Home | Product Categories | SMD ICs | COM-07902

Why did this guy had to mention it! :/

If you are okay with soldering it yourself, Sparkfun sells an SSOP to DIP Adapter Board.

Difficulties in building a capacitive touchscreen

Building a capacitive touchscreen would be analogous to making your own LCD – It requires manufacturing processes you probably don’t have, and requires several hundred IO pins. This is not analogous to an LCD controller – It’s more like an LED driver with which you can build a 7-segment display.

A real capacitive touchscreen has rows and columns which are perpendicular on opposite sides of an electrode substrate (usually glass for a touch-SCREEN-, but possibly PCB material, flex PCB, or a plastic film for an opaque input device). The controller scans each the rows and columns, measuring the capacitance between each pair. A change in capacitance indicates the presence of a finger or stylus at the intersection of the row and column. This requires a lot of electrodes on the glass, a lot of pins on the IC (Far more than 16!), and a fast interface over which to report the data. These processes are usually restricted to high-volume manufacturers, so the chips and datasheets are usually similarly restricted.

What you’d get on a touchscreen project with this part

You could simulate a very low resolution capacitive touchscreen with this part. This one only has two channels, so you could get a 2 pixel grid.

The MPR121QR2 has more channels. Its breakout board has 12 buttons on a PCB, but by arranging these in a 6x6 or 5x7 pattern, you could get a reasonable approximation of a touchscreen. You’d need the electrodes to be small enough that a finger touched a row and a column simultaneously; that part doesn’t do differential row-column scanning.

What is it good for?

This part isn’t even a very good button detector. Other parts have many more channels, and more flexible button-press detection mechanisms, slider and grid configuration options, are cheaper, have a higher update rate, and usually support guard traces. However, the awesome thing about this part is it’s 24-bit, 4 femto-farad resolution measurement capability.

Other parts don’t report this information very well. They focus on processing it for you and provide you with keypress data. The MPR121 provides 10-bit data, stating that “the data range is 0-1024” and not much more. The Atmel Qtouch parts provide 16-bit data, but it’s similarly unclear how this can be interpreted to measure capacitance or how accurate it is. The only information on this measurement is This command returns a 2-byte report containing the most recent measured signal for key ‘K’. The signal is returned as a 16 bit number, MSB first. The Freescale MPR08x series doesn’t give any access to this information at all.

This part includes a 24-bit ADC, with a temperature sensor and voltage reference to make this ADC more accurate. The ADC is exhaustively specified. It doesn’t even mention button sensing. If you decide to attempt manufacture of a touchscreen, this would be a great tool to accurately measure the capacitance developed between channels on your prototype. This is a capacitance sensor (and a very good one at that!), but it’s not a touchscreen, button, or slider controller.