Listen to this article

Welcome to Riverdi University! In this lecture we will talk about interfaces in LCD display modules.

I will try to organize the kinds of display interfaces we have on offer, and how they differ between each other. After this video you will know what kind of external and internal interfaces we have and what their main applications are.

Interfaces in LCD Display modules – SPI, I2C, LVDS, MIPI, VX1, EDP and others.

First, let us start with the division of interfaces into internal and external interfaces in LCD modules. Internal means used inside the device; they usually are embedded interfaces that are not visible, and we do not have access to them as the users of the device. External interfaces, on the other hand, are connected to the device using a cable. Once we have defined internal and external interfaces, both of these categories come as universal or image transfer interfaces.

What is an interface and what is a protocol?

A protocol defines the rules of information exchange, where the interface is the medium. The example here will be language. When I use my voice to communicate with other people, my voice is an interface. Over this interface my voice is being sent to other people’s ears, and the protocol is the language used. Right now, I am using the English protocol. If you understand the protocol, you understand what I am saying. If I switch to a different language, Polish or some other language that you do not understand, you have the same interface, you will still hear me, but because of a different protocol, you do not understand me anymore. In this lecture we will talk only about interfaces, how to connect devices to each other; we will not talk about protocols.

What is an interface and what is a protocol?
Universal interfaces and Image transfer interfaces

Universal interfaces and Image transfer interfaces in LCD modules

Let us try to organize interfaces. For internal interfaces, interfaces that are embedded into the device, we have universal interfaces and image transfer interfaces. Universal means we can send other data through them, not only an image. Being universal, they are usually not perfect for image transfer, because in most of the displays used nowadays, the image transfer is one of the most demanding. The bit rate, the data transfer needed for the image transfer usually is very high, higher that many universal interfaces can give us. If we need to send an image only once per some time, we do not need very high bandwidth. If we do not have live video then we can use some of the internal universal interfaces such as SPI, I2C or even slow interfaces as RS232 or UART, that are very similar.

SPI (Serial Peripheral Interface).

The first universal interface will be SPI (Serial Peripheral Interface). This interface is serial, and it is used to communicate between a host, in SPI called a Master, and devices called Slaves. One host can communicate with many slaves. To select the Slave, we use the Chip select or SS line and then we use two data lines, Master output or Master input and of course the clock, to synchronize the data, because this is a clock synchronized interface.

SPI (Serial Peripheral Interface).

It can be fast, but is not fast enough for live video. The baud rate can be 1 MBd, but it can also be 10 MBd or even 50 MBd on the SPI or QSPI. QSPI is a Quad SPI, a kind of modification of SPI that is faster. But still this interface is very universal, we can use it to connect memory or some input, outputs internally in our device. SPI in the display world is used for simple displays, for small size displays, where we can transfer the image relatively fast, because the resolution is low. The maximum for SPI display would be 3.5 inch, 320 by 240 TFT displays. If we have higher resolution, image transfer will be too slow to use SPI even with a high-speed SPI.

I2C interface

I2C interface

Next, we have the I2C interface. This kind of interface is usually slower than SPI. It uses only two lines, so one is a clock for synchronization, and the other one is a data line. The data line here is bidirectional, so in SPI we have two data lines, one outgoing and one incoming, and in a I2C interface we have only one data line.

If, for example, the Master is sending some data, the Slaves can only receive it and then for the Master to finish it we need to wait a little bit. We can then respond as a Slave to the Master and here also to select the Slave. It works a little bit different because in SPI, as you remember, we had a Chip Select line (CS line) or SS line and in I2C we first need to send the logical address to the interface that is being written by the slaves. In general, this is slow and universal interface used also to connect the simple memory and some other I2S that we have around our microcontroller on the PCB. It is very useful, but usually not used for image transfer. This interface is very popular in the display world for touchscreens. Most of the embedded touch screens that we use have I2C interface because the touchscreen does not generate many data, we only have coordinates of the finger or few fingers at most, that need to be sent back to the microcontroller, to the device processor. The slow baud rate is good enough for the touchscreen, but not enough for the image.

