Driving Old School TFEL Display Panels with a Raspberry Pi Pico

The vast majority of displays that are used in devices today are based on either LCD or OLED technology, because those displays offer the best performance for the money. But many other display technologies have been developed over the years, even if most of them are now essentially obsolete. Plasma TVs, for example, were a very popular alternative to LCD TVs for several years, but they haven’t been manufactured in large numbers since 2015. TFEL (Thin Film ElectroLuminescent) displays are another type that we rarely see today, but ZephRay has managed to drive TEFL display panels using a Raspberry Pi Pico development board.

TFEL displays work using similar technology to EL (ElectroLuminescent) wire and EL ribbon. But instead of exciting the electroluminescent material all at once, TFEL displays have an electrode matrix so that small portions (the pixels) of the electroluminescent material can be lit independently. TFEL displays have a very high contrast and light output, which is their primary appeal. But they also have relatively low resolutions and are monochrome, so they aren’t ideal for many applications today. Even so, they do have an interesting look to them that could be suited to some projects. You just need a way to drive them, which is a challenge because nobody is making TFEL display drivers these days. ZephRay found a way to drive them using a speedy microcontroller and the Raspberry Pi Pico’s RP2040 microcontroller is perfect for the job.

This takes a powerful microcontroller, because it requires bit-banging. Without dedicated controller hardware, the data needs to be sent to the display through software. That is quite a lot of data, because it needs to contain the information for every pixel during every refresh. These particular displays, Plannar EL640.480-AM series panels, refresh 120 times a second. Fortunately because they are monochrome, each pixel is simply either on or off, which requires far less memory and data transfer speed than a color LCD. The Raspberry Pi Pico is quite powerful, with a fast dual-core Cortex M0 processor and 264KB of RAM. But even with that power, ZephRay couldn’t bit-bang fast enough to drive the display without flickering. Thankfully, the Pico also has PIO (Programmable Input/Output) pins. Those can change state independently of the main processor cycles and ZephRay was able to use them to speed up the bit-banging. The result is a flicker-free driving of the TFEL displays using the very low-cost ($4) Raspberry Pi Pico in place of a dedicated hardware controller.