So this just started as a crazy idea, slap a screen and some buttons on a PCB, turn it into a pocket computer. The original had a pi zero for brains.
While having a full linux OS in your pocket can be handy, it also has some drawbacks. First off, your waiting for it to start up before you can do anything useful, which can get really annoying as the pi doesnt have any sleep mode, so a quick session is always bracketed with a startup and shutdown delay. Also the power you get, needs feeding!! Even a reasonable battery will only get you a handful of hours use, and if your working in many short sessions, that can be mostly wasted with the undesirable system usage.
So how much power do you really need??
Well that mostly depends on what you want to use your pocket computer for. You don’t have to look very far back in history to find a chunk of handheld computers that were really popular.
Lets take a look back at an early TRS-80 from 1980 for example, that came out with a 4 bit processor rocking a blistering 256KHz, and 1.5KB of RAM. Coupled with a 1×24 character lcd display, the device allowed programing on the go, and could even interface with other devices like tape storage and printers!!! This was one of the earliest pocket computers and opened up a world of coding on the go
If we go a few years newer, and you come up to devices from Psion. A series 5 is a bigger step up, you gain much in specs, with a 32bit processor running at a mind bending 36MHz, and 16MB of RAM. The screen gains some resolution too, with a 640×240 pixel LCD display adding more space for activities. There was a serial interface again, allowing communication with external devices to increase usefulness
What about the screen??
Sure, it would be nice to have a fancy crisp IPS screen with lots of colours and pixels, but that then increases the demand on the CPU when doing the basic things like editing the code and showing user menu’s etc. Also most screens can be difficult to see in bright situations, such as a bright classroom or outside. Also the backlight on the screen will eat up some of the precious battery
LCD gives better readability when its bright, but then you struggle to be able to see the screen when the device is used in darker conditions. This then means you need a backlight again, so that’s back to eating battery again
e-ink gives you a screen that looks like printed paper, which is readable in almost any environment. This does come with a big drawback that the screen takes a while to refresh, sometimes up to 3 seconds, which wont really work for editing code. There may be ways round that with fast refresh and partial refresh to allow for more usability
How big should it be??
You ask a photographer which is their best camera, and they will tell you its the one they have with them. I think this rings true with anything. Having a device in your pocket will mean that you will use that device. We therefore cant make the device too bit, or it wont really be something you can easily take with you. That means the device needs to be compact, so you cant have a fancy mechanical keyboard and a 12 inch screen
What about the battery??
In keeping with the size requirements, the battery needs to be carefully selected to balance the capacity and size. It is also a good idea to use a standard battery that is easily replaceable by the user. These batteries are also available in reasonable capacities 900-1500mah which would allow for really long runtimes with such a modest CPU and screen