fma1127da Fujitsu Microelectronics, Inc., fma1127da Datasheet - Page 6

• Manufacturer Part Number: fma1127da
• Description: Touch Sensor Controller
• Manufacturer: Fujitsu Microelectronics, Inc.

Touch Sensor Controller
4
Operation Principles
Touch Detection
The FMA1127DA touch sensor controller includes the Impedance Change Detection engine within the device. It detects the impedance difference
between reference and sensor input.
As shown in Figure 1, if the pad is not touched, the impedance of the sensor input Zin should be kept less than the impedance of the reference
Zref. If the pad is touched, as shown in Figure 2, Zin is increased by Ztouch. When Ztouch by touching becomes greater than the difference of Zin
and Zref in the not touched state, i.e., if Zin in touched state becomes greater than Zref by a value higher than 0.078pF, the ICD (Impedance
Change Detection) engine within the chip generates the acknowledged output signal indicating it senses the touch.
Notice the value of 0.078pF or higher is needed to maintain stable output against various noises. The sensor input impedance, Zin, includes
parasitic capacitance of the input line, tuning capacitance of input pin and on-chip input impedance, while Zref includes on-chip impedance, AIC
control values and external tuning capacitance if necessary.
AIC™ (Automatic Impedance Calibration)
Automatic Impedance Calibration (AIC) maintains consistent sensitivity against external environmental changes such as temperature, supply
voltage and current, humidity, and system-level variations. This helps users develop their applications more conveniently by providing the actual
impedance value of each sensor input. For developers, a Tuning Viewer program is provided, which helps to optimize the PCB design and to
decide AIC input parameters. More detailed information is available in the FM1127 Tuning Guide.
The ICD engine residing in the FMA1127DA controls reference impedance values for each sensor input pin by acquiring each input impedance
data. It periodically updates all reference impedance values under the condition that all twelve touch pads remain in no-touched status. This
auto-calibration function absorbs environmental changes and guarantees product stability.
APIS™ Touch Output
When touch pads are arranged too closely to each other, it is sometimes difficult to identify which pad is touched. APIS™ (Adjacent Pattern
Interference Suppression) is a filtering function to identify which pads are intentionally touched. If APIS mode is not defined, all touch data
without APIS filtering are transmitted to the MCU. For example, if the application is a numeric keypad, the user can use the APIS mode1 to get
the strongest output and filter out all other weakly touched inputs. Without APIS, the host may have to do this filtering function. APIS reduces
the burden of the host computing time.
IDC =
Fujitsu Microelectronics America, Inc.
Touch Pad System Model
Figure 1: When a Pad is Not Touched.
Zref
1, if Zin – Zref > 0.078pF
0, otherwise
Zin
Touch Pad
Impendance
Detection
Zin < Zref
Change
Impedance Status
Zin
Zref
Touch Pad System Model
Zref
Zin
Touch Pad
Impendance
Detection
Change
Figure 2: When a Pad is Touched.
Impedance Status
Zin
Zref
Ztouch
Touch
Detected