MT6820 board checklist for no-image debugging

A project log for All About Laptop Display Reuse

LVDS, eDP, connectors and controllers - let's learn it all

ArsenijsArsenijs 06/14/2021 at 11:570 Comments

When it comes to reusing old laptop screens on a budget, MT6820 (also known as MT561) is a great board - for just $5, you get a VGA-capable board, a rewire-able cable and a button board. Here's more info on the MT6820 board, if you're curious.

Let's assume you got one, followed the info available here (or somewhere else) to make it work, and it... doesn't. What do?

Important note - this mainly talks about "no image"/"garbled image" scenarios, I'm not talking about "no backlight" scenarios, since backlight is usually taken care of by an inverter, an entity that has to be debugged separately.

1. Check that the panel voltage is set up correctly (likely, to 3.3V)

This kills the crab screen. If you've powered up your setup with the wrong voltage setting, the panel might be broken already. However, if you're sure your screen is 5V-powered, then it will require 5V to work and likely will not work with 3.3V. Having said that, overwhelming majority of laptop screens are 3.3V-powered.

2. Is the keypad board plugged in correctly?

The keypad board cable is two positions shorter than the connector on the MT6820 board. Here's how you should be plugging it in:

3. Is the LVDS connector rotated the right way?

The LVDS connector on the MT6820 board is a 2mm 2-row header. It's very possible you might plug it in the wrong way around. Look at the bottom of the board, the connector's pinout is listed there - VCC pins should match the red wires when you have the cable plugged in.

4. Is the cable pinout correct?

First of all, the stock MT6820 cable doesn't work with laptop displays unless the store listing where you got it from explicitly states so. Here's how you rewire the cable you got if your panel uses the same connector but a different pinout. Alternatively, here are the cables you can buy, one of them is likely going to be suitable for your panel.

Even if you got the correct cable, it doesn't hurt to check. Go find information on your laptop panel, and if you find a datasheet (likely), check which pinout it uses and whether the cable you got matches the cable that goes with that pinout (there will be a "Suitable cable" link right under the pinout description). If you can't find a datasheet, come to our chatroom and let's see how we can help you.

5. Are you connecting 5V to the board with the correct polarity?

The MT6820 board needs 5V power. Sadly, the MT6820 board doesn't have a dedicated power input connector, you need to either solder to 5V and GND pads somewhere on the board to power it, or use the JST-6 connector that's meant for the inverter. If you accidentally apply reverse voltage to the board, you might have a bit of trouble - I haven't had any of my boards die from that, but it will most certainly not work.

6. Are you setting the resolution+mode jumpers correctly?

On your specific board, jumper settings might be a bit different from the ones available online, in the store listing description, or on my MT6820 description page. I highly suggest you un-peel that dumb sticker on your board that covers the silkscreened pinout table, and see whether the jumpers you've set match the resolution and the LVDS mode that you'd like to see. Also check whether you haven't confused the jumper row orientation - make sure that A in the table matches the A jumper and you haven't accidentally set them in an opposite direction, that's happened to me before.

7. Is your PC (or other source) sending an image of the right resolution to the screen?

The MT6820 board can't do a whole lot of scaling on its own, from what I understand. If you send a 1024x768 image to the board with a 1280x800 screen, it might or might not work, so you better check that the controller (and therefore the screen) receives the data it actually can process.

8. Are you using a dual-link LVDS cable with a single-link LVDS panel?

This is a weird one. I'm yet to write a proper worklog about this problem - but, basically, none of my 1280x800 (single LVDS link) panels work with a dual-link cable, specifically, FIX-30P-S6. I've recently investigated this a bit more, and it's basically a guarantee for 1280x800 panels. If you're using a dual-link cable, you will want to separate and temporarily unplug part of the connector on the MT6820 board side. [TODO: add a picture of how the cable should be separated.]

I will be adding more checklist entries here as I find them, of course.