TSC2005
www.ti.com
SBAS379–DECEMBER 2006
OVERVIEW (continued)
Touch Detect
PINTDAV can be programmed to generate an interrupt to the host. Figure 25 details an example for the
Y-position measurement. While in the power-down mode, the Y– driver is on and connected to GND. The
internal pen-touch signal depends on whether or not the X+ input is driven low. When the panel is touched, the
X+ input is pulled to ground through the touch screen and the internal pen-touch output is set to low because of
the detection on the current path through the panel to GND, which initiates an interrupt to the processor. During
the measurement cycles for X- and Y-Position, the X+ input is disconnected, which eliminates any leakage
current from the pull-up resistor to flow through the touch screen, thus causing no errors.
Analog VDD
Plane
SNSVDD
PINTDAV
SNSVDD
Level
Shifter
RIRQ
51kW
Y+
X+
Pen Touch
Data Available
Control
Logic
High when
the X+ or Y+
driver is on.
Sense
DGND
Y-
High when the X+ or Y+
driver is on, or when any
sensor connection/short
circuit tests are activated.
ON
Vias go to system analog ground plane.
SNSGND
AGND
Figure 25. Example of a Pen-Touch Induced Interrupt via the PINTDAV Pin
In modes where the TSC2005 must detect whether or not the screen is still being touched (for example, when
doing a pen-touch initiated X, Y, and Z conversion), the TSC2005 must reset the drivers so that the 50kΩ
resistor is connected again. Because of the high value of this pull-up resistor, any capacitance on the touch
screen inputs will cause a long delay time, and may prevent the detection from occurring correctly. To prevent
this possible delay, the TSC2005 has a circuit that allows any screen capacitance to be precharged, so that the
pull-up resistor does not have to be the only source for the charging current. The time allowed for this
precharge, as well as the time needed to sense if the screen is still touched, can be set in the configuration
register.
This configuration underscores the need to use the minimum possible capacitor values on the touch screen
inputs. These capacitors may be needed to reduce noise, but too large a value will increase the needed
precharge and sense times, as well as the panel voltage stabilization time.
16
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