Low Power Capacitive Touch Sensing
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This thesis will seek to design a capacitive touch sensor that uses as little power as possible while still having decent performance. The study will start by discussing oscillators and find that relaxation oscillators with a frequency dependent on an RC-circuit is of greatest interest. Thorough simulations and theory will show that it is power efficient for the RC-circuit to oscillate between two voltage levels close to the supply voltage. It will also show that it is only the resistance that affect the power dissipation in the RC-circuit. A Finite State Machine that monitors changes in the period of the oscillator is described and designed. It uses two IIR filters to reject noise from the oscillator and provide an average over time the input can be compared to. A prototype is built and tests establish that both the oscillator and FSM behave as expected. It is found that the response time of the FSM can be stated in sampling periods and that lower bit lengths give faster response time. Power estimations are done and it is found that the FSM uses two orders of magnitude less power than the oscillator. The full design is compared to a low power capacitive touch system currently on the market. Power estimations indicate that the design proposed uses an order of magnitude less power than the commercial implementation it is compared with. The results also indicate that the proposed design has a potential for even more power optimization.