Unicode support in microcontroller applications

A project log for ESP8266 Twitter Client

ESP8266 Twitter client with OLED display

Andrei MehiläinenAndrei Mehiläinen 06/08/2017 at 20:370 Comments

While modern operating systems and web browsers provide extensive support for Unicode, which mobile, desktop and web applications can easily utilize, embedded (microcontroller-based) devices have no such luxury. If the device must be able to display characters other than the Basic Latin (ASCII) ones, things gets difficult. There’s no standard way of doing this and the developer must choose the one which fits her application the best.

One solution is to use display module with a controller which has a built-in font library. Popular Hitachi HD44780 controller, for example, has built-in Cyrillic or Japanese (Katakana only) characters and some characters used in European languages (Latin Supplement). Toshiba T6963C controller has only ASCII characters and some characters from Latin Supplement. Some controller chips, such as RA8806, has larger font libraries and include e.g. Chinese fonts. Still, it is hard to find a controller with full Unicode support. And of course, the user is limited with the font type and size predefined by the chip vendor. Relying on those chips inevitably introduce dependencies. Will the display with this chip be available in future? There are also some external font chips, but those has basically the same weaknesses and limitations as the display controllers with built-in fonts.

Another solution is to use third-party graphics library. Some microcontroller vendors provide free graphics libraries with Unicode support. Those libraries are usually non (easily) portable or not legal to use on other vendor devices. Commercial graphics libraries are usually less device specific, but may still be non-portable to a certain device or it may not be easy to add support for a certain display type. In any case, the usage of a third-party library on a resource constrain device should always be carefully considered. Libraries introduce complexity to the software and the risk of hard-to-find bugs grows. Even if the Unicode support is the only thing needed, you would still have to buy and use the whole library.

In the next post, I will describe how I implemented Unicode support in this project.