Displays are primarily an output device used for presentation of information in text or image forms. In any processing system, it is mostly necessary to have a display to communicate the processed information to the user. Different types of displays are available ranging from a small LED (Light Emitting Diode) to large LCD/LED monitors with touch. The former typically can show only a status of the system while the later can display more information graphically.

In this blog, we discuss about one type of display, the Dot Matrix Displays, more commonly called as DMD, which is essentially a LED based display but packed together tightly to enable basic graphical rendering. We will cover the technology in detail including the principle behind its operation, interfacing techniques and its applications.

Dot Matrix Display (DMD)

Dot Matrix Display is the simple and more commonly used displays for showing advertisement information in shops, clocks, railway departure indicators, bus routes, etc where low cost displays are required with limited resolution. Further it is used extensively by electronic enthusiasts in their hobby projects.

Three 16X32 DMD panels

DMD’s are now becoming more popular in industrial segments due to low cost and durability when compared to LCD technologies. Following are the advantages of the DMD displays used in industries

1. Easy update and dynamic message display to diverse audience
2. Can display in any fonts/languages as generated by software
3. Basic graphics and animation effects possible
4. No need of powerful MCUs
5. Relay messages to employees through one board or company-wide communication system
6. Easy integration with the existing setup with just a tiny gateway between the system and the DMD panel
7. Durable and practically no maintenance
8. Low cost compared to LCD displays

Commonly available display configurations are from 3X5 (15 LEDS) to 128X64 (8192 LEDS). Large sized DMD panels for industry require special design considerations both in hardware and software which is discussed in detail below.

DMD – Design Considerations

Let us assume a case of a DMD panel of size 16×32 i.e. with 512 LED’s. To show an image/display of our liking, it is essential to control the LEDs individually. We should be able to power on or off any particular LED independently of others. Simplest method is to dedicate a control line from MCU for each LED. This calls for 512 GPIO’s from the MCU to control a single panel.

The problem with this approach is that not only it is practically possible to have a MCU with so much GPIO’s but there might be many display which will call for much more pins. It will be cumbersome to manage the wiring as well.

Second easiest way to reduce the number of GPIO’s needed to control the LED’s, Dot Matrix Display panel can be organized as a two dimensional array of LEDs. The LED’s in the Dot Matrix Display can be wired together in such that the anodes of LEDs in a row are tied together and cathodes of LED’s in a column are tied together.

Now that they are organized in a 2D matrix format, it is possible to switch on a particular LED by closing the circuit for the row and column corresponding to that LED. Even as we have control over individual LEDs, still we will not be able to use the model because of the following reasons:

1. Considering 16×32 DMD (i.e. 16 rows and 32 columns, in total 512 LEDs), in a matrix configuration we will still need 16 (row control) + 32 (column control) = 48 GPIOs per panel. Using 3 panels increases this to 144, way beyond pins available in a low cost MCU.
2. Even if we have enough pins, it is not possible to have individual control of LEDs. For example, as shown in the below circuits, turning ON only the first LED in row 1 and both first, second LEDs in row 2 is not possible as closing row and column circuits will illuminate all those connected in common.
   

   Expected to switch ON only three LEDs

   

   Four LEDs ON instead of three

3. Another major consideration is the power requirement. With a 2.1V forward voltage and a typical current consumption of 20mA, each LED will consume 4.2mW. So with all the 512 LEDs powered on simultaneously, the consumption will be around 512 * 20mA * 2.1V = 21.5 Watts. A 3 panel set up will draw upward of 60 W.
4. And none of the MCU’s can power this much.

DMD – Design Philosophy

Considering all these factors, we are in need of a mechanism to work with a sub set of LEDs at a time. First and foremost factor taken in account is that only “we” the humans are going to see the display. We take advantage of a shortcoming in human vision called the “persistence of vision”.

According to this phenomenon, when light strikes the retina, depending on the brightness of the image, retinal field of view and color, the brain retains the impression of that light for about 100 to 60 milliseconds even after the source of that light is removed. So, the eye cannot clearly detect changes in light that occur faster than this retention period and they either go unnoticed or they appear to be one continuous light to the observer.

So based on this, if we power on the LED’s for a brief period of time and keep it off for more time but do it very fast that our human eyes are not able to differentiate it, we will be able to achieve the necessary effect. Let us assume that the 512 LED’s are managed in 4 groups of 128 each, if we can say switch on each 128 LED for 5 ms and keep it off for 15 ms, we will be able to refresh each LED in 20 ms much faster that the eye can notice. Further the power consumption is reduced to a quarter of that when all are illuminated to around only 5.4 W, much manageable.

This is how actually DMD displays are built. The LEDs are organized to smaller groups that are cyclically powered on as needed. Even though this seems to be a lot of work for MCU, actually it is not and even an 8 bit MCU with 6 pins can control the panel with few more tricks. More of this is presented in our next blog.