Touch controller and display driver chips are the two chips that make a touch screen work. The touch controller senses where a finger touches the screen. The display driver chip receives each image from the phone's main chip and sets the brightness of every pixel. In many phones, the two jobs are combined in one chip, called a TDDI chip, short for touch and display driver integration. Laptops use touch controllers in their touchpads too.
Most touch screens sense a finger by its effect on electric charge. Under the glass is a grid of transparent electrodes, thin lines of a clear material that conducts electricity, laid in rows and columns. The touch controller sends a small electrical signal along each row in turn, and measures how much of it reaches each column. A finger conducts electricity. Where it touches, it draws away part of the electric field, so the signal measured at the nearby crossings drops slightly. The controller scans the whole grid 120 to 240 times a second. From which crossings changed, and by how much, it works out the finger's position to a fraction of a millimetre, and sends the position to the system-on-chip. Measuring small, smoothly changing signals and turning them into numbers is analog work, which is why touch controllers are often called mixed-signal chips.
The display driver does the opposite job: it turns numbers into light. A 6.7-inch phone screen with 1080 × 2400 pixels has about 26 lakh pixels. Each pixel has three sub-pixels, red, green and blue, so there are about 78 lakh sub-pixels. On a 120 Hz screen, the display driver receives 120 images a second from the main chip. For each image, it sets the voltage or current of every one of those sub-pixels.
Say a delivery rider in Indore gets a new order alert on his phone while it is raining. He taps "Accept" with a wet thumb. The raindrops on the glass also change the signals, but in a different pattern: a thin film of water changes many crossings a little, while a thumb changes a few crossings a lot. The controller's firmware uses this difference to ignore the water and report only the thumb. Within about 10 milliseconds, the main chip receives the position. The app accepts the order, and in the next frame the display driver draws the new screen.
Phone brands choose these chips together with the screen, as part of smartphone design. The same kinds of chip sense fingers on the touchpads of laptops and on keyboards and other computer peripherals. Display drivers also light the screens of a car's instrument cluster, which shows the speed and warnings in front of the driver. A few companies design these chips, some of them in Bengaluru.