Introduction to TFT Display Interfaces

This article about TFT display interfaces was written by Julia Nielsen. Julia Nielsen is a jack-of-all-trades writer, having written for newspapers, magazines, websites, and blogs for the last 15 years. When she’s not dabbling in the written word, she’s spending time with her beautiful granddaughter. She loves to hear from readers, especially when they offer chocolate.

Display technology has evolved at lightning speed for the last number of years, as opposed to when even the most sophisticated products incorporated numeric or segment displays and alphanumeric or character display technology. The same products also required buttons which have been replaced with resistive and capacitive touch panels.

When color TFT (Thin-Film Transistors) first came onto the stage, they created a buzz in the tech world that hasn’t stop buzzing since. TFT utilizes a type of display that controls each pixel with a transistor, allowing it to individually address each location.

As TFT yields improved with mass production, manufacturing, as well as healthy competition, TFT displays have soared in production performance and dived in price. Because of this, TFTs are considered the de facto standard of displays that boast of full color, brightly backlit (high NIT counts), high video speeds, better viewing angle, specifically for mobile devices and other small devices needing clear displays, such as phones, watches, security systems, and the like.

OLED (organic light-emitting diode) are increasing in popularity, but are still second to TFTs. Much of this is due to the long lead time and shorter half-life of the OLED displays. Although we offer OLED technology, we recommend TFT for the majority of the new design requests we receive.

There are several types of TFT display interfaces which have been designed in the last number of years for all variations of screen size, including LVDS, (Low-Voltage Differential Signaling) parallel, SPI (Serial Peripheral Interface) and I2C or I²C (aka I squared C) display.

Here is an overview of these display interfaces to give you a better idea of the variety of TFT displays that are taking center stage.

LVDS: high speed interface

Low-voltage differential signaling was first designed in the early 1990’s and has seen its popularity mainly in LCD-TVs, industrial cameras, notebook and tablets, and communication systems. LVDS is a technical standard that specifies electrical characteristics of a differential, serial communications protocol, which allows the operation of low power, but very high speed using inexpensive twisted-pair copper cables.

The typical applications that use LVDS include:

High-speed video
High resolution Graphics
General purpose computer busses
Video camera data transfers

LVDS is a differential signaling system, meaning it transmits information as the difference between the voltages on a pair of wires. Its popularity comes from the benefit of reducing noise levels and low power consumption, which results in even more benefits, such as lower heat dissipation and longer battery life; and because the differential drivers can be included on the LVD interface, smaller parts count, lowered parts cost, and increased reliability is a win-win for businesses and consumers.

Commercial and military, as well as aerospace applications also use LVDs in their products for a robust, long-term solution for high-speed data transmission needs. Flat panel displays, printers, digital copiers, and even cell phones incorporate LVDs to provide an excellent display quality. There are different types of LVDS protocols. When looking for the right LVDs, consider data rate, operating temperature range, and supply voltage, using these filters.

Note: Most TFT displays will operate down to -30C without the need of a heater. OLEDs will operate down to -40C without a heater, but OLEDs that are larger than 3.5” are much more expensive and have a longer lead time than TFTs.

Parallel interfaces for TFTs

Parallel interface or parallel port is a type of display interface found on computers for connecting peripherals. In the past, most people associated a ‘parallel’ interface with a printer port. This type of interface refers to a multi-line channel with each line capable of transmitting several bits of data on each simultaneously (bi-directional) or parallel to each line.

Newer PC’s have eliminated parallel interfaces in exchange for fire wire, USB2 and USB3. Parallel interfaces are still the most common for several LCD technologies such as character and monochrome graphics.

Parallel interface is nothing new, going back to the beginning of the 1970’s in its development and implementation. The first printer to use the interface was the Centronics 101 model printer, which became the standard at that time. But because a number of cables were required, Dataproducts and other developers had to create up to 50-pin connectors.

Fast forward to 1981 and IBM introduced their computers and printers with a 25-pin connector on the PC end and a 36-pin connector on the Centronics printer, thus the parallel interface had evolved to using both systems. In 1987, IBM introduced a bidirectional parallel interface. Since then, the parallel interface has evolved, with other companies developing their own, with even more parallel ports, including scanners.

