Cypress has announced its fourth generation of touch screen controllers, claiming it will allow touch screen electrodes to be deposited directly onto LCDs without the need for noise suppression by air gap or electrostatic shield.
Further claims are that the screen will work dripping wet, or when the handheld is connected to noisy chargers.
"We have new analogue hardware that solves display noise. It is an analogue front end that listens to noise through the existing touch electrodes," Cypress marketing director John Carey told Electroncs Weekly.
The controller, which is built around an ARM Cortex core, does one of two things with the detected noise: "It can subtract the noise from the touch signal, which gives up to 1Vp-p immunity, or it can detect the display's vertical and horizontal syncs, and synchronise with them like a phase-locked loop to make its measurements in the quiet times, which gives up to 5V immunity," said Carey.
Noise levels here are measured with a scope connected to a conductive pad on the display.
OLED displays only produce 20mVp-p by Carey's measurements, and are a good, if expensive, solution to touch noise, allowing touch electrodes to be laminated directly on the display or even in the display.
For TFT (thin-film transistor, active matrix) LCDs, the figures are 500mV to 5Vp-p. At the top end, an air gap of up to 0.3mm, a screening ITO layer, or both, is required between touch screen and LCD.
"Customers don't like thickness. 0.1mm is everything to mobile phone guys," said Carey, who went on to assert that the Gen4 touch chips will allow touch electrodes to be laminated directly to any LCD, and also work with any phone charger.
The charger issue is inject common-mode power switching noise that makes its way to ground through the touching finger.
"Everyone deals with EN standard noise levels for phone chargers, but cheap chargers are a lot worse. One common one develops over 31Vp-p at 1-100kHz," said Carey.
Part of Cypress' attack on noise comes from increasing its own signal amplitude.
"Gen 4 is first touch screen controller to do 10V transmit. Every other controller drives at 2.7V," he said. "We take same 1.8V digital and 2.7V analogue supplies, and deliver 10V."
The other is that the firm combines both mutual-capacitance measurement - which gives unlimited multi-touch, poor projection, and a big signal - and self-capacitance measurement - yielding lots of projected field, but ghosting.
"We are still are the only ones that do self and mutual capacitance. We can do 1mm stylus, hover, and waterproofing."
Waterproofing?
"Mutual capacitance struggles with even sweaty fingers," said Carey. "Our works with a dripping wet screen using self and mutual algorithms that can understand what is water and what is touch," he claimed.
Maximum refresh rate is 400 co-ordinate/s, and active power is 2mW with two fingers on the screen and 60 co-ordinate/s - the standard for Windows Phone 7 and Android.
Calibrating the chip for a product has been simplified by a development kit that auto-calibrates parameters from a prototype.
For tuning the capacitance to a particular sensor, the sensor is hooked-up to the chip, which is in turn connected to a PC via an I2C-to-USB bridge.
A GUI on the PC sorts out timing between the chip and the panel.
"You get nearly 90% of performance compared to an expert set-up, and if you don't have serious noise problems or odd requirements, you get 100%," claimed Carey.
Prototype phones using the chip are expected at Mobile World Congress.
