Peripheral Touch Controller (PTC)

Overview

The Peripheral Touch Controller (PTC) acquires signals to detect a touch on capacitive sensors. The external capacitive touch sensor is typically formed on a PCB, and the sensor electrodes are connected to the analog front end of the PTC through the I/O pins in the device. The PTC supports both self- and mutual-capacitance sensors. In Mutual-capacitance mode, sensing is done using capacitive touch matrices in various X-Y configurations, including Indium Tin Oxide (ITO) sensor grids. The PTC requires one pin per X-line and one pin per Y-line. In Self-capacitance mode, the PTC requires only one pin (Y-line) for each touch sensor. The number of available pins and the assignment of X- and Y-lines is depending on both package type and device configuration. Refer to the Configuration Summary and I/O Multiplexing table on the product data sheet for details.

Features

• Low-power, high-sensitivity, environmentally robust capacitive touch buttons, sliders, and wheels
• Supports wake-up on touch from standby Sleep mode
• Supports mutual-capacitance and self-capacitance sensing
  • Mix-and-match mutual-and self-capacitance sensors
• One pin per electrode, no external components
• Load compensating charge sensing
  • Parasitic capacitance compensation and adjustable gain for superior sensitivity
• Zero drift over the temperature and VDD range
  • Auto calibration and recalibration of sensors
• Single-shot and free-running charge measurement
• Hardware noise filtering and noise signal desynchronization for high conducted immunity
• Polarity control, allowing Parallel Acquisition (through the QTouch^® Library) individually controls the polarity of each line
• Driven Shield Plus for better noise immunity and moisture tolerance
  • Any PTC X/Y line can be used for the driven shield
  • All enabled sensors will be driven at the same potential as the sensor scanned
• Selectable channel change delay allows choosing the settling time on a new channel, as required
• Acquisition-start triggered by command or through auto-triggering feature
• Low CPU utilization through interrupt on acquisition-complete

Block Diagrams

PTC Block Diagram Mutual Capacitance

PTC Block Diagram Self Capacitance

Principle of Operation

The I/O lines used for analog X-lines and Y-lines must be connected to external capacitive touch sensor electrodes. External components are not required for normal operation. However, to improve the EMC performance, a series resistor of 1 kΩ or more can be used on X-lines and Y-lines.

A mutual-capacitance sensor is formed between two I/O lines: an X electrode for transmitting and Y electrode for sensing. The mutual capacitance between the X and Y electrode is measured by the PTC:

A self-capacitance sensor is connected to a single pin on the Peripheral Touch Controller through the Y electrode for sensing the signal. The sense electrode capacitance is measured by the PTC:

The PTC is clocked by the GCLK_PTC clock. The PTC operates from an asynchronous clock source and the operation is independent of the main system clock and its derivative clocks, such as the peripheral bus clock (CLK_APB). Several clock sources can be selected as the source for the asynchronous GCLK_PTC. The clock source is selected by configuring the Generic Clock Selection ID in the Generic Clock Control register. For more information about selecting the clock sources, refer to "SAM L10/L11 Generic Clock Controller (GLCK)".

To access the PTC, you must use the Atmel Start QTouch Configurator to configure and link the QTouch Library firmware with the application software. QTouch Library can be used to implement buttons, sliders, wheels in a variety of combinations on a single interface. For more information about QTouch Library, refer to "QTouch Library Peripheral Touch Controller User Guide".

Code Example:

The Low-Power Touch Design Application Note illustrates how to configure the PTC in a Low Power Touch Design.