Since technology has advanced exponentially in the last decade, so has the parallel interface, evolving to include supercomputers that allow for high-performance interfaces and network storage devices. These super performance display interfaces are capable of transferring billions of bits of data per second over short distances on local area networks. Graphical printers, along with a variety of other devices have been designed to communicate with the parallel ports including:

External modems
Webcams
Sound cards
Gamepads
Joysticks
External hard disk drives
CD-ROM drives

Some of the early MP3 players and digital cameras also used a parallel port connection for transferring songs to a device, so you can see how far back the interface has been utilized in electronics.

SPI bus for LCD interfacing

Serial Peripheral Interface allows the serial (one bit at a time) exchange of data between two devices. A master, which controls one or more devices. Each device has its own slave connection. The master can interface with multiple slaves independently.

Once the relationship is established, the direction of control is always from master to slave. A SPI operates in full duplex mode (Data that can be transmitted in both directions on a single carrier, at the same time). To illustrate, let’s look at workstations. A SPI allows one workstation to send data on the line, while the other receives it, thus the term bi-directional, since it allows for data to be sent and received in both directions, and on the same line.

The term SPI was coined by Motorola and is typically used in communication systems between the CPU (Central Processing Unit) and peripheral devices (Any computer device not part of the essential computer, but situated close by). Serial interfaces have an advantage over parallel ones, that of simpler wiring. They can also have longer cables since there is much less interaction or crosstalk among the conductors in the cable. Many types of devices use SPI, such as:

Shift registers
Data storage devices
Sensors
Microprocessors
Printers
Data convertors
Memory chips
Port extenders
Display drivers

A key difference between SPI and Parallel is that with a serial interface, it only allows for transferring data one bit at a time but decreased the pins required, as opposed to the parallel, which allows multiple bits at a time, but requires more pins (8 data pins and 3 controllers). The downside with a SPI is that you can’t read from the display you can only write on it, and it’s typically slower.

I²C

I²C, Inter-integrated Circuit pronounced I-squared-C or I-2-C for a less technical term, is a serial protocol for two-wire interface to connect low-speed devices like micro-controllers, EEPROMS, A/D and D/A converters, I/O Interfaces and other peripherals in embedded systems. It was designed to allow easy communication between components which reside on the same circuit board. I²C only requires two wires: SCL (serial clock) and SDA (serial data). It is a multi-master, multi-slave, single-ended, serial-computer bus, (a communication system that transfers data between components inside a computer or between computers) and was invented by Phillips Semiconductor.

Developed in the mid 1980’s, I²C has been introduced to the market by competitors, along the tech lines of Siemens AG, which is now Intel Mobile Communications, NEC, Texas Instruments, Motorola (later Freescale) and Intersil.

SMbus, (System Management Bus) developed by Intel in 1995, is a subset of I²C, which defines the protocols more strictly. Modern systems employ rules and policies from SMbus, sometimes supporting both systems, requiring minimum reconfiguration. Since 1982, there have been seven revisions to the I²C interface, and has evolved, as every other interface, with new technology always on the horizon.

SPI vs. I2C:

As far as these two TFT display interfaces, we find that SPI is more popular than I2C when designing a custom LCD. We get hit with questions such as:

Why is SPI more popular than I2C?
Which interface (bus type) is best?

TFTs and OLEDs are standard, off-the-shelf displays that come with the interface already chosen for you. In many of the TFTS that Focus Display Solutions offers, the built-in controller allows the user to select from multiple display interfaces. Including RGB (Red, Green, Blue).

As a general rule, the larger the display the better it is to choose a LVDS interface since it transfers data so quickly. LVDS is more expensive than SPI, I2C, RGB and parallel. If you are not sure which display to use, try our online Quick LCD selector tool. The displays in this selector tool are in-stock and can ship the same day.

Need a LCD for a new project? Not sure which technology to choose? Contact a real human at Focus Displays now to begin your design process by calling us at 480-503-4295. Or, you can fill out the contact form and we’ll email or call you immediately.