With the HMI developing, touch sensing is more and more popular.The GPIO-based method was developed as a low-cost way to do touch sensing. NXP had developed this sensing method and provide as TSS library (which combines support for GPIO and hardware-based touch sensing).The GPIO method uses the RC charge time in a capacitor (theelectrodeortouchpad)
Measurement Principles: TSI Method
TSI method uses configurable current sources. Thecurrentsources are active outputs, makingthemfar more robust againstnoise. Currentsources are configurable, makingitpossible toconfigure sampling time. Thesignalslopedependsontheappliedcurrent and thecapacitance. When a fingerapproachestheelectrode, frequencydecreases.
Anotheroscillator, uses a internalunchanging capacitor, thisisourreference, wewill configure it to oscillatefasterthantheexternalone. Bycomparinghowmanyreferenceoscillationswherecountedbythe TSI module per externalreferencescan, we can knowwhen a touchhappened.