ATQT600 Atmel, ATQT600 Datasheet - Page 17

• Manufacturer Part Number: ATQT600
• Description: KIT EVAL TOUCH FOR QT600
• Manufacturer: Atmel
• Series: QTouch™r

Specifications of ATQT600

• Sensor Type: Touch Screen
• Interface: USB
• Embedded: Yes, Other
• Utilized Ic / Part: ATtiny88, ATmega324PA, ATxmega128A1
• Processor To Be Evaluated: ATtiny88, ATmega324, ATxmega128
• Data Bus Width: 8 bit, 16 bit
• Interface Type: USB
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Operating Supply Voltage: 1.6 V to 3.6 V
• Silicon Manufacturer: Atmel
• Kit Application Type: Sensor
• Application Sub Type: Touch Sensor
• Kit Contents: USB Bridge, MCU Cards, Touchpad Cards
• Svhc: No SVHC (15-Dec-2010)
• Mcu Supported Families: ATtiny88,
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -
• Sensitivity: -
• Sensing Range: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

2.4
Touch Sensors Design Guide
Nearby LEDs
The most common form of electrode is a filled circle or rectangle of copper on a PCB, corresponding
loosely in shape to the key graphic. The PCB is then usually glued with an industrial adhesive such as a
two-sided acrylic sheet to the inside of the operator panel.
If LEDs are close (that is, less than 4 mm away) to capacitive sensors, you must give some
consideration to their change in capacitance between on and off states (and the possible change of the
LED driver's output impedance too). It is also necessary to consider the changes in the nature of their
drive circuitry.
If changes in capacitance of the LED and associated drive circuit couple to a touch sensor electrode, it is
possible to cause detection instability or touch keys that stick either on or off when the LED changes
state.
As a general rule, LEDs that are judged as close must be bypassed with a capacitor that has a typical
value of 1 nF
particularly important for LEDs that are pulled down or up to switch on, but are allowed to float when off.
Note that the bypass capacitor does not need to be physically close to the LED itself. Even if the
capacitor is several centimeters away from the LED, it will still serve its purpose. This may aid layouts
that are tight for space around the sensors.
Figure 2-3.
1. One example of acrylic bonding sheet includes 3M type 467MP, although there are other suppliers
2. Non critical value. The idea is to simply provide a constant low impedance path as seen by the sensor, on both
and types which may prove more suitable.
ends of the LED. Low means less than 1 k at 100 kHz
LOW or HIGH,
never Hi-Z
LOW or HIGH,
never Hi-Z
(2)
LED Circuits
if either end of the LED changes to a non-low impedance state at any point. This is
Vdd
HIGH or Hi-Z
LOW or Hi-Z
(1)
Vdd
...or...
General Advice
Vdd
10620D–AT42–04/09
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