When designing a portable device, you want to maximize interoperability and user friendliness, so you want to choose an interface like USB. And when you incorporate USB, you also make your gadgets host-agnostic. It doesn’t matter whether your users connect to a PC, an iPhone, or an Android tablet. Therefore, when you want to connect all of these extra gadgets via USB to your battery-powered go-to mobile devices, what was never a concern in the original USB specification – power consumption – suddenly becomes a top priority when choosing a USB-based solution.
You don’t want to waste the precious battery life of a tablet or laptop just to communicate with the on-board peripherals. And you don’t want to design a simple add-on application for a smart phone that quickly drains its battery.
By choosing the right USB-enabled hardware, you will be able to develop your device with a much smaller energy footprint since a universal M2M interface allows you to exclude almost all external components.
In general, only the host can initiate transfers. Even if there is no communication, the host sends keep-alive messages to the device every millisecond. If the device has data available, it will reply. In this active mode, the device has up to 100 mA of power, and the host expects the device to provide an immediate response to any request. When the host stops sending these keep-alive messages for 3 ms, the device should enter a suspend state and immediately reduce its current draw below 3 mA.
In the suspend state, most of the device can be switched off, and usually we can switch off the most power-hungry parts of the PHY. Even though a 3 mA suspend current should be easily achievable by any modern MCU, there is no reason to keep it that high. MCUs with well-thought-out energy modes, like the Silicon Labs EFM32 Happy Gecko MCU, should be able to achieve less than 3 µA in this mode, including the current draw of the PHY.
However, in active mode, when inspecting the USB communication of a regular keyboard device, active mode is still not very active; most of the time, the device is just waiting for the host to send data. However, whenever the host requests a response from the device, the response must be immediate; that is why most implementations keep the USB peripheral running at 48 MHz at all times to allow sufficient response time. In this particular example, the lines are idle for 97 percent of the time, even though we are enumerated and active.
A USB implementation that decides when the clock is needed and for how long is uniquely optimized for battery-powered applications. Silicon Labs now has two patents pending for designs to make the USB interface truly usable in today’s battery-powered IoT world. Energy-efficient communications, even in active mode, are enabled by using crystal-less USB oscillators and by disabling the power-hungry part of USB connectivity between packets, as shown in Figure 3. This innovative approach greatly reduces system-level power consumption and creates a truly universal M2M interface offering exceptional energy efficiency.
Low-energy USB should be implemented in a way that is completely transparent to developers and to end users. What will be noticeable is significantly reduced power consumption through low-energy modes (LEM), as shown in Figure 4. When this technology is combined with other space- and cost-saving features such as crystal-less USB implementations and clock recovery, developers can realize a truly ultra-low-power universal M2M interface without the need for additional external components.
When examining the evolution of the USB interface, it’s clear that the next step is to make USB the universal and power-friendly solution for battery-powered devices. MCUs like Silicon Labs EFM32 Happy Gecko make the minute decisions necessary to reduce power consumption dramatically, enabling USB to penetrate markets where it has not yet succeeded to its fullest potential.