Design sensors into battery-powered wireless Internet of Things (IoT) devices

The Internet of Things (IoT) is transforming “simulated” events in the real world into actions and reactions of the network. The IoT nodes connected to the network can monitor the simulated events and transform them when the events that need to be reported occur. Report to the application via the Internet to complete the corresponding task. The most prominent category of IoT applications is battery-powered sensors, which are placed in areas without wires to monitor events and communicate with the IoT through wireless networks.In most cases, these products are always-on, battery-operated wireless sensors that support wireless protocols, an MCU, and at least one

The Internet of Things (IoT) is transforming “simulated” events in the real world into actions and reactions of the network. The IoT nodes connected to the network can monitor the simulated events and transform them when the events that need to be reported occur. Report to the application via the Internet to complete the corresponding task. The most prominent category of IoT applications is battery-powered sensors, which are placed in areas without wires to monitor events and communicate with the IoT through wireless networks. In most cases, these products are always-on, battery-operated wireless sensors that support wireless protocols, an MCU, and at least one analog sensor.

The challenge is how to maximize the battery life of the product enough to perceive the environment on a single battery or a single charge.

The challenge can be refined into the following aspects:

1. Competent for real-time perception tasks according to application requirements;

2. Complete sensor measurement while using as little energy as possible;

3. Keep “periodically working” MCU peripherals and let the CPU core sleep as much as possible.

In this kind of application, the typical approach of many MCUs is to wake up the MCU core and then use various peripherals to complete sensor measurements (Figure 1). When there is an event (such as door opening) that needs to be reported, the MCU reports and returns to its periodic work routine. This will consume a lot of power and cannot maximize the battery cruise time, because the running “entire MCU”, including many peripheral devices and unrelated core operations, is consuming power.

In fact, this approach is likely to lead to a poor customer experience: customers put the device in their environment, set it up on the network and enable it, but after a few months, the device is affected by poor battery power management capabilities. stop working.

Design sensors into battery-powered wireless Internet of Things (IoT) devices

Figure 1 The CPU queries and remains active in each measurement, resulting in higher power consumption

1 Ideal battery-powered and wireless sensor node solution for IoT applications

The best solution will deal with every aspect of the above-mentioned challenges, and maximize the working time of the product to complete environmental sensing on a single battery charge.

Considering the above situation, battery-powered IoT sensor equipment should provide:

1. Autonomous and energy-saving sensor management and measurement system;

2. Sensor input/output, threshold and configuration that can be configured independently for each sensor;

3. Low-power, configurable logic engine, will wake up the MCU only when absolutely necessary;

4. Low-power memory used to provide cache for multiple measurements, and to extend the CPU wake-up interval;

5. Low wireless power consumption.

2 Silicon Labs Gecko Low Power Sensor Interface (LESENSE)

A few years ago, Silicon Labs foresaw the importance of battery-powered wireless sensor applications. Since then, we have made large-scale investments in low-energy wireless, MCU and sensor technologies.

Our Gecko MCU has an energy-saving architecture and provides several key systems to enable it to operate more efficiently, and its battery life is longer than other MCUs.

Gecko and Wireless Gecko (hereinafter collectively referred to as “Gecko MCU”) use low-power sensor interface (LESENSE), peripheral reflection system (PRS) and other low-power technologies to operate at extremely low power consumption levels while at the same time Most of the core and MCU are still in deep sleep mode.

Combining the above features with other features can save a lot of power.

Table 1 Requirements for battery-powered IoT sensor systems

Design sensors into battery-powered wireless Internet of Things (IoT) devices

3 Gecko LESENSE details

LESENSE is a highly configurable sensor interface and system that can autonomously and continuously manage and monitor up to 16 resistive, capacitive or inductive sensors, while keeping the entire chip in deep sleep mode, and the core (CPU) is always turned off.

LESENSE includes a sequencer, a counter and comparator unit, a configurable decoder, and RAM for configuration settings and measurement results storage.

1) The sequencer can operate the low-frequency oscillator, and process the interaction with other peripheral devices through the PRS, and can set the sensor’s duty cycle and measurement timing.

2) The counting and comparator unit counts the pulses from the sequencer and compares the information with a configurable threshold.

3) The decoder/state machine receives sensor measurements and takes actions based on up to 16 configurable states and related actions.

LESENSE configurable sensor threshold

It is not a revolutionary concept to wake up the CPU when an external event exceeds the sensor threshold. Essentially, it moves the constant MCU duty cycle from Figure 1 to a single event; when the analog event exceeds a given threshold, the MCU wakes up and performs various actions.

However, LESENSE is different from it in that it provides a complete sensor system in order to manage and monitor sensors and related peripherals, without the involvement of the CPU, and the MCU participation is also the lowest. This is the basic concept of LESENSE, and additional functions further expand the concept.

LESENSE also buffers a configurable number of threshold events without waking up the CPU. This allows the system to monitor external events over a long period of time. LESENSE collects the required peripheral blocks (such as analog comparators, low-frequency oscillators and the sensor itself) autonomously and periodically in order to complete sensor measurements, while the CPU remains in deep sleep mode.

In the conceptual diagram below, LESENSE is configured to allow sensor 1 to exceed its configurable threshold twice before waking up the CPU.

Design sensors into battery-powered wireless Internet of Things (IoT) devices

Figure 2: The input/output of each LESENSE-enabled sensor is independent and configurable.

LESENSE also provides additional functions to manage and monitor up to 16 different sensors with unique thresholds. When using the built-in low-power state machine (decoder), LESENSE can evaluate several events before sending an interrupt to wake up the CPU.

In Figure 3, LESENSE buffers the measurement information of event 1, 2 and 3 of sensor 2, and combines this information with the measurement data of event 1 and 2 of sensor 1 before waking up the core. This simple use case uses LESENSE’s individually configured sensors, low-power memory, and low-power state machine.

Design sensors into battery-powered wireless Internet of Things (IoT) devices

Figure 3: Before the CPU interrupts, multiple sensors and a unique configuration support multiple events.

The sensor node is recalibrated from the LESENSE buffered measurement

Since many sensor systems are implemented under a variety of different environmental conditions, they must be able to operate reliably under constant changes in parameters such as temperature, humidity, power supply voltage, air permeability, and connectivity.

The cache function of LESENSE allows the CPU to recalibrate multiple readings when it is woken up. This can avoid multiple re-calibration events as the situation changes, further save energy and provide a larger system calibration sample set.

Summarize

LESENSE enables Gecko MCUs and wireless MCUs to monitor resistive, capacitive, and inductive (and IR) sensors, while keeping the energy-intensive cores and most MCUs in deep sleep mode. LESENSE can monitor up to 16 sensors that use less than 1 μA and provide configurable thresholds, RAM that can buffer multiple events, and a state machine for configurable wake-up interrupts.

Get to know Gecko MCU, Wireless Gecko MCU and LESENSE:

・ Training videos about LESENSE

・ Training presentation on LESENSE (slides from video)

・ Application description

o Capacitive sensing LESENSE (AN0028)

o Inductive sensing LESENSE (AN0029)

o Resistance sensing LESENSE (AN0036)

o IR sensor LESENSE (AN0053)

o PRS C Energy-saving Peripheral Equipment Reflection System (AN0025)

・ LESENSE C Silicon Labs Community

・ PRS C Silicon Labs Community

・ Gecko MCU (EFM32)

The Links:   SKD160-12 CM25MD-24H