How does one implement a secondary small display on the back of a tablet computer?

Question

I have seen such a feature a few times:

I would like to ask how that is done.

Is such a small display something "off the shelf"?

Thank you!

Answer 1

This answer is a very generalized overview. I cannot get into more detail without getting into how computers actually work, nor without knowing more about the specific device pictured.

Consumer Embedded Devices

Devices of the kind pictured are often an embedded system, designed to do one specific thing. They are cheap to produce, usually self-contained, and have limited capabilities. You can even Google them. Notice how cheap they are?

This is different than your home PC, which is a powerful general-purpose system, capable of doing just about anything.

Processor

The CPU itself may very well be an x86-based or RISC-based chip. The most famous of these is probably the Motorola 68000 which is still sold and in use today (though Motorola is naturally pushing their latest NXP ColdFire microprocessor).

That said, they don’t necessarily have to be all that powerful. The simpler the application, the simpler (and cheaper) the needed CPU.

These processors are used in everything. In your cars. In the robots that build cars. In your home thermostats. In vending machines. In medical devices, both home stuff and stuff found in the hospital. TVs, washing machines, home security systems. Everything from power tools to tractors to heart implants. Everything.

The example you posted a picture of is for a very lucrative market selling stuff that claims to help with child education. I shall not disparage it further, but you know my opinion.

Customization vs. “Off the Shelf”

While some specifics of the machine are customizable, for the most part the computer inside is a mass-produced product. Almost everything there is composed of mass-produced parts.

In fact, it is likely that the only specially-made parts are the plastic case and any pretty company logos silkscreened or stickered to it. Even the game program cartridges that you plug in to it to make it work are often mass-produced. (Oh, and the printed circuit board is probably custom-designed too.)

Display(s)

The display screens are not exempted. The front display pictured in your question is likely an “off the shelf” OLED or TFT display technology. Again, you can find examples on the web for sellers of these kinds of things. I found AdaFruit’s LCDs & Displays page on Google. (Not an endorsement, I just found them on Google.)

I had initially thought the rear display appeared to be a 14- or 16-segment display, which also come in quite a large selection of shapes and sizes, but on looking closer I think it is a monochrome OLED.

Either way, they both connect directly to the hardware and are controlled by having a dedicated memory bus. (That is, they are connected as part of the system’s memory. To modify the display, just write data to the correct spot in memory.)

Software

The software that runs on these computers is specially written for the hardware device. So when something appears on one of the displays, it is because the software knows exactly where in memory to write and which values to write to cause the display to show “Benim adım Marel” (Turkish for “My name is Marel”).

Beyond this exceedingly simplified overview there is a lot to learn. You can always take university classes in hardware design and function, basic programming for microcontrollers, and electrical circuit design.

Typically an embedded consumer device like the one you pictured will be designed first using a PC with dedicated software that can model and emulate the physical device.

Software will be designed and built using a cross-compiler and executed on the emulated device for development and debugging.

There are also several layers of middlemen that can implement various phases of this process for you, from design and prototyping through purchasing and assembly.

I myself know very little about this process. If you, dear reader, know better, feel free to edit my post to correct or add relevant information.

Answer 2

Two-line text displays can be driven using an Arduino - the tutorial I've linked uses an LCD display but there may be other displays using the same interface and looking more like the one in your photo.

Then the Arduino would be either USB or Bluetooth connected, the latter with an interface module (example. Some pretty simple software on the computer then writes to a virtual serial port and the Arduino interprets that for display on the LCD. Similar ideas have been around for some years, often in the front panel of desktop cases using modified drive bay covers.

For completeness, I should mention a related system I built, with a Raspberry Pi and touchscreen communicating over RS232 with an Arduino, that in turn was connected to a PC. You wouldn't want to power that off laptop batteries. Data from the Arduino was displayed on both the PC and the PC; the RPi sent some commands to the Arduino.