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.
BILL WONG
FUJITSU
www.fujitsu.com