This technical brief describes a small application that demonstrates the low-power capacitive touch potential of Microchip’s Capacitive Sensing Module (CSM). The main benefit of this new part is the introduction of a special module for capacitive sensing detection. A more detailed description about this module can be found in AN1171, “Using the Capacitive Sensing Module on the PIC16F72X”.

A simple design was developed to create a board using the PIC16F726 microcontroller with a CSM, a simple LED driver IC Allegro 6276 and 16 surface-mounted LEDs, arranged in a 4×4 matrix. The LED driver was introduced in order to separate the current path of the LEDs from that of the microcontroller, hence a more accurate current consumption measurement on the device’s side. The inputs to the CSM are represented by eight capacitive pads. The VDD line of the microcontroller is shunted by a small 100 Ohms resistor for measuring the current consumption by the device.

The purpose of the board is to allow the user to move a row of three LEDs in the matrix using the arrow buttons, reset their position and vary the frequency at which these LEDs are lit using the slider keys positioned on the right side of the board.

The CSM on the PIC16F72X implements the hardware connection between the WDT and Timer1 gate, and thus the possibility of using the WDT wake from the Sleep event as a time resource for counting pulses coming from the capacitive module to the TMR1 gate. This introduces the power-down mode (Sleep) to be used, which is essential for power saving.

The most effective method used to bring down the current consumption of the device is to keep it as much as possible in Sleep mode. For the lowest current consumption in Sleep mode, all unused I/O pins should be tied either at VDD or VSS.

In non-critical applications, where the concept of enabling the user interface can be used, the proximity detection technique gets the most out of the low-power capacitive sensing concept by increasing the interval in which the MCU stays in Sleep mode.

Below are some general ideas behind proximity detection; the user should:

• Create a state machine-based firmware that can shift from scanning one button to scanning the whole keyboard.
• Increase/decrease the pulse acquisition window by choosing the WDT prescaler value according to the type of scan (proximity or normal).
• Keep the scan rate at a reasonable level to maintain a fast capacitance shift detection.

The firmware developed for the application presented here to illustrate the low-power capacitance sensing concept uses a total amount of 1,930 words of program space and 130 bytes of data memory.

The core firmware that contains the code and data on top of which users can build their application requires approximately 512 words of Flash and 80 bytes of data memory.

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