There are a number of different kinds of displays that can be driven by a microcontroller. This repository contains examples for many of them, along with information about display technologies and some of the more popular libraries for controlling them.
Display technologies range from multi-segment LEDs through high-resolution, full-color displays. The major ones are detailed below.
Multi-Segment LED display - There are many models of multi-segment LED displays, including the classic 7-segment LEDs, alphanumeric displays, dot-matrix diplays, bar graph displays, RGB LEDs, and others. What these share in common is that they will have either a common-cathode or common-anode structure. Common cathode LEDs have multiple anodes, one for each LED segment, and one cathode for all. common anode LEDs have a single anode and multiple cathode for all the segments. Driving these displays requires a control pin for each LED segment. They are usually driven by a multiplexer or LED driver, which can provide both a common interface for all the LEDs (such as an SPI or I2C interface), and a controlled current supply for all the LEDs.
Broadcom/Avago's HCMS-29xx display is multi-segment LED display that has several 5-7 LED matrices with a synchronous serial interface. It has the smallest visibly discrete LEDs in its display that I have encountered.
LCD - Liquid crystal display. LCDs are made up of long-chain molecules in a state between crystal and liquid. When a charge is applied, the molecules align, acting as a polarizer. When paired with a second polarizer, they can either block light or allow it to pass through, appearing either light or dark. A grid of these can form a single-color display. Liquid crystals do not emit light, so a backlight is required to light them up. They come im low-resolution, passive-matrix displays which are usually monochrome or higher-resolution, active-matrix screens which have higher resolution and are usually full color.
OLED - an OLED screen replaces the liquid crystal with a matrix of organic LEDs. This eliminates the need for a backlight, since each pixel generates its own light. For more on OLEDs, see this introduction from ehergy.gov. CNET provides this comparison of LCD vs OLED displays.
ePaper - ePaper displays use a matrix of tiny capsules which are black or colored on one side, and white on the other. Applying a charge to each capsule causes it to turn one way or the other. Unlike LCD or LED displays, ePaper displays maintain their state when powered off. ePaper displays cannot be refreshed as fast as LCD or LED, however. ePaper displays are typically not backlit, and require external lighting. eInk, the primary maker of ePaper displays, has a good FAQ on the technology. Visionect.com has a helpful illustrated explanation as well.
There are a few common methods for driving displays, and a handful of popular manufacturers of driver integrated circuits (ICs).
LCD and OLED screens drive their pixels in one of two ways. A passive matrix uses a grid of wires which control each pixel using a row-column scanning method. Voltage is applied to each column in sequence. Then the rows are scanned. If the pixel on that column at a given row should be on, then the row wire voltage is taken low to create a voltage difference, and the pixel turns on. An active matrix uses a grid of thin film transistors (TFT) instead of a row-column scanning apparatus. TFTs allow for greater pixel density and therefore sharper image quality and better response time for each pixel. Jameco offers a good explanation of passive vs. active matrix driver technology.
For a more formal explanation of LCD and TFT displays, see this page from J-Display
The oldest form of LCD display, patented in the 1980's, is known as Twisted Nematic (TN) LCD, and has limits to its viewing angle. Newer LCD technologies such as in-plane switching (IPS) or plane-to-line switching (PLS) afford wider viewing angles and brighter screens.
There are a number of common display driver ICs on the market. Typically a driver IC will be capable of controlling many different sizes and shapes of display, if they are of the same class. For example, you'll see many TFT displays that use Sitronix' driver ICs, notably the ST7735 and ST7789. Ilitek's ILI9225 chip is also common in TFTs. This means that libraries written for one vendor's display are likely to work for displays from another vendor, if they use the same chipset. This can be convenient, as it means you can sometimes choose the library whose API you find easiest to work with.
There are many LED driver ICs on the market, which give you control over multiple LED segments from a single synchronous serial interface. Some include PWM control over each channel as well. Popular ones include Maxim's MAX7219 and Texas Instruments' TLC5947. Many of the hobbyist electronic vendors carry breakout boards for these chips.
Recently, drivers for LEDs have reduced in size to the point where a driver can drive a single pixel. Usually made of three to four LEDs and a single driver, these are very popular with electronics hobbyists. For more on these, see this repository.
Displays for microcontrollers use a variety of control interfaces. The most common are the ones you see for other electronic modules as well: synchronous serial interfaces like I2C and SPI, or asynchronous serial interfaces. also feature parallel interface, requires a large number of I/O pins from your controller.
