Whether it’s for scrolling on a smartphone or navigating an in-car entertainment system, most people use a touchscreen every day. But how many stop to wonder about the electronics that makes this form of human-machine interface (HMI) possible? Here, Swindon Silicon’s Analogue Design Manager Dr Mike Coulson discusses how Application Specific Integrated Circuits (ASICs) play an integral role in touchscreens.
Touchscreens feature prominently in our daily lives. We encounter them in our mobile phones and cars, but also increasingly in retail, healthcare and industrial settings. New applications and rising consumer expectations call for screens with enough sensitivity to detect a human finger through heavy toughened glass, while wearing thick gloves, or even when hovering above the screen. This drives a continuous evolution of the touchscreen electronics.
How do touchscreens work?
Nowadays, most touchscreens employ a matrix of invisible conductors embedded within the display. At each crossing point the conductors couple to each other capacitively, and part of this coupling occurs through the air above the glass. A nearby finger reduces the coupling at one or more intersections of the matrix, and the touchscreen electronics must recognise when this happens. This is typically done by supplying a voltage stimulus to each conductor in turn, so as to push miniscule currents through its associated capacitances. The touchscreen electronics then measure these miniscule currents as they leave via the other conductors and detect the subtle changes that would indicate a touch taking place.
As each conductor’s signal enters the touchscreen electronics, it is first filtered to reject interference. After being amplified and sampled, the signal is then digitised by an analogue-to-digital converter (ADC). These functions must be provided for every conductor that receives a signal, which can prove a challenge to the size and cost of the chip. Furthermore, any electronics that is replicated between the channels must be well matched in order to simplify the subsequent processing. This calls for tight control over circuit properties and parasitic capacitances. The design challenges do not stop there: as the changes in capacitance are very subtle, great attention must also be paid to electrical noise. This is complicated by the need for the electronics to operate at high frequency in order to deliver an acceptable response time.
The role of mixed-signal ASIC design
It is possible to prototype a touchscreen controller using off-the-shelf integrated circuits (ICs), no doubt supported by an array of capacitors, resistors and other components. This allows the designer to experiment freely to overcome any challenges. However, when it comes to the final product, it’s best to integrate as much of the circuitry as possible into an ASIC. This optimises the overall bill-of-materials (BOM) and allows the touchscreen electronics to be miniaturised in a way that is simply not possible when using standard components.
Modern high pin-count packages allow an ASIC to accommodate many channels in a single part, with excellent matching in properties. Because the cost of replicating identical channels on a single piece of silicon is comparatively low, the ASIC solution can be significantly effective in touchscreen applications. Furthermore, an ASIC is far more difficult to reverse engineer than parts wired together in plain sight, affording greater IP protection.
The ASIC design process itself is based around sophisticated, comprehensive simulation. This simulation goes far beyond that typically used for PCB design, taking into account parasitic capacitance, resistance and inductance alongside the most subtle manufacturing variations in component properties. Noise, which is so critical to touchscreen applications, can be accurately predicted and accounted for. The digital processing aspect of the system can also be integrated onto the IC, either by the use of microprocessor IP or of custom synthesised logic, resulting in the entire signal path being optimised as one.
By carefully refining the specifications of each aspect of the design, silicon area and design effort is expended where it is needed most. In contrast, when designing the system with commercial off-the-shelf ICs (COTS), it is by design that some performance aspects will be over-specified and engineered because these components are sold with a broader application reach in mind and not optimisation.
Designing an ASIC with Swindon
Swindon frequently designs and supplies custom, high-throughput, multi-channel solutions into human-computer interface applications, optimising the overall bill of materials and achieving form factors that are otherwise impossible. But how does a customer embark on their ASIC journey?
Swindon are happy to hear from any potential customer, who have an initial vision for their product and commercial aspirations. Our team will work to understand the concept and background, before quickly assessing whether a custom IC will offer benefits. If so, they will be able to identify the optimum system partitioning to achieve the objectives. Many customers have a well-defined concept but struggle to formalise the requirements for their ASIC, and in these cases the Swindon team uses their expertise to bridge the gap.
Touchscreens are just one application of ASICs but serves to demonstrate how much added value bespoke chips can bring. This is why an ASIC sits behind most touchscreens we encounter, although we may not realise it. In fact, any production-volume electronic product deserves to be auditioned for a Swindon ASIC solution, to enable commercial and technical benefits such as to differentiate its performance from the competition, to improve its ease of manufacture and bill-of-materials, to improve reliability, to reduce size and power, to enhance IP protection and to provide non obsolescence assurances through longevity of supply.
Do you have an application that could benefit from an ASIC? Get in touch with the Swindon Silicon Systems team for a no-obligation first meeting here: https://www.swindonsilicon.com/contact-us/.