AT42QT2640_14 Datasheet, PDF (70/73 Pages) ATMEL Corporation – 64-key 8-slider/wheel QMatrix FMEA

DWELL
Increasing the Ry values alone results in a reduction in charge transfer and sensitivity. This can be recovered
by increasing the charge transfer time, or dwell time, through higher DWELL settings. To achieve the optimum
DWELL setting, start with the maximum value and observe the reference values. Then reduce the dwell, con-
tinue to observe the reference values, and choose the lowest DWELL setting where the reference values are
not significantly reduced from their maximum.
Longer dwell times also result in reduced moisture tolerance, and so a careful balance might be necessary
between these conflicting requirements.
NTHR & THRM
Together NTHR (and PTHR) and THRM allow the threshold to be set. The threshold should be set as high as
possible to prevent false detects but should not be set so high that true touches are not detected. Atmel
QmBtn or Hawkeye software can be used to observe the signals and signal delta on touch, and used to deter-
mine the optimum threshold settings for a specific design.
The threshold will typically need setting at a lower value to accommodate noise than is evident when testing in
the absence of noise. This may initially seem counter intuitive until consideration is given to the fact that heavy
noise can cause undesirable detect dropouts as well as missed touch events. The threshold should be set as
high as possible, but not so high that the noise causes detect dropouts while the key is still touched.
Frequency Hopping – FHM = 3 (sweep)
The immunity of QT2640 based designs to higher noise amplitude can be increased by configuring the fre-
quency hopping with three different frequencies and using FHM = 2. Choosing the three frequencies is a
process of trial and error and varies from design to design, but a considerable increase in tolerable noise
amplitude can be achieved once three appropriate frequencies have been identified.
However, frequency hopping using sweep mode (FHM = 3) is much easier to configure and may provide suffi-
cient immunity for many applications and products.
AT42QT2640 [PRELIMINARY DATASHEET]
70
9684DX–AT42–12/13