[AK4184]
■ Touch Screen Position Detection
XY-coordinate detection (XY-position) from the touch panel is determined by the control command (A1, A0) bits of
the touch screen control command. Coordinate detection is in differential mode. The full scale (ΔVREF) is the
differential voltage between the non-inverting terminal and the inverting terminal of the measured axis (e.g. X-axis
measurement: ΔVREF = VXP – VXN). The input (ΔAIN) of the A/D converter is the voltage between the non-inverting
terminal (VXP) of the non-target axis and the inverting terminal (VXN) of the target axis. (E.g. ΔAIN = (AIN+)-(AIN-)
= VYP – VXN) The input voltage charges the internal capacitor during the sampling time period. No current flows into
the internal capacitors after the capacitor has been fully charged. The required settling time to charge the internal
capacitor array depends on the source impedance (Rin). The source impedance is the resistance of the touch panel
plate. If the source impedance is 600Ω, the settling time needs to be at least 1.5μs. The maximum throughput of the
A/D converter is 125kHz. If the source impedance of the analog input is larger than 600Ω, longer settling time is
required. The position from the touch screen is detected by taking the voltage of one axis when the voltage is supplied
between the two terminals of another axis. At least two A/D conversions are needed to get the two-dimensional (X/Y
axis) position.
AVDD
XP-Driver SW ON
AVDD
YP-Driver SW ON
X-Plate
Y-Plate
X-Plate
Y-Plate
XP
XP
VREF+
VREF-
AIN+
AIN-
VREF+
VREF-
AIN+
AIN-
YP
XN
YP
XN
ADC
ADC
XN-Driver SW ON
YN
Touch Screen
YN
YN-Driver SW ON
a) X-Position Measurement Differential Mode
b) Y-Position Measurement Differential Mode
The X-Plate and Y-plate are connected on the dotted line when the panel is touched.
XP
X-Plate (Top side)
Y-Plate (Bottom side)
XN
YN
YP
c) 4-wire Touch Screen
Figure 3. Axis Measurement
MS0603-E-00
2007/04
10
AKM [ ASAHI KASEI MICROSYSTEMS ]
