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Using Electronic Modules with Jim Rowe 0.91-inch monochrome OLED screen Small monochrome OLED display modules have become widely available at a low cost in the last few years. This one is just 37.5 × 11.5 × 4.5mm but has a 128 × 32 pixel display that’s either white or blue. With its I2C serial interface it can be easily driven by a microcontroller. I n the October 2023 issue, we reviewed the ‘big brother’ of this OLED module, with a display measuring 1.3 inches or 33mm diagonally. It had 128 × 64 pixels – twice that of this module – along with an I2C serial interface. We have used the 1.3inch module in several projects, like the Multi-Stage Buck/Boost Charger Adaptor (October 2022; siliconchip. au/Article/15510). We have also used smaller OLED displays in various projects. For example, a 0.96in module with 128 × 64 pixels was used in these projects: » LC Meter Mk3, November 2022: siliconchip.au/Article/15543 » Q Meter, January 2023: siliconchip.au/Article/15613 » Advanced Test Tweezers, February & March 2023: siliconchip.au/Series/396 And there’s an even smaller 0.49in OLED display with 64 × 32 pixels that we used in the: » SMD Test Tweezers, October 2021/April 2022: siliconchip. au/Article/15276 » Pocket Audio Oscillator, September 2020: siliconchip.au/ Article/14563 The main difference between the current module and all of those others is that the ‘active area’ of its display is wider but shorter: 22.4mm wide by 5.6mm high. Since it has 128 × 32 pixels, that means that it provides a display basically equivalent to the top or bottom half of the 1.3in OLED module. We obtained the module shown in the photos from a supplier on AliExpress for ~$2. Another supplier 78 Silicon Chip on AliExpress had it for ~$3, while it was on eBay for ~$12. Closer to home, Tempero Systems offer it for ~$7, while Core Electronics had a very similar module available for ~$17 (siliconchip.au/link/abw3). All the prices listed above are exclusive of postage costs. Inside these OLED modules The 0.91in (23mm) OLED modules all use a single interface/controller and OLED driver IC, usually either the SH1106 device from Sino Wealth or the SSD1306 device from Solomon Systech. The same controllers are also used in many of the larger modules. Fig.1 is a block diagram of the SH1106 and the SSD1306 interface/ controllers. At upper left is the microcontroller (MCU) interface, which can be configured to interface with an MCU via an 8-bit 6800/8080 parallel interface, a 3- or 4-wire SPI interface or an I2C serial interface. Most of the OLED modules currently available use the I2C interface, including the one we’re looking at here. Display data from the MCU is stored in the Data RAM just to the right of the interface block. The SH1106 and SSD1306 controllers both have around 1024 bytes of Data RAM, enough for a 128 × 64 pixel display. Since the 0.91in OLED only has 32 rows, only half of the Data RAM is used in this module. The Display Controller block to the right of the Data RAM takes data from the RAM and displays it on the OLEDs via the page and segment drivers at the right-hand end of Fig.1. The MCU can also send commands to the controller, which pass from the MCU interface to the Command Decoder block below it in Fig.1. The commands can be used to update the display, turn it on or off, set its addressing mode, set the column starting address and adjust the display Fig.1: the block diagram of the SH1106 and SSD1306 OLED driver controller ICs. The SSD1306 has a slightly bigger internal RAM, letting it store 132 x 64 pixels (four more pixels horizontally than the SH1106). Australia's electronics magazine siliconchip.com.au Fig.2: a common circuit diagram for the 0.91in OLED modules using an SSD1306 controller. contrast and brightness (the latter also determining its operating current). The SH1106 and SSD1306 devices both come in very thin (0.3mm) SMD packages with over 260 contact pads. In the modules, they are mounted directly on the rear of the OLED screen. The module’s circuit Fig.2 shows the circuit of a typical 0.91in monochrome OLED module based on the SSD1306 device. As you can see, it’s very similar to that of the 1.3in OLED module we looked at in the October 2023 issue, although a little simpler. The circuitry to the left of the OLED provides the power supply and assists with the I2C interface. These components are all mounted on the rear of the module’s PCB. Four-pin SIL header CON1 at far left handles both the power input and the I2C interface. REG1 takes the incoming Vcc and steps it down to +3.3V to run the OLED and its controller. The +3.3V line is also used to drive the controller’s reset circuit (it needs to be reset as soon as power is applied) and provides the reference for the 4.7kW pull-up resistors used on the I2C interface lines, SCL and SDA. Before we move on to more practical things like driving one of the modules from an MCU, Fig.3 shows how the SH1106 and SSD1306 controllers save the display data in their Data RAM, and how it is shown on the OLED screen. This is achieved by setting them to what is described as Page Addressing Mode. In this mode, the OLED screen is divided into eight horizontal ‘pages’, where each page consists of 128 vertical segments eight pixels high. The siliconchip.com.au Fig.3: the SH1106 and SSD1306 controllers save their display data into RAM using column-major order. The OLED module’s PCB measures just 37.5mm wide, 11.5mm tall and the module is only 4.5mm deep, making it ideal for compact designs. You can see it at actual size in the adjacent image. Australia's electronics magazine November 2024  79 Fig.4: how to connect the 0.9in OLED module to an Arduino