RS232 interface

The next interface, a very old one nowadays, is RS232. This is also a serial, slow interface, that can be used internally in the device or externally. This interface is not only internal, it can be external, and on the picture above we can see the external connectors, but it is still being used internally because it has a variation – UART.

RS232 interface
UART interface

UART interface in LCD modules

The UART is basically the same as RS232, but it is a fully internal interface. It is pretty slow. We have a TX line and a RX line – a Transmit Line and a Receive Line. We do not have a clock here, we only have a clock to synchronize the device internally, but the clock signal is not sent out. So, we need to synchronize the data that is coming through the lines and to do that we need to set the same baud rate on both sides of the communication line. That means that before we use UART we need to agree first what speed we will use.

That is not a case for SPI or I2C, because we have a clock there that gives the speed to every device. Then each device works according to the clock. In UART we do not have a clock; it is rather not used for image transfer. The UART, or SPI, or I2C can be used for low resolution displays, but for high resolution displays we need an Intelligent Display, a display that will generate the image internally and through these slow universal interfaces we only send commands, or we send the image once, the image is being stored into the internal memory of the intelligent display, that we will use later sending the commands. You can find Riverdi’s intelligent display line on our website: https://riverdi.com/product-category/intelligent-displays/.

These Riverdi products are very advanced Intelligent Displays, made with Bridgetek controllers. The controllers use SPI and QSPI for communication. That means your software, your system, your microcontroller can be simple, you can use SPI interface to drive them, and you can still have high resolution image, even as high as 1280 by 800 in 10.1-inch LCD displays. So, please remember that if you want to use a slow universal interface and have a high-resolution image, you need to use an Intelligent Display.

Internal image transfer interfaces in LCD modules

Next, we have internal image transfer interfaces. The image transfer interface allows us to transfer the image continuously at a very high speed, high enough to refresh the display many times per second. This is called the refresh rate of a display. When you go to a display, monitor, or TV set specification, you will see something like the refresh rate or maximum refresh rate that this display can achieve. If we have 60 Hertz, that is 60 times per second, or 100 Hertz for more advanced displays. That means we need to send the full image 60 times or 100 times in each second. To imagine how much data it is, we need to multiply it by the resolution of the screen. For example, for a 7-inch Riverdi LVDS display with resolution 1024 by 600 it is roughly 600 thousand pixels.

It was explained in the LCD TFT display modules – theory, special features, comparison lecture, LCD TFT displays have three color subpixels, so we have three times more data to be sent via the interface. 600 thousand pixels multiplied by 3 gives us roughly 1.8 million subpixels. To each subpixel we need to deliver data about its ON and OFF state and about its brightness. We usually use 8 bits for that. So, when we have 2 mega subpixel display and 8 bits brightness, we need roughly 16 million bits frame to be transferred and with 100 Hertz refresh rate of a screen we need to do it 100 times per second. The image transfer interfaces need to be really, really fast and they work at a hundreds of Megahertz baud rate or even a Gigahertz rate to be able to transfer this amount of data every second.

LVDS – Low Voltage Differential Signal interface

LVDS – Low Voltage Differential Signal interface

The most common internal image transfer interface in industrial LCD displays nowadays is LVDS – Low Voltage Differential Signal. A very important feature of this interface is that it is differential, that means it is immune to interference and also, that we can use a twisted pair of wires to transfer data. We can send data really fast and it will not be broken down by any noise, interference, what could happen quite common in other interfaces. Please remember the differential signal allows you to send the signals at very high speed and be safe from noise.

RGB interface

The next, older image transfer interface, is called RGB, from the colors: red, green and blue that are sent parallelly to the display. LVDS is a serial interface, RGB is a parallel interface, but the biggest difference is that it is not differential, so it is easier to disturb with noise and you cannot go too high with the speed of this interface. Parallel interface means that we send every bit in a separate line, so in theory this interface could be very fast, but because it is not differential this speed is limited, and we can find the displays with RGB interface only up to maybe 7-inch or 10-inch sizes.

