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5.6.7.3
The channels used for an individual rotor or slider should all be on the same Y line.
The maximum number of Rotors / Sliders supported by the QMatrix acquisition method
depends on the configuration. Refer to the Library_Selection_Guide.xls for details.
Vcc should be kept at 4.5V or lower for reliable operation
Design Guidelines for QMatrix acquisition method systems
AVR Microcontrollers can use a number of clock sources, ranging from high precision external
crystals to less accurate resonators down to simple external RC circuits. Most AVR devices also
come with integrated RC oscillators. This provides a system clock source without additional cost
or board space. When using internal RC oscillators some considerations need to be taken. The
accuracy i.e. frequency of CMOS RC oscillators will vary slightly from device to device due to
process variance.
QMatrix acquisition method uses an internal timer to measure the discharge time of a capacitor,
and any frequency variation or fluctuation in the RC Oscillator will thus show up as a variance in
the measurement data. The application should for this reason be designed and tuned to allow for
such variance in the internal RC oscillator frequency. For most AVR microcontrollers, the rated
accuracy of the internal RC oscillator is 2%, and to have some headroom and guarantee a robust
and stable system, the designer should aim to follow these design rules:
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Reference Value should be in the 150-300 range
Typical delta when touched should be at least 10% of the Reference Value
Recommended threshold should be at least 5% of the reference value and at least 50%
of the typical delta (Higher value gives better robustness)
Hysteresis should be as high as possible in noisy systems (50%)
DI should be set to at least 4
If the design of the system does not comply with the rules above, special attention should be
taken when testing it to make sure that the design meets the desired performance. In systems
with big signal values and small deltas (i.e. less than 10%) it is recommended to either change
component values to conform to the 10% delta rule, or change to a higher precision clock source.
QTouch Studio is the preferred tool when checking and validating any QTouch Designs.
5.6.8 Frequency of operation (Vs) Charge cycle/dwell cycle times:
The library needs different charge / dwell cycles based on the operation and design. The
charge/dwell cycles are determined by the QT_DELAY_CYCLES parameter defined by the user.
The recommended range of charge/dwell cycle times that the user must select based on the
operating clock frequency of the Microcontroller is provided in the table below.
Fine tuning of the QT_DELAY_CYCLES to match the sensor design may be done by monitoring
the reference levels, and finding the charge/dwell time where the reference level has reached
>99% of maximum reference value seen. For QTouch acquisition method, the reference value will
decrease as the QT_DELAY_CYCLES is increased. For QMatrix acquisition method, the
reference value will increase with increase in QT_DELAY_CYCLES. If the cycle time is not
optimum, the design may experience temperature sensitivity.
Possible values:
The following table lists the possible values of QT_DELAY_CYCLES for both QTouch and
QMatrix acquisition method libraries.
46
Acquisition method
Possible values
8207L-AT42-05/12
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