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Microcontroller development kit equipped with inductive sensor allowing for the realization of high-grade metallic buttons

末武清次氏 Mr. Seiji Suetake

 From 2006 to 2007 since its start of being used in smartphones and portable game consoles, touch sensors have become a norm in the blink of an eye. Now application devices are not limited to smartphones and portable game consoles, but are also spreading to major appliances, housing and facility equipment, FA devices, medical equipment, office equipment, bank ATMs, and automatic ticket machines for railway services etc.

 There are multiple technologies to realize a touch sensor. For example, a capacitance method, a resistance film method, an ultrasonic method, and infrared method are used. Among them, the capacitance method is rated highly for its capability of multipoint detection and is used in various electronic devices focusing on smartphones.

 Cypress Semiconductor is a semiconductor manufacturer that drives the electrostatic capacitance type touch sensor market. A microcontroller with an electrostatic capacitance type touch sensor called “CapSense®” is currently available. Cypress has commercialized a microcontroller with a new sensor function “MagSense™” and now offers a compatible microcontroller development kit. In this article, we asked Mr. Seiji Suetake, project manager in the Market Division of the Microcomputer Division of Cypress Semiconductor, about the principle of MagSense, how to use it, and the features of compatible microcontrollers, as well as how to use the development kit. (Interviewer: Katsumi Yamashita = Technical Journalist)

What is the “MagSense”?

Suetake: It’s an inductive sensor. It can detect the proximity of metal objects. We believe it can be used as a user interface of electronic devices. For example, if a threshold value is set, it can be used as a button. With capacitive touch sensors, only plastic buttons could be realized, but with MagSense, it will be possible to realize metal buttons that have a high-class look.

What sort of detection principle does Cypress use?

Suetake: In general, mutual inductance occurs between two metals. This mutual inductance changes with the distance of the two metal objects. In other words, mutual inductance changes as metal objects move closer or further away. With MagSense, this change in mutual inductance is regarded as a change in current to detect the proximity of a metal object.

Could you tell us specifically how Cypress is configuring sensors?

Suetake: For example, a coil is placed on an electronic device using an inductive sensor, and this coil is driven using the microcontroller that has become available. In this state, if the metal object gets close to the coil, the mutual inductance changes, so the current flowing through the coil changes. By measuring the change in current with a microcontroller, it is possible to realize a proximity sensor that detects the proximity of a metal object. If the threshold value is set as previously mentioned, it can be used as a button. It can also be used as a rotary encoder.

The industry’s first microcontroller with a built-in inductive sensor

Can you give us details about the microcontroller equipped with MagSense?

Microcontroller equipped with “MagSense”

Figure 1: Microcontroller equipped with “MagSense”
It is “PSoC 4700”. The CPU core is equipped with “Cortex-M0+” from Arm. In addition, the “CapSense”, a single-slope 10-bit A/D converter, two 7-bit current-output D/A converters, and a comparator are all integrated on one chip.

Suetake: The MagSense technology has been implemented by a microcontroller called “PSoC® 4700” (Figure 1). The CPU core uses the “Cortex®-M0+” from Arm®. In addition to MagSense, it also has CapSense technology, a capacitive touch sensor, and a single-slope 10-bit AD converter built in. Using this A-D converter, it is possible to attach things like a temperature sensor and convert the detected signal into a digital value. In other words, it is possible to sense various physical quantities by using only one PSoC 4700.

 Although there are competitors that have commercialized microcontrollers with integrated capacitive touch sensor functions, the commercialization of microcontrollers with integrated inductive sensor functions is a industry first.

Buttons can also be created using CapSense. I can understand the difference in principle, but can you explain the difference in the sensor functions that can be realized?

Suetake: CapSense is characterized by being resistant to water among capacitive touch sensors. Even if water adheres to the surface that is to be touched, touch can be detected without any problem. However, if the touch surface is completely immersed in water, touch cannot be detected.

 But with MagSense it works without any problems even in water. If the electronic device equipped with MagSense can realize complete waterproofing, the user interface function can be used even in a submerged state. In other words, it will be easier to realize a completely waterproof electronic device.

How do you specifically use PSoC 4700?

Example of use in a washing machine

Figure 2: Example of use in a washing machine
MagSense can be used to create a door open/close detection or rotary encoder using Capsense.

