OLED Screens

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Published On:

Feb 10, 2012

Last Updated:

Feb 10, 2012

OLED screens are electronic screens that use miniature organic LEDs to display the picture. Since the pixels themselves emit light, unlike traditional LCD screens, they have no need for a backlight and therefore can be thinner, more efficient, and even flexible! Another positive feature about OLED’s is their high contrast ratio, that is, you can easily see what is on the screen. This means that you can make pictures and text smaller while still being readable.

Most of the information below focuses on the OLED screens used in embedded circuit design rather than computer/tv display.

Reliability

The lifespan of a monochromatic OLED depends largely on the emitted colour. Here’s a rough guide on the number of operating hours they will last for.

White = 10,000 hours
Blue = 15,000
Yellow = 50,000

Resolutions/Physical Size

OLED display’s can come in really small packages, suitable for small electronic devices.

OLED’s suitable for electronic products typically range in resolution from 96x16 pixels up to 256x96. Larger resolutions exist for stand-alone screens, all the way up to high definition (HD). The physical size of OLED range from 2591mm (widthheightdepth) to 100x50x2mm for electronic products.

Communication/Connection Methods

Most small OLED screens have inbuilt driver IC’s such as the Solomon SSD1306, which takes care of the 100+ lines and allows you to connect the OLED module directly up to a microcontroller. I2C, SPI or parallel are the most commonly used communication methods. Normally they are provided with a solderable FPC connector (a flexible, flat, connector that can be soldered from both sides).

The cable to the OLED screen is usually designed to be soldered flat, or bent 180 degrees and soldered underneath the OLED screen itself. The cable sometimes have a pad pitch of 0.65mm, and you can’t by connector to fit this (it needs to be directly soldered onto the PCB. The soldering can easily be done by hand.

Most OLED screens are pixel-driven, as opposed to character driven such as some LCD screens. This allows much higher configurability, and also means the screen can be installed in any orientation.

Driver IC’s

The Solomon SSDxxxx IC’s are a common choice as a driver IC for an OLED screen. They are usually embedded in the the screen itself (this makes sense, otherwise you would have hundreds of wires to connect up to the screen, one for each row and column), and connected to the external world via a communications protocol such as SPI or I2C.

Solomon SSD1305

Max Resolution: 132x64 pixels

Min Pitch: 45um

Supports up to a 132x64 pixel screen. Similar to the Solomon SSD1306 except it has an internal boost converter (aside from the discrete caps and inductor) that can provide the OLED screen voltage.

Solomon SSD1306

Max Resolution: 128x32 pixels

Min Pitch: ?

Like the SSD1305 except it uses a charge pump instead of a buck converter to provide the screen voltage. This means less external components (only two capacitors required for the charge pump), and no inductor! (who likes choosing/sourcing inductors?).

Just to give you an idea of how graphics data is written to the SSD1306, the GDDRAM structure (the RAM inside the IC which holds information on the colour of each pixel) is shown in the following diagram.

The GDDRAM structure (the RAM inside the IC which holds information on the colour of each pixel) of the Solomon SSD1306 IC.

VCOMH, It May Not Be What You Think

OLED screens that use the Solomon SSD1306 usually have a pin labelled VCOMH, which has the pin description shown below.

The pin description for the VCOMH pin on an OLED screen connector that uses the Solomon SSD1306 IC. This pin is incorrectly labelled as in input, but is actually an output from the internal charge pump for powering the COM segments on the OLED screen.

This can trick you into thinking that you use this pin to set the communication voltage level it will use for I2C or SPI transmissions, as some IC’s have this feature to allow the IC to work with microcontrollers operating at wide range of supply voltages. However, this is not the case, this pin actually output’s a voltage generated by the internal charge pump circuitry for driving the COM segments on the OLED screen. The only thing you need to connect to this pin is a 4.7uF, X5R, 16V capacitor to ground.

Driving Both The Charge Pump And Logic Of The Same Supply

You might notice that some OLED’s specify a voltage range of 3.3-4.2V for the charge pump and 1.8-3.3V for the logic. And then you would think, well I really wanted to run them of the same supply (one of the main reasons why they would include an internal charge pump, right?), so can I run them from 3.3V? Well, although the charge pump input specifys a minimum of 3.3V, you can actually run it at that or lower, sacrificing a little screen brightness in the process (see the below graph). And the logic circuitry seems to handle 3.3V+- the variation of a normal linear regulator without too much trouble.