• Pixelated Capacitive Sensors peripheral
• APB protocol

Gas sensors are widely used to monitor and determine concentration of one, or more gases in the environment. These gases often include $O_2$ and $O_3$, $CO_2$, $CO$, $NO_X$ and volatile organic compounds (VOCs). Air quality around the globe is declining and public health is seriously threatened by indoor air pollution. Typically, indoor air pollutants are composed of a series of VOCs that are generally harmful to the human body, especially VOCs with low molecular weights. Moreover, in some situations, more than one type of VOC is present; thus, a device that can detect one or more VOCs simultaneously would be most beneficial. A sensor able to measure several different gases is often referred to as an "electronic nose". These electronic noses have a wide range of applications in agri-/horticultural, food, environmental, medical, industrial, smart home, and homeland security domains. However, state-of-the-art e-noses consist of different sensors placed on a single PCB connected to separate read-out, A/D, control, and I/O electronics. They are stiff, large, power-hungry, and expensive. Research has been carrying out for many years to miniaturize these types of sensors. Anyhow, insufficiently robust selectivity and sensitivity of chemical sensor technologies after miniaturization are a common limitation to the progressive development of chemical gas sensors toward handheld commercial products that do not require continuous human intervention.

Recently, NXP has proposed a novel method for detecting gas through the changing in capacitance, which aims to overcome these difficulties. Capacitive sensors are widely used in several applications, and they are attractive candidates from the power consumption perspective because they do not consume static power. Their use allows the sensors and the readout circuits to be integrated in a single IC, while being also sensitive to several different gases. This means low area occupation and the possibility of embedding signal processing. With embedded signal processing a variety of novel applications can be achieved in a cost-effective way.

The detection technique introduced by NXP uses compact sensing nodes, called pixels, hence the name Pixelated Capacitive Sensor (PCS). The PCS is made up of a matrix of these pixels placed on the top metal layer of the IC. The pixels are made sensitive to specific gases by covering them with a different sensing ink, i.e., a printable material that changes its dielectric properties when it absorbs gas, VOCs, or water vapor. In particular, a range of sensitive inks will be used for the detection of crop infestation by aphids, mealybugs, etc. by smelling the VOCs secreted by the bugs and the infested plants. This will permit, for example, to check the health of plants inside a greenhouse and can therefore be a very useful tool in the agricultural field.

The PCS is embedded into a microcontroller (MCU), which provides signal processing of the measured signals. The MCU used is LPC845 where LPC is the name of a family of NXP low power microcontrollers. The sensor, the sensor interface and the reading circuit are designed in 140nm CMOS.

The goal of this project has been to design an interface that would allow the Pixelated Capacitive Sensor (PCS) peripheral to be incorporated into an LPC845 microcontroller and to verify its correct operation together with the sensor itself. Furthermore, CIC decimator filters that follow the sensor readout circuit have been designed and the layout of some of the analog circuits inside the SD modulators used in the readout circuit has been created.