at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 46

• Manufacturer Part Number: at42qt1481
• Description: 48-key Qmatrix Ic
• Manufacturer: ATMEL Corporation

5.12
5.13
5.14
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Negative Hysteresis – NHYST
Dwell Time – DWELL
Mains Sync – MSYNC
AT42QT1481
The QT1481 employs programmable hysteresis levels of 6.25 percent, 12.5 percent,
25 percent, or 50 percent. The hysteresis is a percentage of the distance from the threshold
level back towards the reference, and defines the point at which a touch detection drops out. A
12.5 percent hysteresis point is closer to the threshold level than to the signal reference level.
Hysteresis prevents chatter and works to make key detection more robust. Hysteresis is only
used once the key has been declared to be in detection, in order to determined when the key
should drop out.
Excessive amounts of hysteresis can result in stuck keys that do not release. Conversely, low
amounts of hysteresis can cause key chatter due to noise or minor amounts of finger motion.
The hysteresis levels are set for all keys only; it is not possible to set the hysteresis differently
from key to key.
NHYST Typical values:
NHYST Default value:
NHYST Possible range:
The Dwell parameter in Setups causes the acquisition pulses to have differing charge capture
durations. Generally, shorter durations provide for enhanced surface moisture suppression,
while longer durations are required where the keypanel design includes higher-resistance tracks
such as silver and ITO. Longer durations are also usually more compatible with EMC
requirements.
Longer dwell times permit the use of larger series resistors in the X and Y lines to suppress RFI
effects, without compromising key gain
This setup lets the designer trade one requirement for another.
DWELL Typical value:
DWELL Default value:
DWELL Possible values:
The MSync feature uses the WS pin. The Sleep and Sync features can be used simultaneously,
in which case the QT1481 wakes on the mains sync signal, scan the matrix and then returns to
sleep automatically.
External fields can cause interference leading to false detections or sensitivity shifts. Most fields
come from AC power sources. RFI noise sources are heavily suppressed by the low impedance
nature of the QT circuitry itself.
Noise such as from 50Hz or 60Hz fields becomes a problem if it is uncorrelated with acquisition
signal sampling; uncorrelated noise can cause aliasing effects in the key signals. To suppress
this problem the WS input allows bursts to synchronize to the noise source. This same input can
also be used to wake the part from a low-power Sleep state.
The noise sync operating mode is set by parameter MSYNC in Setups.
0, 1 (6.25 percent, 12.5 percent)
1 (12.5 percent)
0, 1, 2, 3 (6.25 percent, 12.5 percent, 25 percent, 50 percent)
1 (188 ns)
1 (188 ns)
0 – 15 (125 ns – 9.9 µs
(Section 2.9 on page
)
, accuracy is ±10 percent
11).
9621B–AT42–06/11