MPUs and MCUs
Save board space and system costs in battery powered applications with small footprint MCUs
By Stian Sogstad, MCU8, Microchip Technology D
eveloping battery powered and connected applications such as security systems and wireless medical monitoring devices is mainly a matter of optimizing
design complexity and power efficiency - after all, the longer the required battery life, the lower the average power consumption must be.
To achieve these power requirements, while enabling devices that are reliable and have a long life, small and power efficient microcontrollers (MCUs) are ideal. Some also offer smart, sophisticated built-in features and functionalities, allowing them to handle most tasks required by the application. As well as reducing the need for external passive components in a sensor node design, they also offer low power and built-in features for extra flexibility and simplicity.
For example, battery powered sensor nodes for a home security system will often use Passive InfraRed (PIR) motion detectors. These sensors detect changes in the amount of infrared radiation “seen” by the sensor elements, which varies depending on the temperature and surface characteristics of the detected object. When a person passes between the sensor and the background, the sensor detects a temperature change caused by the person’s body. This change is then converted into an output voltage (VPIR(t)). Other objects with the same temperature as the background, but with different surface characteristics, will cause the sensor to detect a different emission pattern, as seen in Figure 1.
The output signals from a PIR sensor are typically very low and less than 1 mV. To ensure a correct detection, the analog signal needs to be amplified before being sampled by the analog-to-digital converter (ADC). Typically, this is achieved using high gain Op Amp stages, adding to complexity, component count and costs and reducing power efficiency.
40 April 2022 Components in Electronics
www.cieonline.co.uk
Figure 1. PIR sensor motion detection principle
Cutting design complexity An alternative is to base a PIR sensor node design on a small footprint MCU with features such as a 12-bit differential ADC with programmable gain amplifier (PGA). This reduces external components, board space and bill-of-material (BoM) cost. For example, the PIR Click sensor from MickroE is a printed circuit board (PCB)
containing passive components to make a working PIR sensor node. The click board is based on an Op Amp solution, including ADCs, resistors and capacitors, and works out of the box. A PIR sensor can benefit from an MCU like Microchip Technology’s ATtiny1627, which features a 12-bit differential ADC and PGA. This can cut the number of external components as there
is no need for an external Op Amp. This, plus an external ADC, also allows passive components like resistors and capacitors to be removed.
By using such an MCU, the PCB layout of the PIR Click can be simplified. Figure 2 illustrates how components can be removed (X) and how new connections can be made (blue lines).
Figure 2. Example modifications to the PIR Click and schematics
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