In my first project log, I had designed a PCB for interfacing with the touchscreen digitizer, based around Atmel's mXT2952, which was the only device I managed to find that had:
- datasheets available
- enough drive / sense pins for the iPad's screen (40 drive, 30 sense)
- a reasonable minimum order quantity
Unfortunately, the mXT2952 is a tiny 5mm x 10mm 162-pin BGA, with 0.5mm pitch. This is a tiny, tiny package, and they made it extremely dense -- many BGA chips have a few rows of pins, some blank spaces, then a few more rows of pins. To fit a trace between two pads, you need 3 mil traces and 3 or 3.5-mil spaces. To fit a via between four pads, you need vias that are less than 0.25mm diameter / 0.15mm drill. While there are some PCB manufacturers online that claim to have these specs, the cheapest I was able to find was about $150 according to their online quote form (others seemed to be $300 and up). When I engaged with the cheap one, they told me they couldn't actually do 0.15mm drill holes.
So I sat on this problem for a while, thinking maybe I'd eventually bite the bullet and spend several hundred dollars on a more expensive manufacturer.
The other night, I searched the web again for "capacitive touchscreen IC", and saw some results for the Goodix GT911 and GT9271. The latter is used in the Pinetab, and it's one of the first touchscreen controllers I considered. However, the GT9271 only has 32 drive / 20 sense pins - not enough for the iPad's touchscreen, and the GT911 has even fewer.
Long ago, I had tried looking on the Goodix webpage about their touchscreen controllers, which indicates that they have controllers that should work for even larger displays. This page has zero links or product numbers on it, however.
I searched around to see if I could find a full listing of all the ICs in the gt9xx family, and searched for every part number I found. Eventually I stumbled on the GT9110 and GT9113 (which seem to be nearly pin-compatible). Both of these have datasheets available online (not from goodix.com, of course), were available for purchase on AliExpress, and have a reasonable 0.4mm-pitch 88-pin QFN package.
I've created a new PCB design based around the GT9113 (should work for the GT9110 also - some of the broken-out pins become NC on the 9110).
As always, if you have any feedback, please let me know!