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AT42QT1040 [DATASHEET]

9524D–AT42–05/2013

4.

Detailed Operations

4.1

Adjacent Key Suppression

The use of AKS is selected by the connection of a 1 M

 resistor (RAKS resistor) between the SNSK0 pin and either

Vdd (AKS mode on) or Vss (AKS mode off).

Note:

Changing the RAKS option will affect the sensitivity of the particular key. Always check that the sensitivity is

suitable after a change. Retune Cs0 if necessary.

4.2

Discrete Outputs

There are four discrete outputs (channels 0 to 3), located on pins OUT0 to OUT3. An output pin goes active when

the corresponding key is touched. The outputs are open-drain type and are active-low.

On the OUT2 pin there is a ~500 ns low pulse occurring approximately 20 ms after a power-up/reset (see Figure 4-1

for an example oscilloscope trace of this pulse at two zoom levels). This pulse may need to be considered from the

system design perspective.

The discrete outputs have sufficient current sinking capability to directly drive LEDs. Try to limit the sink current to

less than 5 mA per output and be cautious if connecting LEDs to a power supply other than Vdd; if the LED supply is

higher than Vdd it may cause erratic behavior of the QT1040 and back-power the QT1040 through its I/O pins.

Table 4-1.

RAKS Resistor

RAKS Connected To...

Mode

Vdd

AKS on

Vss

AKS off

The RAKS resistor should always be connected to either Vdd or Vss and should not be

changed during operation of the device.