AT42QT1010
The HeartBeat indicator can be sampled by using a pull-up resistor on the OUT pin (Figure 3-6),
and feeding the resulting positive-going pulse into a counter, flip flop, one-shot, or other circuit.
The pulses will only be visible when the chip is not detecting a touch.
Figure 3-6. Obtaining HeartBeat Pulses with a Pull-up Resistor
VDD
HeartBeat™ Pulses
5
Ro
VDD
1
3
4
6
OUT
SNSK
SNS
SYNC
VSS
2
If the sensor is wired to a microcontroller as shown in Figure 3-± on page 9, the microcontroller
can reconfigure the load resistor to either Vss or Vdd depending on the output state of the
QT1010, so that the pulses are evident in either state.
Figure 3-7. Using a Microcontroller to Obtain HeartBeat Pulses in Either Output State
PORT_M.x
1
3
4
6
OUT
SNSK
SNS
R
o
Microcontroller
PORT_M.y
SYNC
Electromechanical devices like relays will usually ignore the short HeartBeat pulse. The pulse
also has too low a duty cycle to visibly affect LEDs. It can be filtered completely if desired, by
adding an RC filter to the output, or if interfacing directly and only to a high-impedance CMOS
input, by doing nothing or at most adding a small noncritical capacitor from OUT to Vss.
3.9.3
Output Drive
The OUT pin is active high and can sink or source up to 2 mA. When a large value of Cs
(>20 nF) is used the OUT current should be limited to <1 mA to prevent gain-shifting side
effects, which happen when the load current creates voltage drops on the die and bonding
wires; these small shifts can materially influence the signal level to cause detection instability.
9
9541G–AT42–03/10
ATMEL [ ATMEL ]
