[Editor's Note] Since the launch of Apple's iPhone, the touch sensor market has begun to grow in earnest. Recently, its application has expanded to various display products such as laptops as well as smartphones. The touch market has expanded significantly as the electrostatic capacitive touch sensor technology that enables high speed and multi-touch has been applied to smartphones. In the midst of this, we met with Jin-Yong Lee, Manager of Infineon Korea, a global semiconductor solution company, to learn about the principles of capacitive touch sensors and the PSoC™ 4100S Max series with the 5th generation touch algorithm 'Multi-Sense Converter (MSC)'.

■ The principle of the capacitive touch sensor

A capacitive touch sensor is a sensor that detects whether a human hand has touched something by detecting the change in charge between electrodes.

It is divided into the Self-Cap method, which detects the addition of charge, and the Mutual-Cap method, which detects the loss of charge.

The Self-Cap approach implements a single touch sensor. This is a method of detecting whether a hand has touched the sensor pad connected to the GPIO of the MCU by repeatedly charging and discharging the charge and increasing the time taken for the charge and discharge when a person's hand touches it.

It is generally used in touch buttons as a way to check the change in charge between a single sensor and the surrounding GND.

Unlike Self-Cap, Mutual-Cap requires two signal lines called TX/RX.

At this time, to implement one touch sensor, two GPIOs are required on the MCU.

This is a method of charging a capacitor by receiving the charge sent from TX from RX and then discharging it again. However, when a person's hand touches it, the amount of charge transferred from TX to RX decreases, changing the charge/discharge time of the capacitor.

It is a method that checks the amount of change in charge formed between the TX and RX sensors, and is mainly used in touch screens, although it is also used in touch buttons.

■ Advantages and disadvantages of capacitive touch sensors

The advantages of capacitive touch sensors can be largely divided into three categories: durability, price, and design.

First, mechanical buttons become deformed with continued use and eventually need to be replaced, but touch sensors are free from such issues.

Second, while mechanical buttons increase in price as the number of buttons increases, touch buttons are competitive in price because they can be connected by just connecting one more GPIO.

Finally, the touch sensor allows for a clean design of the product's exterior without any buttons exposed to the outside, giving customers more choices when designing their products.This is wide.

On the other hand, capacitive touch sensors do not recognize when touched by non-conductive objects. Because of these characteristics, capacitive touch sensors and pressure sensors are sometimes designed in combination.

■ You said that PSoC™ is not a touch IC but a general MCU with touch functionality. What is the difference between PSoC™ and other MCUs?

When PSoC™ was first introduced in Korea in the early 2000s, it was mainly used as a touch sensor, so many people recognized PSoC™ as a touch sensor, but the touch sensor is a peripheral block included in the MCU called PSoC™.

PSoC™ is an abbreviation for “Programmable System On Chip” and is a product of Infineon’s MCUs that range from 8-bit to 32-bit and from Cortex® M0 to M4 dual core.

It has built-in programmable analog, digital, and I/O subsystems, and depending on the product, it has various built-in peripherals, and commonly includes a touch sensor block.

In addition to the three modes of Active, Sleep, and Deep-Sleep provided by the Arm® core, it is a low-power specialized MCU that supports four additional modes of System LP, ULP, System Deep-Sleep, and Hibernate, supporting a total of seven power modes.

In particular, PSoC™ 4 and PSoC™ 6 have products with built-in BLE and also have built-in SDIO that can interface with Wi-Fi products, making them suitable for low-power IoT devices.

The programmable analog block has a built-in programmable touch sensor block, ADC, Opamp, comparator, etc., making it suitable for sensor interface.do.

The PSoC™ 4100S MAX, one of the PSoC™ 4 series that has been updated with the 5th generation touch sensor block, has been newly released.

In addition to the improved touch block, the PSoC™ 4100S MAX product includes a 12-bit SAR ADC, two opamps, and a comparator.

It has 16 16-bit TCPWM blocks that can be configured as Timer, Count, PWM, etc., and 5 SCB blocks that can be configured as I2C, SPI, UART, etc., so it can be used as a general touch sensor IC, but it is also expected to be used as a sensor hub because it can input other sensors and interface with other devices.

■ Please explain the 5th generation touch algorithm 'Multi-Sense Converter (MSC)' block

There are several updates compared to the existing product.

First, the existing touch block could not be linked with DMA, so there was a problem that CPU usage increased when scanning the sensor, but the newly updated 5th generation touch block can be linked with DMA, so CPU usage can be reduced even when scanning the sensor.

Second, the existing method supplied charge by turning on and off a current source called IDAC, but in this process, there was a part where jitter noise was involved due to the on and off.

In the MSC block, the parts that cause jitter noise are eliminated by controlling the supply of charge using a variable capacitor method called CapDac.

Finally, charge and discharge There is a comparator inside that determines the timing. In the conventional method, the comparator was in single mode, but MSC operates in automatic mode. This can eliminate common mode noise flowing into GND.

In this way, the MSC block can have the effect of improving the SNR (signal-to-noise ratio) compared to the existing one due to the signal improvement and noise removal effect.

■ Please briefly explain the demo content that will be covered in the webinar on June 21st.

This is a demo for a smart speaker that controls the speaker by detecting hand movements through Proximity without actual touch. Gesture proximity detection at a height of 15 cm was implemented through improvement of SNR when using the MSC block.

Swiping to the left plays the previous song, swiping to the right plays the next song, up and down gestures adjust the volume, and if your palm remains still for a certain amount of time, it plays and pauses.

■ Lastly, please say a word to the readers.

Capacitive touch sensors started out in mobile devices and are now expanding into the automotive industry.

This is possible because the stability and reliability of the electrostatic capacitive touch sensor have been secured.

Infineon has been developing and growing domestic touch sensors, and is still not resting on its laurels and is constantly thinking about how to provide better solutions to customers. We will continue to strive to help you with integrated MCU solutions, not just simple touch sensors.

thank you