Suetake: For example, let’s consider a washing machine. With MagSense, it is possible to detect the opening and closing of the door you put the laundry in and realize the rotary encoder that sets the washing/drying time (Figure 2). Furthermore, the CapSense can create buttons for various function selections. In addition to this, because it also has a PWM control function, it can also drive an LED indicator that indicates the operating state of the washing machine. A single PSoC 4700 can provide all of the washing machine user interface features.

How many inductive sensors can be built with one PSoC 4700?

16 Inductive Sensors Can Be Built

Figure 3: 16 Inductive Sensors Can Be Built
Up to 16 inductive sensors can be built with one PSoC 4700. When using a single-function IC (Inductance-to-Digital Converter IC) manufactured by a competitor, an external circuit such as a multiplexer or microcontroller is required, resulting in a complicated circuit.

Suetake: One PSoC 4700 can drive 16 (channel) coils (Figure 3). In other words, you can create 16 inductive sensors. In fact, a multiplexer circuit is mounted at the input section and a method of measuring/detecting the current flowing in a maximum of 16 coils by time division is used.

Can MagSense and CapSense be used simultaneously?

Suetake: It can be used simultaneously. Therefore, it is also possible to make a metal button and a plastic button in one electronic device.

What is the resolution of the detection distance?

Suetake: The resolution of distance measurement is 195nm. It’s possible to detect an equivalent of 16 bits.

Able to realize performance comparable to a single function IC

Where does Cypress stand in terms of performance and usability compared to competitors

Suetake: At the moment, microcontrollers with built-in inductive sensors do not seem to have been commercialized by competitors. However, single-function ICs of inductive sensors have been commercialized. Compared to these single-function ICs, the performance of the MagSense technology equipped PSoC 4700 is better. For example, the resolution is 16 bits for MagSense, but 12 bits for competitors. For example, the resolution is 16 bits for MagSense, but 12 bits for competitors. The number of channels is 16 for MagSense and 4 for competitor products. The current consumption is 40μA for MagSense and 72μA for competitor products. It’s no exaggeration to say that the performance that the MagSense technology can achieve is quite high.

What can be used as a development tool?

Can be designed using “PSoC Creater”

Figure 4: Can be designed using “PSoC Creater”

Suetake: Similar to existing PSoC products, “PSoC Creater” is compatible (Figure 4). As inductive sensors that can be configured using MagSense, we have 3 types prepared; “button”, “proximity sensor”, and “rotary encoder”. Just by selecting one on from the development tool, it is possible to realize the desired function. Things like threshold value can be freely set by the user on PSoC Creater. It’s also possible to fine-tune the threshold value while holding down the button on the actual device. After executing all these settings, the program can be generated automatically.

What is included in the development kit?

Development Kit “CY8CKIT-148”

Figure 5: Development Kit “CY8CKIT-148”
It consists of a board with a microcontroller, buttons, and a rotary encoder, a metal target, a USB target, and a quick start guide.

Suetake: The development kit “CY8CKIT-148” includes a board with buttons and a rotary encoder built in, a metal target, a USB target, and a quick start guide in addition to being equipped with PSoC 4700 (Figure 5). The buttons built into the board are built on top of a flat coil, with metal buttons that bend slightly when touched by humans. A touch can be detected simply by touching this metal button. It is possible to realize a metal button while using the same sense of use as the button realized with CapSense.

Isn’t the inductive sensors’ design highly difficult?

Suetake: Detailed information on the design is described in the application notes. The recommended design is also available, and it can be implemented relatively easily if it is made with reference to it. The basic idea for the design is exactly the same as CapSense.

MagSense Microcontroller Development Kit

PSoC 4700S Inductive Sensing Evaluation Kit “CY8CKIT-148”

The PSoC 4700S Inductive Sensing Evaluation Kit which was also introduced in this manufacturer’s interview is currently available.
The CY8CKIT-148 PSoC 4700S Inductive Sensing Evaluation Kit is a low-cost hardware platform that enables the design and debugging of PSoC 4700S MCUs. The kit includes a button and proximity sensor using MagSense™, Cypress’s latest inductive sensing technology. In addition, FPC connectors are provided to evaluate various interfaces such as rotary encoders.