AT42QT1481_14 Datasheet, PDF (45/73 Pages) ATMEL Corporation – 48-Key QMatrix FMEA IEC/EN60730 Touch Sensor IC

5.13
Dwell Time – DWELL
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 (Section 2.10 on page 12).
This setup lets the designer trade one requirement for another.
DWELL Typical value:
1 (188 ns)
DWELL Default value :
DWELL Possible values:
1 (188 ns)
0 – 15 (125 ns – 9.9 µs), accuracy is ±10%
5.14
Mains Sync – MSYNC
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, scans 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 50 Hz or 60 Hz 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.
The sync occurs only at the burst for the lowest numbered enabled key in the matrix. If it does not sleep at the end of
the matrix scan, the QT1481 waits for the sync signal for up to 100 ms after the end of a preceding full matrix scan,
then the matrix is scanned in its entirety again. If the QT1481 sleeps, it waits indefinitely for the mains sync.
The sync signal drive should be a buffered logic signal, or perhaps a diode-clamped signal, but never a raw AC
signal from the mains. If mains sync is enabled and sleep is disabled, the QT1481 synchronizes to the falling sync
edge. However, if both mains sync and sleep are enabled, the QT1481 is sensitive to a low level on WS. A first
matrix scan occurs when the low level is first detected, and further matrix scans occur for as long as WS is held low.
It is therefore recommended that WS is driven low for less than a single matrix scan time.
Since Noise sync is highly effective and inexpensive to implement, it is strongly advised to take advantage of it
anywhere there is a possibility of encountering low frequency (50/60 Hz) electric fields. Atmel QmBtn software can
show such noise effects on signals, and therefore assist in determining the need to make use of this feature.
If the sync feature is enabled but no sync signal exists, the sensor continues to operate but with a delay of 100 ms
from the end of one scan to the start of the next, and hence has a slow response time. A failed Sync signal (one
exceeding a 100 ms period) causes an error flag (see command 0x06). From reset, the QT1481 first reports a mains
sync error after initialisation followed by a delay of 100 ms waiting for the sync signal. This time interval may be
determined by adding 100 ms to the initialisation times stated in Section 2.14 on page 14.
MSYNC Default value:
MSYNC Possible range:
0 (Off)
0, 1 (Off, On)
AT42QT1481 [DATASHEET]
45
9621EX–AT42–07/2014