at42qt1060 ATMEL Corporation, at42qt1060 Datasheet - Page 9

• Manufacturer Part Number: at42qt1060
• Description: Qtouch? 6-channel Sensor Ic
• Manufacturer: ATMEL Corporation

2.11.2
2.11.3
3. Wiring and Parts
3.1
3.2
9505E–AT42–02/09
Cs Sample Capacitors
Rs Resistors
Detect Integrator
Burst Length Limitations
The device features a fast detection integrator counter (DI filter), which acts to filter out noise at
the small expense of slower response time. The DI filter requires a programmable number of
consecutive samples confirmed in detection before the key is declared to be touched. There is
also a fast DI on the end of the detection (see
applied at the start of a detection if a detection on any other channel has already been declared.
In a balanced system common signals are regarded as thermal shifts and are removed by the
relative referencing drifting, if enabled. This means that the burst lengths must be similar. This
can be checked by reading the reference values (Address 52 – 63) and making sure that they
are similar. The absolute maximum difference is that the maximum value of reference is less
than three times the minimum value amongst all the channels. It is recommended having the
burst lengths (references) as close together as possible, through better routing and layout.
For example, if the keys have references of 250, 230, 220, 240, 200 and 210, this is acceptable.
If the keys have references of 250, 230, 220, 240, 200 and 710, the efficiency of the relative
referencing drifting will be affected. The last key’s (710) layout should be changed or relative
referencing be disabled. The closer the references are in value, the better the relative
referencing drifting performs.
If only normal drifting is enabled, the burst lengths can have bigger variations.
The normal operating limit of burst lengths is between 16 and 1536 counts. A value out of these
limits causes the respective key to be disabled and not measured until a calibration. Signal value
for an out-of-limit key is zero.
Cs0 – Cs5 are the charge sensing sample capacitors; normally they are identical in nominal
value. The optimal Cs values depend on the thickness of the panel and its dielectric constant.
Thicker panels require larger values of Cs. Typical values are 2.2 nF to 10 nF.
The value of Cs should be chosen so that a light touch on a key produces a reduction of ~10 –
so large that the key signals exceed ~1000, as reported by the chip in the debug data.
The Cs capacitors must be X7R or PPS film type, for stability. For consistent sensitivity, they
should have a 10 percent tolerance. Twenty percent tolerance may cause small differences in
sensitivity from key to key and unit to unit. If a channel is not used, the Cs capacitor may be
omitted.
Series resistors Rs (Rs0 – Rs5) are inline with the electrode connections and should be used to
limit electrostatic discharge (ESD) currents and to suppress radio frequency (RF) interference.
They should be approximately 4.7 kto 20 k each.
Although these resistors may be omitted, the device may become susceptible to external noise
or radio frequency interference (RFI). For details of how to select these resistors see the
Application Note QTAN0002, Secrets of a Successful QTouch
Touch Technology area of Atmel’s website, www.atmel.com.
20 in the key signal value (see
Section 6.22 on page
Section 6.20 on page
23). The chosen Cs value should never be
™
Design, downloadable from the
23). The fast DI will not be
AT42QT1060
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