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AVR259: ATtiny40 QTouch Reference Design
Features
• Up to 11 Atmel® AVR® QTouch® Channels - Single I/O pin per Channel ® 2 • TWI (Philips I C Compatible) Interface • SPI Interface • Two Analog Outputs (PWM) • Buzzer • LEDs • Supports QT600 Touch Debug Interface • TPI Programming Interface • Powered from USB or External Supply • Patents & Trademarks: ® ® - Atmel QTouch (patented charge-transfermethod) ® - AKS (patented Adjacent Key Suppression®) technology

8-bit Microcontrollers Application Note

Preliminary

1 Introduction
The reference design is to demonstrate the capabilities of ATtiny40, which supports QTouch, the latest capacitive touch technology. The technology requires only a single I/O pin per touch channel. The reference design supports two analog output channels usingPWM, buzzer and LEDs indicating touch, and TWI or SPI communication interfaces. It also allows complete configurability, and supports offboard QTouch sensors. The application note includes the firmware supporting seven keys, TWI slave interface (I2C compatible), buzzer and LED control. Figure 1-1. ATtiny40 QTouch reference design

Rev. 8332A-AVR-10/10

2 Hardware
The reference designsupports 11 Atmel® AVR® QTouch® Keys, and has provision to hook-up off-board touch sensors like keys, sliders and wheels. The latest QTouch technology requires only one I/O pin per channel, and do not require any external components. The QTouch channels are on the ADC pins. The two analog channels are controlled via PWM from ATtiny40. These channels provide the analog values when the corresponding keyis touched. This is ideal to replace an existing tactile switch (resistive ladder) TV control keyboard, or for any other application which does not require a communication interface to indicate a key touch or release. The buzzer and LEDs can be configured to provide an indication on power up or key touched/released status. Figure 2-1. System Block Diagram

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AVR259
8332A-AVR-10/10

AVR2592.1 QTouch
The latest acquire method is integrated in the AVR259, and requires the ADC for the measurement. The touch pins should be on the ADC pins for the capacitive measurement. The ADC module should be released for the library operation by the user application. The library will internally back up the user setting of ADC, reconfigure it for touch measurement, and once the measurements of allthe channels are completed, restore the backed up registers. The acquisition is very fast, typically in the range of 100µs to 300µs per channel at 4MHz. The acquisition method is singe channel at a time. Because the capacitive acquisition is primarily integrated, a Cs capacitor is not required as with the traditional QTouch method. The series resistor is required, however, for edge rate controland for EMC/ESD reasons. The sensor design remains the same as for the traditional QTouch sensors. Figure 2-2. QTouch Circuit Diagram

The latest QTouch method charges a sense electrode of unknown capacitance to a known potential. The electrode is typically a copper area on a printed circuit board. The resulting charge is transferred into an internal measurement circuit. By measuring the chargeafter one pair of charge-and-transfer cycles, the capacitance of the sense plate can be determined. Placing a finger on the touch surface introduces external capacitance that affects the flow of charge at that point. This registers as a touch.

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8332A-AVR-10/10

Figure 2-3. Charge Transfer Waveform

Signal processing in the decision logic makes QTouch robust and reliable. False triggeringdue to electrostatic spikes, momentary unintentional touch, or proximity is eliminated. QTouch sensors can drive single or multiple keys. Keys of different sizes and shapes can be used to meet both functional and aesthetic requirements. QTouch devices feature automatic drift compensation to account for slow changes due to ageing or changing environmental conditions. They have a dynamic range of...