Chapter 5:
Basics of controllers for capacitive touch sensors
5.2 The touch controller industry, its caveats, and the operational principle of the capacitive touch controller
The caveats of the touch controller business
Based on the previous analysis, the importance of selecting the appropriate touch microcontroller should be obvious. Unfortunately, despite its importance, it is a choice that is very hard to make, and more often than not is made without the engineers having all the information they need to make an informed decision. To gain access to the information they need, if they are lucky and the information is publicly available, engineers are forced to resort to the tedious task of browsing through controller documentation.
The scarcity and elusiveness of information surrounding touch controllers can be in most cases traced back to one (or both) of two of these two reasons: the complex nature of the tasks they perform and the fierce competition in the microcontroller industry. We will briefly discuss both of these points.
First, let us consider the technical challenges controller manufacturers (typically IC makers, also referred to as semiconductor companies) have to face when designing and developing these chips. The core information the controller has to be able to extract is the change in the capacitance on the touch sensor between the “touch” and “no-touch” states. Being able to do this is the bare minimum it needs to be able to do in order to successfully register touches.
Under normal conditions, measuring changes in capacitance is not particularly challenging. For example, one could easily measure the change in capacitance (called ΔC) if the capacitance changes for 10pF to 100pF. The process would be straightforward and no special equipment would be needed.
That however, is not the case in capacitive touch sensing. In these applications, changes in capacitance are much smaller, at most a few picoFarads (which highlights the importance of a good touch sensor design that enables the maximization of ΔC). This makes the task of the controllers much harder, as very sensitive circuitry is required in these cases, which is expensive and can have significant development costs. A common method microcontrollers use to achieve this, is the charge transfer method that we will analyze in the following pages.
Unfortunately, the slight change in capacitance is not the only hurdle IC markers’ engineers have to overcome when designing a controller. Noise in sensors and circuitry presents a substantial source of errors and should be addressed. Again, following the proper touch screen design principles can contribute to the mitigation of this problem. But IC makers need their controllers to be as versatile as possible, and as a result cannot rely on the noise elimination skills of touch sensor designers to provide the ideal conditions for their controllers to work. Moreover, we should keep in mind that noise is omnipresent in electrical circuits. So even if a touch sensor designer took all the necessary precautions to shield the design from the system’s noise (which is called inherent noise), transmitted noise generated by any of the electric devices in its environment could interfere with it.
Figure 1. A visual representation of signal and noise [Source].
To mitigate this problem, IC makers act in two different ways. First, they provide general instructions to touch sensor designers (commonly referred to as design guidelines in the industry) that alert them of the most common design choices that lead to excessive noise which interferes with the operation of the controller. An example of such a design choice would be poor grounding design. The design practices IC markers urge touch sensor designers to follow or avoid aim to prevent these problems. The problem with these guidelines is their generic nature. Touch sensor designs differ dramatically case-by-case. As a result, there is no guarantee that touch sensor designers will be able to meet their design requirements if they follow these design guidelines. On the other hand, if they do not, the compatibility between their design and the controller cannot be guaranteed.
As a result, IC makers have to take further action. The other measure they take is trying to limit as much as they can the impact noise has on the operation of their controllers. The quality of any signal can be measured with a metric called signal-to-noise ratio, or SNR. The higher the SNR the clearer the signal, while the lower the SNR the more noisy the signal is. Naturally, we would like the SNR of the input of the controller to be as high as possible. But as we saw in the previous analysis, capacitive touch systems can be rather noisy which leads to a decreased SNR, especially if the best design practices are not followed. IC makers try to get their controllers to work even in situations where SNR is not ideal, by implementing proprietary techniques.
Both of the reasons mentioned above contribute to the increase of R&D costs and the development of proprietary technologies by IC makers that allow them to extract the most out of their chips. Since the effort and the amount of money that goes into these designs is tremendous, it makes sense that IC makers are not willing to disclose specifics regarding the operations of their controllers to the public. That leaves touch sensor designers lacking the information they need to make the best design decisions, and are forced to rely on rules of thumb.
