LCDs Will Drive You Up The Wall — Part 2

Brian Benchoff
Apr 23, 2020 · 4 min read
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A small 1.14" SPI-based display. Image: Adafruit

In the previous page of this guide, we took a look at the types of display interfaces available to hardware designers. The first installment of this guide looked at low-power, low-resolution, and (potentially) small displays. These are the types of displays you’d connect to a small microcontroller for an IoT thermostat, for instance. Or a smartwatch. Something like that.

For this installment, we’ll be taking a look at vastly more complicated display interfaces, and those usually mean larger, higher-power displays. That’s not a guarantee, though: small, low-power displays can and do use one of the following display interfaces. But we’re going with generalities here, not specifics.

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Before we dig into the specifics of these display formats, it’s a good idea to discuss VGA. Yes, the old blue video connector on your old computer, and the only way that corporate laptop can connect to the projector in the conference room.

VGA is a display standard, but it’s not quite a digital standard. In a way, it’s a lot like old broadcast TV.

There are only five connectors of any importance on a VGA connector. There are three analog pins for red, green, and blue, and also an HSYNC and VSYNC. For each frame a monitor displays, there is one pulse onf the VSYNC line. For each line of that frame, there is one HSYNC pulse. In between those HSYNC pulses, the analog pins spit out data for each pixel as an analog value. It’s simple, but it works.

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A parallel RGB display. Image: Raystar Optronics

The RGB display — an utterly un-Googleable term unless you say ‘RGB TFT display — is the digital successor to the VGA format. It differs in that where the VGA format spits pixels out in an analog format where each pixel is a distinct analog value for red, green, and blue, the RGB interface uses eight digital lines for red, green, and blue. That’s 24 different wires, along with an HSYNC and VSYNC.

The disadvantage to this display format should be obvious. You’ll need at least 26 pins on your microcontroller or SoC to control this display. The timing isn’t too difficult, with the maximum clock being 50MHz. This is withing the realm of a low-power microcontroller if you’re good with code.

In the early 90s, manufacturers needed more video, more bandwidth, and larger screens. They came up with LVDS, or Low-Voltage Differential Signaling.

Basically, LVDS transmits data serially over just a few wires. These wires are always in pairs, because of the way LVDS works.

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Instead of sending a signal down a single wire, LVDS uses a differential pair, sending the positive of a signal on one wire and the negative of a signal on the other wire. This is then reassembled by a receiver at the other end.

The benefits of this arrangement mean LVDS is nearly immune to interference. The downside is that it’s very complicated, and almost requires custom ICs. Texas Instruments has a wide portfolio of LVDS transmitters and receivers, some of which can convert more traditional signals into something an LCD panel can understand.

Up until now, it was possible for a simple microcontroller to ‘speak’ to any LCD panel using these technologies. Not so with LVDS. The speeds required for LVDS are fast, and you’ll really only find them in custom, dedicated hardware. Can an FPGA do it? Sure, but FPGAs are never the cheapest option.

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The DSI connector on a Raspberry Pi

Also in the early 90s, manufacturers recognized displays on portable devices had higher requirements. The smartphone hadn’t been invented, but that’s just the problem MIPI, more specifically the Display Serial Interface (DSI) solves.

Like LVDS, DSI uses differential signalling for resistance to interference and sends commands and data to the display.

Unlike LVDS, DSI is widely supported by nearly every OS, with device drivers available for Linux and Windows. It’s a common protocol, and can be found in everything from tablets to infotainment systems in cars.

Any hardware project you take on will need some sort of user feedback. Whether that’s through LEDs, buzzers, or a high-resolution display, you need to choose the right technology that will make it work. They all have their benefits and drawbacks but with the right know how you can get the right display for your project

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