RGB interface

12 inch is the total maximum, and 12-inch LCD display with RGB interface will be lower resolution like 800 by 600, so very low for this display size. That is the reason 7-inch is a size above which LCD displays are being switched from RGB to LVDS interface. You can see that among our products (if you go to the Riverdi website and to the IPS display tab), are displays without the controller, you will see that small displays like 3.5-inch, 4.3-inch and 5-inch are with RGB interface, then when you go to the 7-inch LCD displays you will find RGB and also LVDS and MIPI displays (I will talk later in this lecture about MIPI), but when you go to the 10-inch or bigger displays, you will find only LVDS displays because our 10-inch LCD displays are high resolution 1280 by 800 and it is impossible to build it with the RGB interface. Please remember that RGB is rather low speed and not very immune to noise, so we use it for the smaller size displays and when we do not use high resolution.

MIPI – Mobile Industry Processor Interface

MIPI – Mobile Industry Processor Interface

An internally embedded image transfer interface that is getting more and more popular right now is MIPI – Mobile Industry Processor Interface. This kind of interface is used usually in mobile applications, tablets or mobile phones but it is slowly entering industry as well. In Riverdi we offer 7-inch MIPI displays, but please be careful with other MIPI displays on the market. Many of them come from mobile phones or the tablet market, and TFT glass availability may not be stable as this market changes really fast, every six months or every year. When you buy a 7-inch Riverdi display with the MIPI you are safe because this is an industrial display.

That is why we have a very limited number of MIPI displays, because we want to be sure that what we have in our offer is always available for a very long time. Longevity is one of Riverdi’s core values, we do not want to deliver anything that will not be supported for a minimum 3 to 5 years later because many of our customers are building industrial, medical or military devices and they need displays to be available a very long time. Anyway, MIPI is a very important and growing interface in the display field.

Vx1 image transfer interface

Next, we have the Vx1 interface. It is a LVDS kind of interface, like MIPI, so low voltage differential signal. It is a very high-speed interface, usually used in large high-resolution screens, for example 55 inch or larger 4K TVs. When you buy this kind of a TV set right now, probably the embedded interface inside will be just Vx1. So, Vx1 is a very fast interface used for high bandwidth, with high refresh rate and high-resolution displays, 4K and above.

Vx1 image transfer interface

eDP image transfer interface

The last internal image transfer interface is Embedded DisplayPort, eDP in short. We call it the new LVDS because many new industrial displays are coming with the eDP. If you go through industrial manufacturers of TFT LCD displays, you will see that there are more and more models available with the eDP. eDP is also a native interface that you can find in some new processors, like Intel or AMD based processors. They may have the embedded DisplayPort as a native display interface, that means you do not need anything extra to connect a display to the processor, so it drives the cost down.

So, we have the processors on the market, now we need displays on the market with embedded DisplayPort. Many laptops or monitors right now use embedded DisplayPort as an internal interface instead of LVDS. LVDS is still the most popular industrial LCD display. All the internal image transfer interfaces like MIPI, Vx1 and eDP are variations of LVDS, where the protocols are different, and the signals are a little bit different. For example, for eDP we can have lower noise and reduction of power; all of them have some advantages over regular LVDS, but they are all of LVDS type.

eDP image transfer interface
External interfaces

External interfaces

Now we can jump to external interfaces. We covered many internal interfaces, universal like SPI, I2C, RS232 UART and image transfer very fast interfaces like LVDS, RGB that was a parallel one, MIPI Vx1 and eDP Embedded Display Port; and now let us look at external interfaces, the ones that we usually have access to. For example, it can be HDMI to connect our computer with the monitor or TV then DVI, usually used for monitors only, VGA that is an old image interface for monitors and DisplayPort that is a new HDMI, and universal interface USB-C, the most common and universal interface nowadays used to connect devices to each other.

