Electronic Design

Fujitsu Takes Digital Approach To Capacitive Touch Sensing

Capacitive touch systems typically operate using analog-to-digital converters (ADCs) to indirectly measure changes via voltage. Fujitsu’s FMA1127 touch sensor controller uses a more digital approach to detecting fingers, measuring the difference of the change of state of the RC portion of a pair of monostable, multivibrator flip-flops (see the figure).

The chip uses a common counter/timer mechanism to measure the difference in time between the pair of edges for the flip-flop outputs. The position of the edges is based on the capacitors—one fixed and one variable. The variable capacitor is based on the touch sensor that is detecting the controlling device.

The approach is faster and more accurate than the ADC approach. It can use up to 15 times less power and improve response time by a factor of 2.5 because of its simplicity. The chip only uses 0.15 mA, with 70 µA when idling and 10 µA when in sleep mode. Response time is 0.2 ms. The FMA1127 also provides 8-kV electrostatic discharge (ESD) protection per input for chip/contact pad and 15 kV system/ air. The touchpad does not require a ground, and the chip does not require an external clock.

Inside the FMA1127, things are a little more complex since the system is normally used to measure not just one switch but many. It can also detect multiple touch and relative positions using a touchpad. The chip incorporates a microcontroller as well, allowing more advanced processing. For example, the FMA1127 can provide detailed positioning and strength information or simply a binary indication of a button press. The binary indication can handle hysteresis, debouncing, and adjacent buttons. Button inputs can be linked to digital outputs on the chip, providing a simple interface to external devices. Touch strength resolution is 8 bits.

The FMA1127 uses an I2C interface for setup and nondirect I/O access. If only direct I/O is used, a small, six-pin single-chip microcontroller could be used to initialize the system. The chip’s automatic impedance calibration (AIC) allows different sensitivities to be set for each input. An external microcontroller can change calibration intervals and other parameters. It also can pause and resume AIC operation.

The chip comes in a number of versions, with the top end available in a 40-pin quad-flat no-lead (QFN) package with 12 direct I/O outputs and 12 sensor inputs. Larger arrays can be constructed by ganging up to four chips together. Pricing starts at $1.40.



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