The Heartbeat Behind Portable Medical Devices
Ultra-low-power, mixed-signal MCUs help developers deliver medical devices to a market that demands the very best in performance and affordability in the smallest form factors.
The medical industry is undergoing a dramatic transformation as governments, healthcare providers and consumers seek ways to make healthcare more affordable and accessible. This transformation is also occurring in the medical electronics industry as patient care is increasingly moving away from hospitals and into home environments through telehealth services enabled by cost-effective personal medical devices.
Technology Advances Accelerate Transition to Telehealth
The rapid growth of the personal medical device market stems from a variety of factors: steadily aging populations requiring more frequent health monitoring; skyrocketing costs of traditional, physician-directed medical care; growing consumer awareness of the benefits of wellness products; widespread availability of personal medical devices online and in retail outlets; and the increasing sophistication, ease of use and affordability of these consumer healthcare products enabled by continuous advances in semiconductor technology.
Because medical electronics are getting smaller, more portable and more intuitive to use, consumers are embracing the benefits of telehealth especially since it has the potential of reducing their healthcare costs. Due to recent advances in embedded wireless technologies, it is now possible to enable consumers to remotely transmit information to their healthcare provider automatically through personal medical devices. These types of advances are helping to make remote telehealth services a viable alternative to traditional in-hospital care and doctor’s office visits.
From a healthcare provider point of view, telehealth can greatly improve the efficiency and reduce the costs incurred by doctors. Insurance companies are also actively influencing this trend by encouraging access to medical care in the home, which enables reductions in medical costs while also in many cases enhancing patient care.
The electronics industry is helping to accelerate the transition to telehealth services by providing significant advances in enabling semiconductor technology for personal medical devices. To succeed in the competitive home healthcare market, portable medical devices should offer the following features:
- Intuitive, easy-to-use human interface
- Highly reliable and safe operation meeting stringent governmental regulations
- Easy, secure RF connectivity (such as sub-GHz, 2.4 GHz ZigBee or Bluetooth Low Energy)
- Low-power operation, which is essential for long battery life
- Support for a wide range of supply voltages (especially lower voltages)
- High measurement accuracy
- Small form factors
- Affordable cost
Mixed-Signal Microcontrollers Are at the Heart of Portable Medical Devices
To provide these product features at economical prices, medical device developers must reduce system cost by minimizing the number of discrete components within the design. Semiconductor suppliers are also tasked with supplying feature-rich embedded-control solutions that enable increased performance and reliability within strict power and cost budgets. At the heart of these portable medical device designs are mixed-signal microcontrollers (MCUs) that deliver exceptional processing performance at ultra-low supply currents.
Ease of use is also essential for portable medical products because it reduces errors in measurement resulting from operator error. Such patient-friendly devices should require minimal user interaction for proper operation, a simple user input (for example, fewer buttons and streamlined software menus) and large, easy-to-view displays (e.g., large LCDs with backlighting). To support these features, MCUs must provide field-programmable, non-volatile memory storage (typically in-system programmable flash memory), as well as flexible I/O configurations to make the best use of limited pins.
While many portable medical devices today simply display health monitoring results and leave the interpretation and logging to end users and their physicians, newer devices feature simple connectivity to log and transmit results automatically. Typically, these more sophisticated products will connect to personal computers or mobile health appliances with software that can track results, or they will securely transmit information wirelessly to medical professionals, caretakers or Web-based applications—a practice known as telemedicine.
USB Enables Standardized Data Transmission
The healthcare equipment market widely uses the universal serial bus (USB) interface to enable standardized transmission of data and messages, regardless of device manufacturer. Personal medical device manufacturers can choose from a variety of 8- and 32-bit MCUs with on-chip USB controllers, as well as single-chip, plug-and-play connectivity bridge solutions that support USB to UART, USB to SMBus and I2C, and even USB to I2S (making it easy to add audio features to end products). USB connectivity bridges (such as the USB-to-UART controller shown in Figure 1) greatly simplify the process of implementing USB in personal medical device designs. The bridge chips are often pre-programmed with all the necessary USB software, eliminating the need for the developer to have deep expertise in the complexities of implementing the USB specification.
|Figure 1: Single-chip USB-to-UART controllers simplify USB implementation.|
Wireless transmission of data will make connectivity even easier and more convenient for telemedicine applications. RF transmitters and transceivers working in concert with MCUs can enable wireless connectivity for a wide range of portable medical device applications. In addition, wireless MCUs—single-chip devices that integrate a low-power MCU core with a high-performance RF transceiver—are now widely available to medical device developers. Whatever the connectivity method or system architecture used, communication protocol stacks will require more code space in the MCU. As a result, more memory in smaller footprint devices will be in increasing demand.
Biometric Monitoring Defines Analog Voltage Measurement Requirements
While choices in high-performance yet energy-efficient MCUs and wireless communications options remain very important, all medical devices will measure some physical parameter to quantify some aspect of a person’s health (e.g., blood pressure or oxygen levels). This requires the ability to sense and measure light (for blood oxygen), conductivity (for blood glucose), pressure (for blood pressure) and temperatures, and these measurements must be extremely accurate and consistent.
