Atmel QTouch Library User Guide

‣ 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 4If the design of the system does not comply with the rules above, special attention should betaken when testing it to make sure that the design meets the desired performance. In systemswith big signal values and small deltas (i.e. less than 10%) it is recommended to either changecomponent 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.Frequency of operation (Vs) Charge cycle/dwell cycle times:The library needs different charge / dwell cycles based on the operation and design. Therecommended range of charge/dwell cycle times that the user must select based on the operatingclock frequency of the Microcontroller is provided in the table below.Fine tuning of the cycle times to match the sensor design may be done by monitoring thereference levels, and finding the shortest cycle time where the reference level has reached >98%of maximum reference value seen with a much longer cycle time. If the cycle time is too short thedesign may experience temperature sensitivity.Possible values:The following table lists the possible values of QT_DELAY_CYCLES for both QTouch andQMatrix acquisition method libraries.Acquisition methodPossible valuesQTouchAny value from 1- 255 for 8bit AVR3,4,5,10,25,50 for UC3 and ATSAMlibrariesQMatrix 1,2,3,4,5,10,25,50Example:When operating at 4 MHz, 1~10 cycle charge times are recommended (0.125us to 1.25us).Frequency ofMicrocontroller(MHz))Table 2 : Frequency of operationmicrocontroller Cycle time Suitable Charge Cycle times (or)(us)Suitable Dwell Cycle times(us)1 1 1 to 2 cycles (1us to 2us)2 0.5 1 to 5 cycles (0.5us to 2.5us)4 0.25 1 to 10 cycles (0.25us to 2.5us)8 0.125 1 to 10 cycles (0.125us to 1.25us)10 0.1 2 to 25 cycles (0.2us to 2.5us)16 0.0625 2 to 25 cycles (0.125us to 1.5625us)20 0.05 3 to 50 cycles (0.15us to 2.5us)48 0.02083 5~50 cycles (0.104us to 1.04us)>48