A partial solution to this problem microcontroller manufacturers deploy comes in the form of Field Application Engineers (FAEs). FAEs are engineers with a specialization in capacitive touch sensing that are hired by semiconductor companies in order to support their clients. Their expertise allows them to help their clients steer their project in the right direction to ensure seamless cooperation between their designs and the microcontrollers. While this solution can lead to excellent results, it has an important drawback: it is not scalable enough.
Semiconductor companies are called to support many customers from many different backgrounds. From the hobbyist looking to buy a handful of chips for a small-scale project, to the conglomerate that looks to find the microcontroller of their next mass-produced smartphone, and everything in between. Due to their specialization, the services provided by FAEs will never be enough to support all cases and their time is valuable. Thus, from a financial point of view, it makes sense that not all customers will be eligible for their services. Only those that generate significant revenues for the company have dedicated FAE support , which leaves the rest of the customers having to rely on the generic documentation. However, some microcontroller manufacturers want to change this.
The case of STMicroelectronics: How STM32 customers work with FIeldscale SENSE
Figure 2. STMicroelectronics – Fieldscale partnership gives designers the ability to ensure the compatibility of their designs with the STM32 controllers using Fieldscale SENSE.
STMicroelectronics is a prime example of a company that is looking to shift the way business is done in the capacitive sensing industry. STMicroelectronics has set out to support its customers with an ecosystem of tools that will enable them to choose the right controllers for their projects. To achieve this, ST has partnered with Fieldscale, and has made available its flagship microcontroller series, STM32 to Fieldscale Sense.
Fieldscale, as an ST Authorized Partner, enables touch sensor designers to ensure the compatibility of their designs with the STM32 controllers via their offering, Fieldscale SENSE, allowing them to significantly reduce their design and validation cycles leveraging the power of simulation. You can read more about this partnership here. Users can select the Touch Sensor Controller embedded in an STM32, virtually configure its parameters, get the equivalent circuit of a sensor in netlist format, and then use it on a system level analysis to simulate the Counts from the STM32 microcontroller just as they would be measured in the lab.
As we discussed earlier, touch sensors emit an electrical signal following a touch (or gesture) event. This is just the first step towards translating the touch event into actionable data for the system. The signal emitted by the touch sensor is an analogue signal. Unfortunately, analogue signals are not very useful for the rest of the system. On the other hand, digital signals are. So, what we need is a device that transforms the analogue signal emitted by the touch sensor, into a digital one, that can be used by the rest of the system. Ideally, the same component would be able to extract as much information as possible regarding the actions of the user.
That is exactly what a touch controller does. A touch controller receives as input the analogue signal, and transforms it into digital data as output, while extracting information about the user’s actions (e.g. the touch location or its duration). Essentially, the main task a touch controller performs is that of an analogue-to-digital converter (ADC converter).
Charge transfer method: the basis of capacitive microcontroller operation
As discussed earlier, the actual operation principles of most controllers remain close-guarded secrets. While it must be certain that each IC maker implements a series of heuristics in the implementation, there is an operation principle that appears to be quite popular, the charge transfer method.
Figure 2. The charge transfer method.
The charge transfer method takes advantage of the electrical properties of the capacitor charge to measure the capacitance of the electrodes in each sampling cycle. The electrode charges are transferred to another, larger capacitor called the sampling capacitor. The charge transfer cycle is repeated a number of times, until the voltage of the sampling capacitor reaches a voltage threshold. The number of transfer cycles it takes for the sampling capacitor to reach that threshold, is an indication of the electrode’s capacity. The number of transfer cycles is commonly referred to as “Counts”.
To recap, having read this chapter, you should have a solid understanding of the role of a microcontroller in a capacitive touch system, and its relationship with the touch sensor. You should have also realized its importance and the environment that surrounds these devices. This gentle introduction to this complex topic ends with the most basic methodology controllers leverage, the charge transfer method. In the following chapters, we will dive into deeper topics regarding controllers, such as tuning parameters and touch sensor capabilities.
Key takeaways from this section:
Due to the nature of the microcontroller industry, IC makers cannot disclose all the information engineers need to ensure cooperation between their design and the controller.
Some semiconductor companies have come up with innovative solutions to help enginners, with the example of STMicroelectronics standing out.
The charge transfer method is one of the most widely used methods of operation of touch microcontrollers.
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