Datasheet
33
9502A–AT42–07/08
AT42QT2160
The burst length is the number of times the charge-transfer (QT) process is performed on a
given key. Each QT process is simply the pulsing of an X line once, with a corresponding Y line
enabled to capture the resulting charge passed through the key’s capacitance Cx.
Increasing burst length directly affects key sensitivity. This occurs because the accumulation of
charge in the charge integrator is directly linked to the burst length. The burst length of each key
can be set individually, allowing for direct digital control over the signal gains of each key
individually.
Apparent touch sensitivity is also controlled by the Negative Threshold level (NTHR). Burst
length and NTHR interact; normally burst lengths should be kept as short as possible to reduce
scan time and limit RF emissions, but NTHR should be kept above 6 to reduce false detections
due to external noise. The detection integrator mechanism also helps to prevent false
detections.
Note: setting a burst length of zero for a specific key, disables that key.
Typical values: 8 to 32 (32 to 128 burst pulses)
Default: 4 (16 burst pulses)
7.22 Address 70...71: GPIO/GPO Drive
If the GPIOs are set to outputs, the drive for the individual GPIO is set according to the
corresponding bit in GPIO Drive bytes. Setting the bit to 1 will drive the corresponding GPIO pin
to Vdd, while setting it to 0, will drive the corresponding GPIO pin to ground.
Enabling PWM on a GPIO pin will override the drive on the pin.
Shared X line GPOs will be only driven when not doing any measurements. During
measurements, burst pulses will be driven from the X lines, make sure that the driven device will
not be affected.
Default: 0 (All driven low)
7.23 Address 73: GPIO Direction
Sets the direction of the GPIOs: 1 = driven outputs, 0 = floating inputs.
If set as inputs, the GPIO will only be read every 16ms (fixed cycle time).
Table 7-23. GPIO/GPO Drive
Addressb7b6b5b4b3b2b1b0
70 X7 X6 X5 X4 X3 X2 X1 X0
71 0 0 0 GPIO3 GPIO2 GPIO1 0 0
Table 7-24. GPIO Direction
Addressb7b6b5b4b3b2b1b0
73 0 0 0 GPIO3 GPIO2 GPIO1 0 0