USB C universal external interface

Let us start with the universal one that will be USB and particularly USB-C, one of the best interfaces that we have ever designed because this interface is a really fast one and also very universal. That means it can not only transfer data, not only but because it is fast enough to transfer image, but because it can also transfer a lot of power.

USB C universal external interface

You can transfer up to 100 Watt of power, because you can increase voltage and current. In a regular USB it is usually 5 Volt and 0.5 or 1.0 Amp, so a few Watt only, but for USB-C you can go with the voltage as high as 20 Volt and with the current 5 Amp in total so we can have even 100 Watt of power. That means this interface is not only for data, but for real power transfer. You can not only charge a phone, but you can also charge your laptop through USB-C. If you buy a new laptop right now, maybe you will not get a regular power connector, but a USB-C only. The USB-C is a very smart interface, if you connect the devices, they can negotiate with each other which one has more power. For example, if we connect a charger to a laptop, the charger has more power and will charge the laptop, but if you connect the laptop with the same interface to your mobile phone, then they will discuss with each other, and of course the laptop will be charging the phone. So, it is a really smart and fast interface to be used and we can build displays with the USB-C. You can find monitors on the market that already have a USB-C instead of HDMI. This kind of monitors can be powered from your computer so they have only one cable, both for image transfer and power from USB, and for sure in the future we will see more and more USB-C implementations.

HDMI image external interface

HDMI image external interface

Let us now move on from universal USB interface to image transfer interfaces. The first, most common one on the market is HDMI – High-Definition Multimedia Interface. A very popular and easy to use interface. Multimedia, because it transfers not only image, but also sound, so if you connect your computer to your TV set, you need one cable for both, the video and the sound. We have a few variations of HDMI connectors, standard HDMI, mini-HDMI, and micro-HDMI, a little bit different connector, but the pinout and everything else is the same.

You have app. 15% of the article left. That content is exclusive for our Riverdi University members only. Please fill out the Riverdi University Membership form below and join our community!

DVI image external interface

The next one is DVI – Digital Visual Interface. The first DVI was not a multimedia interface, that means it did not have audio. Some variations can transfer audio, but it is non-standard, so DVI is only for image transfer. It is a digital interface, very similar in signals to HDMI. The latest variation like DVI-I, that means integrated interface, can have a digital and analog part for VGA compatibility, but on the picture above there is a DVI-D, only digital, where we do not have the pins for analog VGA interface, that you can sometimes find in your desktop computer but not in laptops anymore.

VGA image external interface

The oldest video interface that is still being used but is less and less popular, is the VGA – Video Graphic Array interface. This is an analog interface, not a digital one like all the other interfaces that we have talked about in this lecture. Analog means that we do not send the bits over the interface, but we send the voltages values over. The analog signals are not very stable, they are quite easy to disturb, so the transfer here cannot be very high. If you have a high-resolution display, or if you have a noisy environment, VGA is not the best solution to use, and it is becoming less and less popular.

DP (Display Port) image external interface

The last external interface that we can find in our devices nowadays is a DisplayPort. DisplayPort is similar to HDMI or DVI. It can also transfer not only image, but also the sound and it is faster than the HDMI. Usually, the DisplayPort is used for high resolution displays, for new monitors and TVs with 4K or 8K resolution where it is really hard, or impossible even, to achieve such resolution using HDMI interface.

Please remember to SUBSCRIBE to our YouTube channel and fill out the MEMBERSHIP FORM, to be informed about our Riverdi University materials and live events!

Join our Riverdi University Membership community! Please complete your name and email address to unlock the content.
I agree to the Riverdi Sp. z o.o Terms & Conditions and Privacy Policy. I also agree to receive emails from Riverdi Sp. z o.o and I understand that I may opt out of Riverdi Sp. z o.o subscriptions at any time.