Tag Archive for: SPI

I’m trying to figure out how multi-touch controllers work and have a few questions: Why are certain multi-touch controllers unable to sense more than 2 touches? What are the device limitations? How does a 10-point sensing controller overcome this?

Hi Raji,

So, on this Valentine’s Day you want a date with Multi-Touch? Or are you looking to make that special commitment? Either way, I can help you out …

As Touch Guy says, the best predictor of tomorrow is what happened yesterday. In the long 25 year reign of resistive touch, the electronics went from dedicated touch controllers, to single ASIC’s, and finally to touch controllers as a simple four or five wire connection to the LCD driver chip. For multi-touch, we are in the early stages transitioning from dedicated controllers to ASIC’s. All of the dedicated controllers communicateTouchGuy_Basic with the host via USB. In Touch International’s case, the communication format is Microsoft Windows 7 tablet, which makes the touches look like a whole mess of absolute mice to the host. Using this protocol format, both Win CE 7 and Win 7 will work using the built in driver. Linux also has a number of open source drivers available, and if you follow the common implementations, this will be easy.

Today you can pick your life partner (or at least your touch screen ASICS), as there are about twenty multi-touch projected capacitive ASICS available, with even more coming. Of course, each has its strengths and weaknesses, some of which are cost, signal strength, noise rejection, pen sensitivity, allowable thickness of cover glass, speed, number of detectable touch points, availability, and so on. And, if you are looking to make that life-long commitment, but are worried about compatibility, then rest assured that of the ASIC’s I know, all have both I2C and SPI interfaces; a few have USB built in, but a USB peripheral chip can be added to any of the interfaces.

Now, if you’re looking to make multiple commitments (think HBO’s Big Love), there is no I2C dependency and you can run multiple partners on the same bus. Some of the ASIC’s have the address hard-wired in, and some ASIC’s allow the programming of the address. Regardless, you can talk to multiple suitors at the same time. Be aware, however, that there is no “standard” touch format on the I2C, as there is with our USB implementation, so how you communicate your touches will be up to you in some cases, and up to the partner chip in others.

And Touch Guy is off to bestow the ladies of his life with chocolate covered cherries.