Rainbow Electronics AT42QT1040 - Manual (Page 8)

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9524A–AT42–03/09
AT42QT1040
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 occuring 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.