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AT42QT1070-SSUR

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2.10 Guard Channel
A guard channel to help prevent false detection is available in both modes. This is fixed on key 0 for standalone
mode and programmable for comms mode.
Guard channel keys should be more sensitive than the other keys (physically bigger). Because the guard channel
key is physically bigger it becomes more susceptible to noise so it has a higher Averaging Factor (see Section 5.9 on
page 18) and a lower Threshold (see Section 5.8 on page 18) than the other keys. In standalone mode it has an
Averaging Factor of 16 and a Threshold of 10 counts.
A channel set as the guard channel (there can only be one) is prioritised when the filtering of keys going into detect
is taking place. So if a normal key is filtering into touch (touch present but DI has not been reached) and the key set
as the guard key begins filtering in, then the normal key’s filter is reset and the guard key filters in first.
The guard channel is connected to a sensor pad which detects the presence of touch and overrides any output from
the other keys.
Figure 2-1. Guard Channel Example
2.11 Signal Processing
2.11.1 Detect Threshold
The device detects a touch when the signal has crossed a threshold level and remained there for a specified number
of counts (see Section 5.10 on page 19). This can be altered on a key-by-key basis using the key threshold I
2
C
commands.
In standalone mode the detect threshold is set to a fixed value of 10 counts of change with respect to the internal
reference level for the guard channel and 20 counts for the other four keys. The reference level has the ability to
adjust itself slowly in accordance with the drift compensation mechanism.
The drift mechanism will drift toward touch at a rate of 160 ms × 18 = 2.88 seconds and away from touch at a rate of
160 ms × 6 = 0.96 seconds. The 160 ms is based on 20 × 8 ms cycles. If the cycle time exceeds 8 ms then the
overall times will be extended to match.
2.11.2 Detect Integrator
The device features a fast detection integrator counter (DI filter), which acts to filter out noise at the small expense of
a slower response time. The DI filter requires a programmable number of consecutive samples confirmed in
detection before the key is declared to be touched. The minimum number for the DI filter is 2. Settings of 0 and 1 for
the DI also default to 2.
The DI is also implemented when a touch is removed. This uses the Fast Out DI option. When bit 5 of Address 53 is
set the a key filters out with an integrator of 4.
Guard channel
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2.11.3 Cx Limitations
The recommended range for key capacitance Cx is 1 pF – 30 pF. Larger values of Cx will give reduced sensitivity.
2.11.4 Max On Duration
If an object or material obstructs the sense pad the signal may rise enough to create a detection, preventing further
operation. To prevent this, the sensor includes a timer which monitors detections. If a detection exceeds the timer
setting the sensor performs a key recalibration. This is known as the Max On duration feature and is set to
approximately 30 s in standalone mode.
In comms mode this feature can be changed by setting a value in the range 1 – 255
(160 ms – 40,800 ms) in steps of 160 ms. A setting of 0 disables the Max On Duration recalibration feature.
Note: If bit 4 of address 53 is clear then a recalibration of all keys occurs on Max On Duration, otherwise individual
key recalibration occurs.
2.11.5 Positive Recalibration
If a keys signal jumps in the negative direction (with respect to its reference) by more than the Positive Recalibration
setting (4 counts), then a recalibration of that key takes place.
2.11.6 Drift Hold Time
Drift Hold Time (DHT) is used to restrict drift on all keys while one or more keys are activated. DHT restricts the
drifting on all keys until approximately four seconds after all touches have been removed.
This feature is particularly useful in cases of high-density keypads where touching a key or hovering a finger over the
keypad would cause untouched keys to drift, and therefore create a sensitivity shift, and ultimately inhibit touch
detection.
2.11.7 Hysteresis
Hysteresis is fixed at 12.5% of the Detect Threshold. When a key enters a detect state once the DI count has been
reached, the NTHR value is changed by a small amount (12.5% of NTHR) in the direction away from touch. This is
done to affect hysteresis and so makes it less likely a key will dither in and out of detect. NTHR is restored once the
key drops out of detect.+
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3. Wiring and Parts
3.1 Rs Resistors
Series resistors Rs (Rs0 – Rs6 for comms mode and Rs0 – Rs4 for standalone mode) are in line with the electrode
connections and should be used to limit electrostatic discharge (ESD) currents and to suppress radio frequency
interference (RFI). Series resistors are recommended for noise reduction. They should be approximately 4.7 k to
20 k each.
3.2 LED Traces and Other Switching Signals
Digital switching signals near the sense lines induce transients into the acquired signals, deteriorating the signal-to-
noise (SNR) performance of the device. Such signals should be routed away from the sensing traces and electrodes,
or the design should be such that these lines are not switched during the course of signal acquisition (bursts).
LED terminals which are multiplexed or switched into a floating state, and which are within, or physically very near, a
key (even if on another nearby PCB) should be bypassed to either Vss or Vdd with at least a 10 nF capacitor. This is
to suppress capacitive coupling effects which can induce false signal shifts. The bypass capacitor does not need to
be next to the LED, in fact it can be quite distant. The bypass capacitor is noncritical and can be of any type.
LED terminals which are constantly connected to Vss or Vdd do not need further bypassing.
3.3 PCB Cleanliness
Modern no-clean flux is generally compatible with capacitive sensing circuits.
If a PCB is reworked in any way, clean it thoroughly to remove all traces of the flux residue around the capacitive
sensor components. Dry it thoroughly before any further testing is conducted.
3.4 Power Supply
See Section 6.2 on page 22 for the power supply range. If the power supply fluctuates slowly with temperature, the
device tracks and compensates for these changes automatically with only minor changes in sensitivity. If the supply
voltage drifts or shifts quickly, the drift compensation mechanism is not able to keep up, causing sensitivity
anomalies or false detections.
The usual power supply considerations with QT parts apply to the device. The power should be clean and come from
a separate regulator if possible. However, this device is designed to minimize the effects of unstable power, and
except in extreme conditions should not require a separate Low Dropout (LDO) regulator.
It is assumed that a larger bypass capacitor (such as1 µF) is somewhere else in the power circuit; for example, near
the regulator.
CAUTION: If a PCB is reworked in any way, it is highly likely that the behavior of the
no-clean flux will change. This can mean that the flux changes from an inert material
to one that can absorb moisture and dramatically affect capacitive measurements
due to additional leakage currents. If so, the circuit can become erratic and exhibit
poor environmental stability.
CAUTION: A regulator IC shared with other logic can result in erratic operation and is
not advised.
A single ceramic 0.1 µF bypass capacitor, with short traces, should be placed very
close to the power pins of the IC. Failure to do so can result in device oscillation, high
current consumption and erratic operation.
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AT42QT1070-SSUR

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Manufacturer:
Microchip Technology
Description:
Capacitive Touch Sensors QTouch 7 Channel Touch Sensor IC
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