A display with an SPI interface will feature most or all of the following pins:
- Voltage - Microcontroller voltage supply. 3.3V or 5V voltage, depending on the display
- Ground - Microcontoller ground
- SDI - Serial Data in. Connects to the microcontrollers SPI SDO pin
- SCK - Serial Clock. Connects to the microcontroller's SPI clock pin
- CS - Chip Select. Connects to the microcontroller's SPI CS pin
- D/C - Data/Command. Connects to whatever pin the microcontroller has assigned for this function.
- RESET - a reset pin, connects to whicheve pin the microcontroller has assigned for this function.
- BUSY - an output pin to indicate that the display controller is busy. connects to whicheve pin the microcontroller has assigned for this function. This pin is less common on TFT displays than on ePaper displays.
- Backlight - most TFT screens have a pin which enables or disables the backlight of the screen. The naming for this is not standardized: BLK, LITE, TE are all in use. Read the module's datasheet for details.
Adafruit tends to add an SD card reader and sometimes a static RAM chip to its modules, which are also controlled by the SPI bus. When these are present, you'll see chip select pins for them, usually labeled CCS or SCDS for the SD card and SRCS for the SRAM. If you're not using the SD card or SRAM in your application, you won't need these.
Note: the electronics industry has used the terms "master/slave" to refer to controller devices and peripheral devices for decades without regard for the historical context of, and offense caused by, those terms. As a result, you will see the terms MOSI/MISO/SS in data sheets to refer to the pins of an SPI device While a modern standard naming scheme has not yet emerged to replace these, there are proposals in discussion. The Open Source Hardware Association has this proposal, for example. Make Magazine has this proposal. The debate is not resolved, and you will likely see some variations on the terms. The SDO, SDI, and SCK terms are the most widely accepted terms with the least historical baggage, but unfortunately, it's still necessary to be aware of the other possible terms for pins in SPI.
Displays with I2C interfaces typically have the following pinouts:
- SDA - Serial data, connects to the microcontroller's I2C SDA pin
- SCL - Serial clock, connects to the microcontroller's I2C SCL pin
- RESET - a reset pin, connects to whicheve pin the microcontroller has assigned for this function. Not all screens implement this pin.
- Hitachi HD44780 LCD display. See the Arduino LiquidCrystal library. These 2x16 character LCD displays are ubiquitous in the hobbyist market and come in many starter kits for the Uno. They are a passive-matrix LCD with a parallel interface (6 pins) that runs on 5 volts. They will typically not run on 3.3 volts. Each character is a 5x7 pixel matrix, so these are very low-resolution displays. They can usually be foung for $10-$15, which was a bargain in the early Arduino days. Nowadays, if you need an inexpensive 2-line display, some of the OLED displays like the SSD1306 are a better bargain.
- 8x8 LED Matrix
- 7-segment LED displays
There are some display modules which have an asynchronous serial (UART) interfaces. These typically have a microcontroller on the display module itself, which is interfacing with one of the types of interfaces above. These modules typically have a communications protocol that is unique to the vendor. They are convenient, but more expensive than their synchronous serial or parallel counterparts.
Finding the right display library for your Arduino or Arduino-compatible display can be challenging. Vendors who design and sell their own breakout boards tend to publish libraries that are compatible with their own boards. Smaller vendors may not make their own libraries, relying on third-party libraries instead. The Arduino site lists over 300 display-related libraries. The ease-of-use and adaptability of those libraries varies widely. The ones I've found most useful are Adafruit's GFX library and Oli Kraus' U8g2 library.
Since there is a relatively small number of driver chip manufacturers (Hitachi, Ilitek, Solomon-Systech, and Sitronix among them), different vendors' boards often use the same driver hardware. This means that the libraries from one vendor can support the hardware from another. When you shop for displays, it's worthwhile to check what the driver is for each one, and see if there's a compatible library from your favorite library writer.
Adafruit_GFX is a hardware-independent graphics library written to work with all the Arduino-compatible displays that Adafruit sells. They complement this with display specific libraries like Adafruit_SSD1306 for SSD1306 OLED libraries, Adafruit_EPD for ePaper displays, Adafruit_ST7735 for some TFT libraries, and others. The advantage of the GFX library is that you get a common drawing API regardless of which display you're using. It uses the Arduino Printable interface too, so commands like print() and println() work with this library just like they do in the serial monitor. There's a good guide to the GFX library as well. Sparkfun's got their own complement to the GFX library, Hyperdisplay.
squix.ch has a useful font generator for the Adafruit_GFX library.
u8g2 is designed as a universal library for many different displays. It supports a wider range of displays than any other I've used so far. It has its own graphics API which is more or less similar to Adafruit's, and a wide font set as well. There's also U8g2_for_Adafruit_GFX, a library which allows you to add U8g2 fonts to any Adafruit_GFX-based library.