Uno or similar. Fig.5: connecting the OLED module to a Micromite Plus Explore 64 are just as simple as an Arduino. If instead you’re operating the module with a Micromite Mk2 or BackPack, then the SCL pin of the module connects to pin 17 of the Micromite, and the SDA pin connects to pin 18. An example photo of the OLED module connected to a Micromite via a breadboard. The underside of the OLED module shown enlarged for clarity. All components except for the screen are mounted to this side. 80 Silicon Chip Australia's electronics magazine pages are themselves arranged vertically, with page 0 along the top of the screen, page 1 immediately below it and the remaining pages descending. With the 0.91in OLED module, though, the pages and segments are used rather differently. In this case, only every second segment byte of each page is used (segments 0, 2, 4 and so on), and only four bits are used in each segment byte (bit 0, bit 2, bit 4 and bit 6). These four data bits are then used to display the four upper pixels in that segment of the OLED. The data for the lower four pixels of that OLED segment come from the next page in the controller’s RAM, which is organised in the same way: only every second segment is used, and only the four bits are used in each segment byte. I think you’ll agree that this all seems a bit weird, but that’s the way data is organised in the 0.91in OLED modules. Now we can turn our attention to what is involved in driving one of these modules from an MCU like an Arduino Uno or a Micromite. Connecting it to an Arduino Connecting the OLED module to an Arduino Uno (or compatible) is quite straightforward, as you can see from Fig.4. The GND and Vcc pins connect to the GND and 3.3V pins om the Arduino, while the SCL and SDA pins connect to the Arduino’s A5 (SCL) and A4 (SDA) pins, respectively. You can also connect the OLED module to an Arduino Uno R4 Minima, simply by connecting the module’s SCL pin to pin 17 of the Minima and the SDA pin to the Minima’s pin 16. As for software to drive the OLED module, if you go to www.arduino. cc and look at the library listings for ‘Display’ applications (siliconchip.au/ link/abw5), you will find quite a few libraries intended to do this job. The first one I found was Adafruit’s SSD1306 library, with the latest version (V2.5.9) able to handle OLED displays with either 128 × 64 or 128 × 32 pixels. It also relies on using their GFX library. The Adafruit library comes with five example sketches, including one called SSD1306_128x32_i2c.ino – which is the one most suitable for use with the 128 × 32 pixel OLED module. siliconchip.com.au When you run this sketch, it gives you a series of graphics and text displays, including those shown in the article lead and at left. As you can see, the 128 × 32 OLED’s display is quite small, but can display a useful amount of information. Connecting it to a Micromite It’s also quite easy to connect the OLED module to a Micromite MCU. Fig.5 shows the connections needed for a Micromite Plus Explore 64 and, as you can see, they are just as straightforward as driving the module from an Arduino. Connecting the module to a Micromite Mk2 or LCD Backpack V1/V2/V3 would be almost the same, except the module’s SCL pin would be connected to pin 17 of the Micromite and the SDA pin to pin 18. As with an Arduino, you also need some software. It turns out that this isn’t quite as easy as with the Arduinos, as it’s much harder to find any Micromite OLED driver software. As I related in the October 2023 article, I could write an MMBasic program to display text and simple graphics on the 1.3in OLED module, with some much appreciated help from fellow Silicon Chip staff member Tim Blythman. Since the 0.91in OLED modules use the same SSD1306 controller, I decided to try adapting that program to work with them. But that approach didn’t work with the 0.91in module, even when I tried quite a few modifications to the program – the OLED’s display remained stubbornly dark. So once again, I asked Tim for help (sorry, Tim). And as before, he provided a lot of help. Tim searched around The Back Shed (www.thebackshed.­ com/forum) and found some valuable information I had missed concerning MMBasic programming of the various OLED modules. He found a driver written by MM­Basic programming guru Peter Mather and soon came up with his own working program by combining elements of Peter Mather’s driver with a few ideas taken from my program for testing the 1.3in OLED module. Tim sent me his new program by email, and when I tried it out, I found it worked very well. So I added a few comments, plus code to display a full four lines of text instead of the single line that Tim had provided. You can see the display produced by this program at lower right. The program is called “091in OLED TB version.bas” and you can download it from siliconchip.au/Shop/6/454 As before, it’s a fairly simple program, and as it stands it only demonstrates how to drive the OLED module to display text and very simple graphic symbols. It doesn’t let you type text in via the Micromite console and display it directly on the OLED, as that would involve a fair bit of additional code. Hopefully it will make it easy for those who want to display up to four lines of text and basic symbols on the screen of one of the 0.91in OLED modules from a Micromite to do so. I’d like to thank Tim Blythman for help in producing this MMBasic program for the Micromite. Useful links • Interfacing the 0.91in OLED with an Arduino Uno: siliconchip.au/link/abw6 • OLED breakouts: siliconchip.au/link/abw7 • LCDwiki MC091GX user manual: siliconchip.au/link/ abw8 SC siliconchip.com.au Australia's electronics magazine November 2024  81