Displays We Love Hacking: LVDS And EDP

There are times when tiny displays no longer cut it. Whether you want to build a tablet or reuse some laptop displays, you will eventually deal with LVDS and eDP displays. To be more exact, these are displays that want you to use either LVDS or eDP signaling to send a picture.

Of the two, LVDS is the older standard for connecting displays, and eDP is the newer one. In fact, eDP has mostly replaced LVDS for things like laptop and tablet displays. Nevertheless, you will still encounter both of these in the wild, so let’s start with LVDS.

The name “LVDS” actually comes from the LVDS signaling standard (Low-Voltage Differential Signaling), which is a fairly generic data transfer standard over differential pairs, just like RS485. Using LVDS signaling for embedded display purposes is covered by a separate standard called FPD-Link, and when people say “LVDS”, what they’re actually talking about is FPD-Link. In this article, I will also use LVDS while actually talking about FPD-Link. Barely anyone uses FPD-Link except some datasheets, and I’ll use “LVDS” because that’s what people actually use. It’s just that you deserve to know the distinction so that you’re not confused when someone mentions LVDS when talking about, say, industrial machinery.

Both LVDS and eDP run at pretty high frequencies – they’re commonly used for color displays with pretty large resolutions, so speed can no longer be a constraint. eDP, as a successor technology, is a fair bit more capable, but LVDS doesn’t pull punches either – if you want to make a 1024 x 768 color LCD panel work, you will use LVDS, sometimes parallel RGB – at this point, SPI just won’t cut it. There’s a lot of overlap – and that’s because LVDS is basically parallel RGB, but serialized and put onto diffpairs. Let me show you how that happened, and why it’s cool.

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Using Cheap Displays With The Raspberry Pi

The Raspberry Pi B+ has a native VGA connection. Sure, it’s hidden away in binary blobs and device trees, and you need to wire up the GPIO pins just right, but it’s possible to connect a VGA monitor to a Raspi B+ natively. For the brave, smart, or foolish, this means you can also drive raw DPI displays. [Robert] had a few of these dirt cheap displays sitting around and decided to give the entire thing a go. It worked, and he’s written down how to do it.

One of the chip architects for the Raspberry Pi, [Gert van Loo], was exceedingly clever when designing the Pi. There’s a parallel interface in the chip that, when combined with a few dozen resistors, can drive a VGA display in addition to the HDMI display. Screens with a Display Parallel Interface are actually pretty similar to what the VGA spec calls for. The problem is, hardly any of this is documented for the Raspberry Pi, and finding it means trawling through forums.

[Robert]’s example circuit uses a 5″ display from Adafruit, a 40-pin breakout, and a bunch of prototyping wires. Setup requires grabbing a cut down version of the device tree used for the Raspi VGA breakout board, setting the output format, rgb order, and aspect ratio of the display, and wiring everything up.

What’s interesting here is that [Robert] reproduced this project from scratch, and found that any display with a 40-pin DPI connector will work with the Raspi, provided you have a datasheet. That’s pretty cool; these displays can be cheap, and since we don’t yet have a proper DSI display for the Pi, this will have to do for now.

Video below of [Robert]’s inspiration for this build, [Ladyada].

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