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:
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 Composer is the preferred tool when checking and validating any QTouch Designs.