Infineon has augmented touch sensing and liquid level sensing it a new series of MCUs.

Devices in the 4000T family have hardware and software to implement: enhanced capacitive touch sensing, inductive touch sensing and capacitive liquid sensing.

“Developers now have endless possibilities to develop new HMI and sensing solutions, from sleek metallic designs with touch-on-metal buttons, to waterproof touch-buttons and innovative ways of sensing liquids,” said Infineon.

Infineon hover touch capacitive sensingThe enhancement of capacitive touch sensing, implemented in the fifth generation of the company’s ‘Capsense’, is that it can detect a nominal touch many millimetres from the metal sensor surface. “In addition, fifth generation Capsense enables sensing at 10x lower power consumption and offers a 10x higher signal-to-noise ratio than previous devices,” said the company.

If 10mm diameter conductive sense pads are spaced 10mm apart on a PCB, an approaching finger can be reliably sensed at a distance 10mm from the PCB surface – these numbers can be increased and decreased proportionately. In practice a non-conductive panel with user graphics would be mounted over the PCB at this distance. Infineon uses ‘hover touch’.

Infineon flyback inductive touch sensing

The inductive touch sensing scheme uses planar spiral track on a PCB as a sensing element, spaced from a slightly flexible metal diaphragm used as a touch surface. The necessary air gap between spiral coil and metal surface depends on application, and is generally in the range 0.2 to 4mm.

Sensing is such that the miniscule deflection that occurs with finger pressure on a sub-mm thick stainless steel sheet can be detected – sheet thickness and button size have to be selected to match expected detectable force.

On the electronics side, Infineon has shunned more conventional resonant frequency shift sensing to detect the change in inductance, and has instead chosen to detect fly-back voltage when the inductor is pulsed.

For the fly-back technique, it claims: “better sensitivity than resonance method to sense small deflections, broad operating frequency [and] low sensitivity to temperature changes”.

Two MCU pins are needed per touch sensing inductor, both connected to the inductor. One pin drives the inductor via an resistor with a square pulse, whilst the second senses the resulting voltage curve via an RC filter using an on-board ΣΔ ADC.

As well as touch sensing, the inductive technique can be adapted for detecting the movement of metallic targets.

“Inductive sensing enables use-cases such as touch-over-metal, force-touch and proximity sensing,” said Infineon. “It is complementary to capacitive sensing, and enables metal-based and waterproof designs such as metal touch buttons on refrigerators, or interfaces for underwater devices such as cameras and wearables.”

Infineon capacitive liquid level sensorLiquid level sensing has been implemented using a capacitive technique, with multiple capacitive patches (parallelograms in the diagram right) separated from the liquid by an insulator, and each connected to a separate MCU pin.

The associated level-sensing algorithm is claimed to be resistant to temperature, humidity and surface foaming, and it can handle irregular-shaped containers. Infineon sees it being used in robot vacuum cleaners, washing machine soap dispensers and coffee machines.

On the processing side, the 4000T ICs are based around a 48MHz Arm Cortex-M0+ CPU with single-cycle multiply. This is parnered with up to: 64kbyte flash, 8kbyte ram, 16 self-capacitive inputs, 64 mutual-capacitive inputs and eight inductive inputs, plus an array of timers, PWMs and serial interfaces.

For development, middleware includes board support packages, sensing algorithms and a peripheral driver library.

Operation is from 1.71 to 5.5V, averaging 200µA for active touch detection and touch tracking, dropping to 6µA for ‘always-on’ sensing.

For similar sensing, more memory and more IO, ‘4100T Plus’, a second series of MCUs, is due in the second quarter.