HUM: GRAPHIC LCD DISPLAY 128X64

You are a beginner with Dasduino. Or electronics? A specific module caught your eye, but you do not know how to use it? Do not worry, HUM is here for you! How to Use Module (HUM) is a blog tutorials series by Soldered where you will find all you need in order to begin working with your favorite module. Tutorials include: technical characteristics, work principle, instructions on how to connect module with Dasduino and the basic code. Everything else is left to your imagination.

In this tutorial we will write about graphic display, why and when to use it, how to connect it to Croduino and how to program it to display what we want. The reason why to use graphic display instead of a regular 16×2 LCD speaks for itself, and that is the fact that besides text, we can also display graphics, including symbols, tags, images and even the text which has a different font than the one on a regular display. Another reason why to use a graphic display is the fact that it is possible to perform a very simple GUI (Graphic User Interface) which makes controlling and adjusting some of your projects much more intuitive and easier. As the first step towards that, let’s learn how to control our display.

In the picture you can see the back side of our graphic display. You can see that there are 20 pins used for power supplying the display, communication, adjusting contrast, and background LED lighting. But, there is another very important thing on it, which is not visible at first, and that is the selection between the parallel and serial communication. Namely, this display (more precisely, the chip, i.e. display controller) supports serial SPI communication and parallel communication. About the controller a bit later in the tutorial.

By switching the middle pad with the pad next to the letter S, the display receives serial communication, and if the middle pad is switched with the pad next to the letter P, the display uses parallel communication. The look of these pads (JP2) and their location on the display is visible in the following picture:

In order to be able to connect LCD and Dasduino, it is necessary to know which function does an individual terminal on the 20 pin connector have. The following image shows the function of each of these 20 pins.

Some displays have the choice of parallel and serial communication on the connector itself (PSB Pin). In that case, it is not necessary to switch the previously mentioned pads. In case of serial communication, RS pin is actually CS (Chip select), RW is DIN (Data in or MOSI with the SPI communication), and E as SCK (Serial Clock), pins DB0 and DB7 are not used. If you want simplicity and each used pin is important, and you can risk a bit slower display, we recommend using serial communication (SPI), but if you want to connect something else besides the display to the SPI, we advise you to use display in the parallel mode (it is not that the display will not work, but some modules do not like to share SPI with another module, so you should bear that in mind). There are three more pins left to explain, and those are RST, VOUT and V0. RST pin is the pin that restarts the whole display to its initial state. Some libraries use this pin to initialize (i.e. restart) the display itself during the initialization of the library. If a library (or even some Arduino code) does not require that pin, it is necessary to connect it to the voltage power supply (+5VDC) in order for the display to be in a normal mode ready to receive commands and data. V0 pin determines the desired contrast, but the adjusting itself is a bit different than the one of 16×2 display. With it, we have connected the variable resistor between V0 and mass (GND). In this case, the variable resistor is connected between V0 and VOUT because for adjusting the contrast on this display we need a negative voltage, which can be generated by that same display. Some displays already have a built-in SMD trimmer for that purposes, so in that case, we do not need to connect any resistor between V0 and VOUT, only adjust the small SMD trimmer that will adjust the LCD’s contrast.

It is mentioned that the display has a controller on it (one of the three epoxy circles on the back side of the display), and it is about the ST7920 integrated circuit, the two remaining chips are ST7921 that increase the number of pixells the controller can manage. The controller itself has several work modes, and one of them is that it can be used as a regular alphanumeric display that has a significantly larger font than the regular 16×2 display or can even work in the extended set of commands, and work as a graphic display. We will remain here on the extended set of commands that allows “drawing” different shapes on it. If you want to deal with the first work mode, the way you can do it can be found in the display’s datasheet. 

Finally, we will say what types of LCD displays can be found. Some main division is that there are three types: monochromatic (black-white), grayscale (can display shades of one color, e.g. gray) and color. Our display belongs to monochromatic ones because it can display only the pixel that is turned on or off. Those who can change the intensity of the turned on pixel are grayscale displays and they are difficult to obtain and expensive, therefore they are rarely seen in some projects. And lastly, the most interesting ones are color displays, which exist in different versions (serial, parallel communication or even startup from an SD card), of different resolutions and with different amounts of color bits per pixel (from 8 to 24 bits), with or without touchscreen. Such displays are easily accessible.

In this part of the tutorial, we will show you how to connect Dasduino to the LCD. We should note that there will be two schemes, one of them showing how to use LCD in serial communication, and the other how to use it in parallel.

A scheme for serial (SPI) communication, which is used by the ST7920 GFX Library.

Scheme for parallel communication, which is used by the u8g library.

As we have already said, we will use two operation modes, parallel and serial communication. In order to make writing of the code easier, we will use libraries (one is ST7920 GFX Library which is based on the well-known Adafruit GFX Library (check the instructions for Adafruit GFX library here), only for now it does not support parallel communication, and the other is U8G library which supports both serial and parallel communication, but is somewhat more complicated to handle). If you do not know how to install libraries, we advise you to check our tutorial about it.

First we will deal with the ST7920 GFX Library, so open Arduino IDE, then FIle->Examples->ST7920_GFX_Library->ST7920_graphic_test and upload the code to Dasduino. If everything is fine, the display will show the test program for display. If that does not happen, check the connections.

Now that we know that everything is working, let us make a simple display of some image and text.