# Reverse-engineer pinout of capacitive touchscreen

I'm interested in reusing the capacitive touchscreen of my old iPad. (As a learning experience -- I'm aware that this project will cost me more than just buying a new capacitive touchscreen would.)

The digitizer has a ribbon cable: (Picture from the iFixit teardown)

This connects to two 51-pin FPC connectors, seen in the bottom right of this picture, also from the iFixit teardown:

My understanding of capacitive touchscreens is that they have a set of conductors going horizontally across the screen, and a set of conductors going vertically across the screen, called the X lines and Y lines. These then connect to a controller, which measures mutual capacitance between each pair of (X, Y) lines, and does some algorithmic magic to turn that into touch events.

Since this connector has so many pins (more than just a serial interface), I'm assuming it brings the X and Y lines to the logic board, where it will connect to a touchscreen controller.

Without destroying anything, how do I determine the pinout of this connector?

• If you can refine the search, it's becoming common that pin out diagrams are available at places like Alibaba. For example, this page for a 51 pin 480x800 ips 3.97 lcd touch screen module with MCU/RGB interface has a pinout that might be exactly what you want - assuming your touch screen is 480x800. – JBH Dec 22 '20 at 0:13
• the touch panel probably uses only 4 or 5 pins – jsotola Dec 22 '20 at 5:05
• @jsotola Then what are the other 97 pins for? The LVDS connector for the LCD and backlight is a separate cable. As far as I know, this 102-pin ribbon cable is only for the touch panel. – Evan Krall Dec 22 '20 at 6:47
• I was under the impression that capacitive touch screens didn’t actually need that many “lines” (that would probably be more for a resistive touchscreen). On a small scale, just connectors in the corners would be enough to detect a change in capacitance across the screen. There are probably more lines to handle the relatively large area as well as multitouch. You should probably look up touch screen controllers, those will probably give you the relevant details. – jcaron Dec 22 '20 at 8:54
• The year down says the logic board bears a Texas Instruments CD3240A1 touch screen controller. Can’t seem to find a day sheet for that though, all results point to iPad. – jcaron Dec 22 '20 at 8:59

After reading this blog post series, I realized that schematics for this model of iPad are available if you search around for them. This helps me, but doesn't help someone else trying to reverse-engineer a digitizer pinout for a different product, so here is some information that may be helpful:

• There are a few types types of pin: ground, transmit, receive, and driven shield pins (which my device doesn't have.)
• 5mm is a common electrode spacing (i.e. spacing between adjacent transmit lines or between adjacent receive lines). The mXT1664T3 datasheet also lists 3.8mm, 6mm, and 6.5mm. You may be able to estimate how many X and Y lines there will be based on this.
• The X spacing and the Y spacing will likely be the same.
• The ribbon cable may have connections to one or both ends of the receive lines. (On this device, there are two connections for each receive line (one for each end?), except for MT_PANEL_IN<29> which only comes to one pin.).
• The ribbon cable should be laid out in a way that minimizes mutual capacitance between transmit lines and receive lines -- transmit lines will be grouped together, receive lines will be grouped together, and there will likely be ground or driven shield pins between transmit and receive.
• Transmit lines will have relatively high voltage signals on them (e.g. 18V on this iPad), whereas receive lines will have small signals (millivolts).
• Adjacent pins on the connector will usually map to adjacent lines on the display. (Though not necessarily monotonically increasing. On mine, one section of cable counts from 0 to 29 then back down to 24.)

# Visual inspection

To minimize capacitance from transmit to receive lines within the ribbon cable, my device has wide ground traces separating transmit lines from receive lines. These are faintly visible on the cable itself as slight variations in thickness. This may give clues as to the groups of pins.

In the photo below, I've highlighted the transmit pins in red, the ground pins in green, and the receive pins in blue.

# Identifying ground pins

Find another ground on the device, and use a continuity tester to identify which pins are connected to ground.

None of the pins on my digitizer ribbon cable connected to any of the case screws on the digitizer, but on the logic board side I was able to identify two ground pins. The two ground pins on the digitizer had about 320Ω between them.

# Identifying transmit pins

These pins are driven by the touchscreen controller. On this device, they're driven with an 0-18V square wave, with a few dozen cycles to one transmit pin before moving on to the next.

To identify where the line connected to a particular pin is, you can tape a wire to the screen, parallel to the transmit lines. This wire should capacitively pick up some of the signal from the transmit lines, which should be visible on an oscilloscope. The strongest coupling will happen with the closest line.