Mixed-signal MCUs selected for personal medical device applications must give superior analog voltage measurement results in the presence of noisy digital processor and communications signals in small spaces. This is one of the most challenging engineering problems faced by semiconductor suppliers, and such specifications will be scrutinized by product engineers, especially when faced with low battery voltages for the IC. Measurements must be low in noise and distortion (good signal-to-noise and distortion ratios) and highly linear. Analog-to-digital converter (ADC) operation must be allowed even when the MCU is in operation as the end user will expect to observe functions during measurement, and it is likely the MCU will interpret results in real-time. Furthermore, all MCU features should be available even at the lowest battery voltages; what good is the MCU if you cannot make a measurement over the full battery life? In short, MCU suppliers in this market must integrate accurate analog measurements without compromise.
Portable Devices Demand Long Battery Life
The next design challenge for the MCU supplier is the demand for long battery life in the end product. “Portable” generally means that the device is battery-powered. Added features typically increase power consumption, but developers will not design a portable medical product that requires end users to use large, heavy batteries or to change them frequently.
MCUs must support three parts of a low-power strategy: Low power while in active mode, low power while in standby mode and reduced time in an active state. The portable device, and thus the MCU, will be in an off or lower power state most of the time; however, it will often maintain some sort of function such as clock/calendar or alarm when not in use. While active power consumption is important, minimizing the time awake is the key to extending battery life. MCU designers must engineer ways to wake the MCU clock and analog circuits for fast measurements and then allow the MCU to settle back to a low-power state. For example, making a voltage measurement with an ADC requires a voltage reference. Such voltage references typically require tens of milliseconds to turn on and stabilize before a measurement can be made. During this time, the MCU is on and draining the battery.
Today’s ultra-low-power MCUs wake up in microseconds as shown in Figure 2, allowing accurate ADC measurements to begin quickly. In most MCU designs, the ADC can rapidly accumulate many measurements without CPU intervention for improved results while further minimizing awake time. The less time spent awake, the less current is drawn on the battery while still delivering good performance.
|Figure 2: Fast wake times and short operation intervals will extend battery life.|
Another important trend in MCU design involves supporting new battery use configurations and technologies (see Figure 3). Rechargeable batteries are popular and typically need higher voltage support, and integrated on-chip voltage regulators are mandatory. An emerging trend is to use only one alkaline battery to reduce product size or to save cost when the end user expects the supplier to ship an installed battery. Until recently, this approach required the added cost and space of a discrete dc-dc switching regulator to boost the alkaline battery voltage for proper MCU operation (alkaline batteries have a useful life to 0.9V). Not only do these switching regulators create a large amount of noise in voltage measurements, they must remain on at all times to allow the MCU to wake from sleep mode, thereby draining power and reducing battery life.
|Figure 3: MCUs should support a wide range of voltages supplied by batteries.|
Advanced ultra-low-power MCUs that integrate a dc-dc regulator are designed to address these voltage issues. The result of this integrated approach is lower noise, less cost, reduced footprint and better control, enabling the dc-dc regulator to remain off while the MCU is in its low-power state, which further extends battery life. Even though this dc-dc regulator is integrated, it should still output the boosted voltage supply externally to the rest of the system for a true low-power, single-battery solution.
All This, and Small Form Factor, Too
While MCU suppliers continue to innovate and integrate power- and battery-saving features, the trend toward energy efficiency would not be fully advantageous if the cost and footprint of the MCU grew substantially. The goal is to help the embedded developer deliver a lower cost, smaller end product, as well as reduce power consumption. Such solutions must reduce the bill of materials and size. The best MCUs will deliver plenty of performance, integrated connectivity, memory and superior analog peripherals in the smallest form factors. In other words, semiconductor suppliers must provide increased functional density without compromise.
Note the ultra-low-power 32-bit MCU example in Figure 4 with flash code storage scaling to 256 kB, up to 32 kB of configurable RAM, an ADC and DAC, low drop-out (LDO) regulators, a dc-dc buck converter, low-power charge pump and an array of digital timers and interfaces. This compact mixed-signal MCU design allows a complete measurement and interface system on a single chip without sacrificing performance or battery life. Portable medical device designers will be careful to choose low-power MCUs like this with the right set of peripherals and energy efficiency to achieve the optimal cost, power and performance benefits. Another key factor in selecting the optimal MCU for a telehealth application is to determine what core processing performance is required for the personal medical device. For example, a blood pressure monitor will have lower core processing requirements than a blood glucose meter. Therefore, a 25 MHz 8-bit MCU may be sufficient for a majority of these types of applications. On the other hand, a 32-bit MCU may be more appropriate for much higher core processing requirements if the application can justify the higher cost of these devices.
|Figure 4: Ultra-low-power 32-bit MCUs can provide a high degree of mixed-signal integration.|
Embedded developers and product managers are under continuous pressure to push the envelope of cost, size, power consumption and performance in their next portable medical device designs. The answer is to use highly integrated mixed-signal MCUs to deliver products to a market that demands the very best in performance and affordability in the smallest form factors. Ultra-low-power mixed-signal MCUs will provide the heartbeat for the next generation of portable medical devices, and healthcare equipment makers that deliver optimized products that meet consumer needs will enjoy the benefits of this fast-growing market segment.
Shahram Tadayon is a field marketing manager for Silicon Labs’ Embedded Systems products. Mr. Tadayon joined Silicon Labs in 2002 as a product marketing manager for the ISOmodem product line, and he has also served as a product marketing manager in the company’s AM/FM receiver product line and as a senior product marketing manager for microcontrollers. Previously, Mr. Tadayon held design positions at Hewlett-Packard, focusing on circuit design. Mr. Tadayon holds a master’s and bachelor’s degree in electrical engineering from the Massachusetts Institute of Technology (MIT).