Use a cellular and GPS SiP to implement asset tracking for agriculture and smart cities
Developers can override the default low-power sub mode, switching instead to a constant latency sub mode. In constant latency sub mode, the PMU maintains power to some resources, trading an incremental increase in power consumption for the ability to provide a predictable response latency. Developers can invoke a third power mode using the external enable pin, which powers down the entire system. This capability would typically be used in a system design that uses the nRF9160 SiP as a communications coprocessor controlled by the host system’s main processor. These power optimization features enable the SiP to achieve the kind of low power operation needed to ensure extended battery life in an asset tracking device. For example, with the microcontroller in the idle state and the modem powered down, the SiP consumes only 2.2 microamps (μA) with the real-time counter active. With the microcontroller and modem both off and power maintained only to the general-purpose input output (GPIO)-based wakeup circuitry, the SiP consumes only 1.4 μA. The SiP continues to achieve low power operation while executing various processing loads. For example, running the CoreMark benchmark with a 64 MHz clock requires only about 2.2 milliamps (mA). Of course, as more peripherals are enabled, power consumption rises
accordingly. Still, many sensor- based monitoring applications can often operate effectively at reduced operating rates that help maintain low power operation. For example, current consumption for the integrated differential successive approximation register (SAR) ADC drops from 1288 mA to less than 298 mA when switching from a high accuracy clock to a low accuracy clock for sampling in either scenario at 16 kilosamples per second (Ksamples/s). The device also uses other power optimization features for its other functional blocks including GPS. In normal operating mode, continuous tracking with GPS consumes about 44.9 mA. By enabling a GPS power saving mode, current consumption for continuous tracking drops to 9.6 mA. By reducing the GPS sampling rate from continuous to every two minutes or so, developers can significantly reduce power. For example, the GPS module consumes only 2.5 mA when performing a single-shot GPS fix every two minutes. The device’s support for other power saving operating modes also extends to the nRF9160 SiP’s modem. With this device, developers can enable modem features supporting special cellular protocols designed specifically to reduce power in battery-powered connected devices.
Figure 2: The Nordic Semiconductor nRF9160 SiP combines an SoC with application processor and LTE modem with other components needed to implement a compact low power cellular-based design for asset tracking or other IoT applications. Image source: Nordic Semiconductor
How the nRF9160 SiP achieves low power cellular connectivity The nRF9160 SiP combines its extensive hardware functionality with a full set of power management features. Its included PMIC is supported by a power management unit (PMU) which monitors power usage and automatically starts and stops clocks and supply regulators to achieve the lowest possible power consumption (Figure 3). Along with a System OFF power mode, which maintains power only to circuits needed to wake the device, the PMU supports a pair of System ON power sub modes. After power-on-reset (POR), the device comes up in the low-power sub mode, which places functional blocks including the application processor, modem, and peripherals in an idle state. In this state, the PMU automatically starts and stops clocks and voltage regulators for different blocks as needed.
Utilizing low power cellular protocols As with any wireless device, the largest contributor to power consumption, besides the host processor, is typically the radio subsystem. Conventional cellular radio subsystems take advantage of power saving protocols built into the cellular standard. Smartphones and other mobile devices typically use a capability called discontinuous reception (DRX), which allows the device to turn off its radio receiver for a period of time supported by the carrier network. Similarly, the extended discontinuous reception (eDRX) protocol lets low power devices such as battery-operated asset trackers or other IoT devices specify how long they plan to sleep before checking back in with the network. By enabling eDRX operation, an LTE-M device can sleep up to about 43 minutes while an NB-IoT device can sleep up to about 174 minutes, dramatically extending battery life (Figure 4). Another cellular operating mode, called power save mode (PSM), enables devices to remain registered with the cellular network even while they are in sleep mode and unreachable by the network. Normally, if a cellular network is unable to reach a device within some period of time, it will terminate the connection with the device and require the device to
with mechanisms designed to protect sensitive data. In addition, a secure key management unit (KMU) provides secure storage for multiple types of secret data including key pairs, symmetric keys, hashes, and private data. A separate system protection unit (SPU) also provides secure access to memories, peripherals, device pins and other resources. In operation, the SoC’s microcontroller serves as the host, executing application software as well as starting and stopping the modem. Other than responding to start and stop commands from the host, the modem handles its own operations using its substantial complement of integrated blocks including a dedicated processor, RF transceiver, and modem baseband. Running its embedded
firmware, the modem fully supports 3GPP LTE release 13 Cat-M1 and Cat-NB1. Release 14 Cat-NB2 is supported in hardware but requires additional firmware to operate. Figure 3: The nRF9160 SiP includes a PMU that automatically controls clocks and supply regulators to optimize power consumption. Image source: Nordic Semiconductor
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