Chapter 6:
Basics of controllers for capacitive touch sensors
6.1 Ideal touch sensing circuit for the touch transfer method
As we’ve seen in chapter 5, the charge transfer method is the most popular operational principle of capacitive touch controllers. Due to its popularity, it makes sense to develop an example controller, whose operation is based on the charge transfer method and implement its ideal circuit, which we can then use in SPICE tools for further analysis.
Such a schematic can be seen in Figure 1. The example controller is used to emulate the role of an actual micro-controller unit (MCU) in capacitive touch sensing applications. The charge transfer capacitive measurement technique requires the use of multiple analog switches, which are embedded in the GPIO pins of the MCU.
Figure 1. The ideal circuit of a MCU that uses the charge transfer method.
We can use this schematic to obtain the acquisition sequence of the controller. The acquisition sequence varies on a case-by-case basis. We can incorporate the differences between the various cases by altering the parameters of the schematic circuit, depending on the application. To do that, we can alter the number of sensors connected to the sensor, and the external components (capacitors) of the system.
Before taking a close look at the acquisition sequence, let’s discuss the clock frequency of this ideal controller.
Clock frequency and dividers in MCUs
The charge transfer method operates at a certain frequency. Thus, a hardware clock has to be embedded in our model of the MCU, to control the frequency rate of the method. This clock operates at a predetermined base frequency. We’ll annotate the parameter that indicates the base frequency of the clock pulse as FREQ_CLK.
Moreover, a clock divider may be used to further divide the base frequency by a constant ration. We’ll annotate this parameter as DIV.
Furthermore, we’ll use two more parameters in our analysis. The charge transfer cycle is split into two phases: the Charge phase and the Transfer phase. The parameter CHARGE_STATE controls the duration of the charge phase of the cycle, and the parameter TRANSFER_PHASE which determines the duration of the transfer phase.
The relationship between the parameters we’ve defined above can be seen clearly in Figure 2.
Figure 2. Relationships between the parameters of the circuit.
Charge phase
In the charge phase of the cycle, the switch S1 is closed and the switch S2 is open. During this phase, the electrode is charged to VDD voltage for the duration of the cycle, which is controlled by the CHARGE_STATE parameter of the controller. Figure 3 illustrates the equivalent circuit during the charge phase.
Figure 3. The equivalent circuit during the charge phase.
Transfer phase
In the transfer phase, the switch S2 is closed and the switch S1 is open. During this phase, the charge that was stored in the electrode Cx is transferred to the sampling capacitor, Cs. The duration of this phase is controlled by the TRANSFER_STATE parameter of the controller. Figure 4 illustrates the equivalent circuit during the transfer phase.
Figure 4. The equivalent circuit during the transfer phase.
End of acquisition
In every cycle, the charge stored in the sampling capacitor, and subsequently its voltage level, Cs is increased. The acquisition ends when the voltage of Cs reaches a certain threshold. Usually, the value of this threshold is a fraction of the VDD.
As we briefly discussed in the previous chapter, the number of cycles until the end of the acquisition is called Counts. The value of counts is proportional to the electrode’s capacitance: the greater the capacitance, the larger the value of counts and vice versa. Figure 5 illustrates voltage of the sampling capacitor as a function of time, during the acquisition.
Figure 5. Sampling capacitor voltage over time.
Key takeaways from this section:
The charge transfer method’s equivalent circuit can be used in SPICE analysis.
Charge transfer acquisition can be split in 2 parts: the charge phase and the transfer phase.
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