Wednesday, February 3, 2021

Displays -- good news and bad

In the midst of all this talk about the probe nose and the plumbing thereof, it's worth a note about our displays. You might recall that we've been limping along with a commodity "Waveshare" LCD and a Raspberry Pi. This is bulky, and whaddya know, Waveshare discontinued the model we were using! These are described in this post from November 2019.

I designed a new display board based on Jeremy Cole's design and the latest Raspberry Pi Compute Module 4. From the outside, it looks like this:

It's super small, and I've decided to eschew the knob to shrink the form factor even more. I designed a circuit board for it, which looks kind of like this:

At the top right of the leftmost photo, you can see two 0.1" pitch headers. One is for wired installation (accepts+5VDC in and UART signals) and one is an "expansion" header (provides +3VDC out and breaks out several of the GPIO signals). Both are sized so that a screw terminal block can be used.

Note also the two Hirose DF40C-100DS-0.4V(51) connectors to mount the CM4. We shall return to these....

When assembled, the insides are arranged something like this:
So with optimism -- and a whistle on my lips and a spring in my step -- I ordered some PCBs from PCBWay and some components from the Interwebs, and set myself the task of soldering all this stuff together.

Under the best of circumstances, this work is dastardly difficult. The 40-pin LCD connector and the TFP401 HDMI decoder chip are a pain in the neck to get right. But then, I have unsteady hands.

The Hirose connectors, though -- these are another ball of wax of a different color entirely. The connector pitch is 0.4mm, and instead of regular pins, they are actually made up of a whole stackup of vertical "plates" that suck up solder bridges in between them like crazy.

The results looked horrible. I even bought a cheap USB microscope and 3D printed a stand for it, just so I could examine my boards and prove to myself just how horrible they were:

Finally, I gave up and ordered a full PCB build from PCBWay. The cost, compared to buying my own components, is actually amazingly reasonable. When you add in the cost of my hassle and my loss of faith in myself and humanity at large, the extra cost is cheap indeed. And the folks at PCBWay are so incredibly helpful and nice!

But of course, this turns out to be the Chinese spring festival from February 6 through 18 -- who knew? I wish my PCB manufacturing friends a happy time. But meanwhile, I must wait for my boards. Do stay tuned, though. If they work, they ought to be super cool!

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