Capacitive Touch Switch for Automotive Applications 4

The output frequency is used to time a pulse-width modulator (PWM), whose output is a waveform consisting of alternating low-level and high-level cycles, with actual values determined by specific applications. This signal functions as a counter logic gate, where the high-level logic gate allows the counter to increment at a rate of fref. A falling edge of this logic gate signal generates an interrupt, enabling counter read and reset operations. As previously mentioned, when the charging current is 5mA, the comparator threshold is 1.3V, and the sensor capacitance is 30pF, an output frequency of 128kHz is generated. With a 6MHz reference clock, the accumulated counter value per cycle is 46, and a dual-cycle logic gate yields a counter value of 93. For 10 cycles, the counter value would be 468. Clearly, higher accumulated counter values correspond to greater resolution or sensitivity. Larger counter values can be achieved by:  ● Increasing the reference frequency  ● Reducing the oscillator frequency  ● Increasing the number of cycles used for the logic gate signal  A configurable mixed-signal array offers a cost-effective solution for implementing capacitive switch sensors. Figure 4 illustrates the block diagram.  diode

It includes configurable analog functional blocks for constructing relaxation oscillators and digital functional blocks for implementing periodic measurement hardware. More importantly, an I/O analog multiplexer has been added. Each pin is equipped with a switch that can connect it to an analog bus. The I/O analog multiplexer is a large crossbar switch, enabling

Connect any pin to the analog array of a control system. Connected to this bus is also a programmable current source and a discharge switch. This functional combination allows all 28 I/O pins to be used as capacitive sensor inputs. Figure 5 shows a complete capacitive touch sensing system.  diode