Feature: RF
Figure 1: Functional block diagram of the Silicon Labs EFR32BG22
MCU current consumption Unless otherwise indicated, typical conditions are: VREGVDD = 3.0 V. AVDD = DVDD = IOVDD = RFVDD = PAVDD = 1.8V from DC-DC. Voltage scaling level = VSCALE1. TA
=
25°C. Minimum and maximum values in this table represent the worst conditions across process variation at TA
= 25°C. Then we come to the all-important sleep modes and
the current consumption profile for each, as well as which SoC resources are running during each. Sleep currents can range from hundreds of nanoamps to a few microamps, and probably the most critical consumption figure is with RAM retained and the real-time clock operating. Taking the example of the EFR32BG22 SoC, the EM3 mode deep-sleep current is 1.05µA with 8kB of RAM retained and the real-time clock (RTC) running. Fully understanding the application requirements and the
anticipated duty cycle between active and sleep modes will provide an estimate of the average power consumption. An SoC with a low deep-sleep current will suit a low duty cycle, and a high duty cycle application will better suit an SoC with the lowest active consumption. The evolution of processor geometries has significantly
aided Bluetooth SoC design in the recent past. A more optimised technology node, 40nm in the case of the EFR32BG22 SoC, has permitted the integration of an on- die DC-DC converter, which significantly improves power management, reduces overall current consumption and extends battery life. Other technical advances, such as the use of low-leakage-
current SRAM, also contribute to minimising the deep-sleep power consumption. The EFR32BG22 SoC has a total of 32kB of on-chip RAM with independently-selectable SRAM blocks. Another Bluetooth SoC attribute that can have an impact on power consumption is leakage current under different operating
temperatures. Leakage current is typically quoted at 25o C and
rises with temperature. For many smart-home applications, temperature is unlikely to be an issue. However, some industrial IoT sensors, for example, might be used in higher ambient temperatures, resulting in reduced battery life. As mentioned earlier, a full understanding of the SoC
architecture will also assist when reviewing which functional blocks of the device can use RTC and power gating to shut down certain functions when not required. Fractional power savings may not appear of particular importance, but collectively they will significantly help improve battery life. Getting the best out of a selected Bluetooth SoC also
depends on being able to optimise the wireless transceiver activity. Optimisation requires a sufficiently technical and comprehensive software development toolkit (SDK) and a fully integrated development environment (IDE). With these tools, developers can optimise the radio performance, configure the DC-DC converter, and manage the Bluetooth protocols and profiles.
Comparative analysis Taking a high-level system approach to benchmarking Bluetooth SoCs from different suppliers can significantly assist in comparing devices. To support the assessment process, the Embedded Microprocessor Benchmark Consortium (EEMBC) has developed the IoTMark-Bluetooth LE benchmark profile. The profile provides the means to determine power consumption through sleep, active and connected modes by modelling a real-world IoT edge-node device consisting of an I2
C sensor and a BLE radio. Incorporating BLE connectivity into an embedded design
starts with an in-depth investigation of potential SoC devices. Rather than taking at face value the headline figures
www.electronicsworld.co.uk February 2021 29
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