Today Wi-Fi products can do everything from sending email to streaming audio and video- linking users to the Internet from their home, office, coffee-shop or even from a plane 30,000 feet in the air. All this is essentially enabled today by the Wi-Fi connection on laptops and mobile phones.
The next wave of Wi-Fi applications will likely be driven by the integration of Wi-Fi into embedded systems-the billions of dedicated electronic systems that run on microcontrollers. The applicability of robust wireless local and Internet connectivity provided by Wi-Fi has prompted a rush of Wi-Fi integration into embedded systems. These systems are the heart of a broad range of devices in various vertical segments, including healthcare, industrial, automotive, smart energy and smart buildings. In this article, we will cover the integration of Wi-Fi connectivity into an embedded design and discuss come of the challenges and benefits of doing so and how to overcome those challenges.
Wi-Fi's original intention was to provide wireless extension to data oriented Ethernet. The software protocol is designed to work as a layer-2 network interface. The Wi-Fi protocol is sophisticated and typically has a requirement to run a Wi-Fi driver on a host processor with a minimum size of 100KB. Integration of these complex Wi-Fi drivers typically requires long design cycles and expertise in Wi-Fi.
Embedded systems are characterized by limited computing resources with limited host memory. They perform dedicated functions like interfacing to sensors or peripherals. They are characterized by a variety of micro-controllers ranging from 8-bit versions, with a few kilobytes of instruction memory, low-speed serial interfaces UART/SPI to 32-bit higher-end versions with SDIO/USB interfaces. Most embedded systems run a simple RTOS or no operating system at all; a host microcontroller may not even have TCP/IP on it.
Embedded Wi-Fi poses multiple challenges such as seamless ease of integration, how to future proof Wi-Fi on resource constrained systems and shorter design cycles. Moreover, embedded systems demand wireless devices that offer ease of integration, little or no load on the host microcontroller, low power operation, low cost, quick wake-up from low power modes, small footprint, and guaranteed interoperability in current or future wireless network scenarios. Their users also wish to be free from demands associated with wireless engineering.
The integration of a Wi-Fi interface into these systems should not only have a minimal impact on the system configuration and resources used, but also on the design methodology itself. Embedded users are acquainted with tool-chain and development environment of microprocessors. If Wi-Fi can act as a peripheral to microcontroller platforms providing an option of ready-to-use starter kits with fully compiled libraries integrated to development environment, it will also provide an excellent starting point for a large number of embedded system designers to "Wi-Fi" their products.
Embedded Wi-Fi block diagram:
Figure 1 shows a WLAN subsystem integrated into the embedded system. The microcontroller's UART or SPI interface is used here to connect to the WLAN module.
Design considerations for integrating Wi-Fi to an embedded system
Interfacing a WLAN subsystem to a microcontroller-based device requires considering several factors including the physical and electrical specifications, choice of interface, host load, the software architecture, power-save mechanisms, wireless configuration, wireless performance, and certification. We look at these below.
Hardware integration:
Figure 2 shows the main components of the Wi-Fi subsystem and integrated as a chip or module. Fully integrated self-contained Wi-Fi modules or integrated chips require a single 3.3V supply and provide a simple external interface consisting of the antenna and the host-interface. High levels of integration, with on-chip DC-DC, zero-host-architecture and power-amplifier integrated into the RF. The choice of host interface is generally made from one of several low-power interfaces including SDIO, SPI, and UART. The modules are FCC/IC/CE certified and enable modular certification for the systems into which they are built-in. Self-contained WLAN modules are usually calibrated during manufacture, and the software controlling the device would use the calibration data during normal operation. Using pre-calibrated wireless modules helps avoid the complexity and cost of calibrating an assembled embedded system during its manufacture.
Software integration:
Software architecture is an important aspect of the integration of a wireless LAN into an embedded system. Figure 3 shows the typical complete software stack related to data transfer over 802.11 WLAN. This software option is used when a host microprocessor has an operating system and has the capability to handle a Wi-Fi host driver.
On the other, most embedded systems are based on low-end microprocessors with no operating system or TCP-IP stack capability. Figure 4 shows the delineation of software functionality between the host microcontroller and the WLAN module in such cases.
Ease of deployment (Wireless configuration):
Embedded Wi-Fi modules are sometimes integrated into end devices that lack a display/keyboard to configure the wireless link (e.g., security password and other link parameters). This limitation can be overcome if the end device has a serial or USB connector; in that case, the link can be configured through that interface. In a few embedded systems (e.g., wireless tags) the provision of that connector would violate the system form-factor- in such cases the "wireless" configuration mechanisms have to be deployed. One of the possible implementation could be:
The embedded Wi-Fi device is put to a factory-reset state, starting with a default configuration. New credentials like SSID, passphrase for new connection are wirelessly transferred through a configuration application running on a laptop or a smartphone.
Full featured and future proof for long-life span:
Embedded systems (wireless thermostats, sensors, etc.) demand long-life span. Wi-Fi technology evolved from802.11b in 1997 with a migration to 802.11g in 2003 and then to latest state of the art 802.11n technology in 2009. Designing an embedded system with future proof technology like 802.11n is a very important design consideration. The following are a few advantages of 802.11n devices over legacy 802.11b/802.11g Wi-Fi devices:
1. Future-proof without cost overhead
Single antenna 802.11n incurs zero cost-overhead over legacy 802.11b or 802.11b/g solutions and future-proofs the Wi-Fi system!
2. Lower power
The higher available data-rates (up to 65Mbps) enable smaller durations for packet transfers and hence less time spent in "active" mode, thus reducing average current consumption. In-fact single-antenna 802.11n solutions transfer 20x more data than 802.11b solutions and 2x more data than 802.11g solutions on the same battery!
3. Better coexistence
Increasingly, 802.11n embedded systems are deployed into enterprise and commercial 802.11n networks. These networks use Wi-Fi for voice, video and other quality-of-service intensive applications. Legacy solutions, especially 802.11b solutions, disrupt such networks since they use the same wireless medium. In addition to this, 802.11n access-points use ‘legacy protection' mechanisms in the presence of legacy 802.11b or 802.11g solutions. Legacy solutions affect the network throughputs and quality of service. Network capacity is preserved with 802.11n devices.
4. Higher throughputs
Due to higher PHY data-rates and advanced MAC mechanisms including packet-aggregation and block-ack to reduce overheads, single-stream, 802.11n implementations can achieve 40-45Mbps of host goodput. This is almost double compared to legacy 802.11g implementations.
5. Higher range
STBC in the down-link (AP to STA) and MRC in the uplink (STA to AP) enable 6-9dB better performance in multipath for single-antenna 802.11n stations over legacy 802.11b and 802.11b/g stations.
Development path with Wi-Fi starter kits:
Wi-Fi Starter and evaluation platforms provide a user-friendly introduction of embedded Wi-Fi modules, for example Connect-io-n modules from Redpine and its integration with MCUs from leading vendors (Atmel, Cypress, Freescale, Renesas,etc). These kits include an API library to control and configure the Wi-Fi module, sample code, and easy-to-use demo applications integrated to MCU software/hardware tools.
This approach makes the integration of Wi-Fi connectivity into an embedded design as easy as using any peripheral on microcontroller and aid penetration of Wi-Fi connectivity to billions of embedded devices.
www.redpinesignals.com