of real-estate constraints on the PCB as well as power
   consumption constraints, we decided to not integrate Wi-Fi and/or
   Bluetooth

 - the only keypad technology able to meet our thickness goal is using
   integrated tactile switches

 - battery capacity had to be adjusted to the maximum available
   internal volume, using standard battery dimensions

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