SoC microcontrollers power portable medical device innovation
March 01, 2008
Addressing the microcontroller needs of the portable medical space is a challenge for semiconductor companies.
Our call for articles on System-on-Chip (SoC) applications included a request for what's happening with small devices in the medical area. As seen here, "portable" is relative – some of these applications are in vivo, with very small devices made possible by highly integrated SoCs. Kevin illustrates the technology driving this miniaturization.
Portable medical equipment is improving health care for millions of patients. Products such as blood glucose monitors, heart rate monitors, ingestible gastrointestinal (GI) tract monitors, pain-blocking implants, and a host of other devices have enhanced the quality of life for those with chronic or acute diseases and conditions. Portable automatic defibrillators save lives in emergency situations. Sport watches can wirelessly record users‚Äô heart rates, measure distance, count footsteps, and perform other functions to maximize aerobic workout benefits. The coming years will continue to bring many new, innovative products for medical applications that will vastly improve the delivery and effectiveness of health care.
Practically all of these portable products need a low-power microcontroller (MCU) to take commands from the user or operator and provide readings and status updates. Because almost all of these products run on batteries, a key consideration is lengthening battery life. These products vary in the battery lifetime required, but 2-10 years between battery changes is not unusual. In addition, these space-constrained products generally require highly integrated analog components to condition and convert the signals into the digital domain so they can be processed and interpreted.
Addressing the microcontroller needs of the portable medical space poses a challenge for semiconductor companies. While engineering is generally about making trade-offs between opposing features, specifications, and space constraints, such trade-offs are generally difficult to make in the portable medical arena. The requirements in this market are often incongruous, such as the need for a small form factor along with high functionality, low power consumption along with high-performance analog, and long battery life along with high processing capability. These products require an Analog-to-Digital Converter (ADC), adjustable gain, power management, and Liquid Crystal Display (LCD).
This article will discuss some of the innovative products in the medical market, common trends in the medical equipment industry, and how SoC microcontrollers can be used to improve future devices.
The introduction of ultra-low-power, highly integrated MCUs has spawned new products that can alleviate pain and improve patients‚Äô care and quality of life.
The Eon spinal cord stimulator from Advanced Neuromodulation Systems, a St. Jude Medical company, is an example of an implantable device designed to treat chronic pain (Figure 1). The implanted device sends mild electric pulses to leads located near the spinal cord that mask or interrupt the sensations of pain as they travel from the nervous system to the brain, replacing painful sensations with a more pleasing sensation called paresthesia.
Another interesting, recently introduced product, the SmartPill (Figure 2), is designed to be swallowed by the patient to monitor conditions in the GI tract. While traversing the GI tract, it wirelessly transmits data to a portable data collection unit. Previous methods of collecting data through the GI tract were much more invasive and/or inconvenient for the patient. This device must meet extremely small form factor and battery-life requirements.
As the SmartPill system reveals, low-power wireless protocols, such as RFID, ZigBee, 802.15.4, and other proprietary protocols have spawned a new level of innovation in the medical space. In addition to patient-oriented products, multiple companies have recently announced wireless protocol-based asset-tracking systems designed to track hospital equipment and personnel.
Portable medical equipment can greatly benefit from the level of integration modern SoCs with onboard MCUs provide. Without a high level of analog integration to offer more capability in a small space, many of these tiny products would not be possible.
Figure 3 shows a block diagram of a modern microcontroller that can be used as an SoC for portable medical products such as glucose monitors or pulse oxymeters. The integrated microcontroller is the only IC needed to run this device. This type of microcontroller, often used in medical applications, may contain the following features:
- High-performance ADC, typically 12 bits and greater
- Op-amps for signal conditioning such as automatic gain control
- Digital-to-Analog Converter (DAC), sometimes used for feedback
- Segment-based LCD driver
- High-performance reference voltage for the ADC
- Integrated flash and RAM memory
- Power management and supervisors, where these products can typically be run directly off a coin cell lithium battery or two alkaline batteries
Among the advantages of integration are reduced PCB area, cheaper manufacturing, easier procurement, and power minimization. The integrated flash and RAM can be used not only for program and data memory but also for data logging memory if the flash is programmed in-circuit.
The MSP430FG4270 MCU is a good example of a highly integrated device that generates cost advantages. It contains an ADC, DAC, programmable gain op-amps, high-precision voltage reference, and LCD driver. Using separate chips to achieve this chip‚Äôs high level of analog performance would add approximately $1 to the Bill Of Materials (BOM), not to mention the additional procurement, logistical, and manufacturing costs. These additional expenses are easy to disregard during the design process but can be substantial, especially if manufacturing is shut down due to one or more components being out of stock.
Having options for memory size is also a significant benefit for designers. Because memory size and die size have a roughly linear relationship, the cost of an MCU is proportional to the memory size. The ability to select memory size based on programming needs not only optimizes the BOM cost but also reduces risks if the preliminary estimates for programming memory are low. This ability also allows different grades of end products with varying amounts of functionality using only firmware and BOM options.
The longer a device can operate between battery charges, the higher the end-user satisfaction or patient comfort will be. Also, in most portable medical equipment, the MCU will spend the majority of its time in standby mode (asleep). This means that the ability to achieve an ultra-low standby current is a primary factor in determining a device‚Äôs battery life.
For products such as data loggers that frequently go into and out of standby, another critical parameter is the time it takes to wake up from standby. An MCU often can use just as much energy when coming out of standby as when the device is fully processing. A fast wake-up time is essential because during the wake-up period the MCU cannot do useful processing, meaning that energy used coming out of standby mode is effectively wasted. In addition, a modern MCU needs to wake up quickly in response to triggering events. For example, some of TI‚Äôs MSP430 MCUs can wake up to fully operational mode with stable clocks in less than 1 ¬µs, making near real-time operation out of standby a reality.
Since every cycle in which the MCU is active uses energy, minimizing processing time is critical to extending battery life. Low-power MCUs include many features that help minimize this time. For example, products need data to be collected from an ADC for a period of time before data is analyzed. If the MCU is active while data is collected, the processor uses energy while providing little value. Some MCUs can use direct memory access to minimize the time a processor must be active by allowing the ADC to collect data and store it into memory. Once the required number of samples is collected, the processor is interrupted out of standby; then it analyzes the data and outputs the result.
As mentioned previously, minimizing power is another advantage of integration. The MCU can have simpler and better control in enabling and disabling peripherals like op-amps, DACs, or ADCs and selectively turning these off when they are not needed.
Operational power is another crucial parameter. Even though an MCU may spend a relatively brief period in an active state, wake-ups from standby mode to check status such as battery voltage level can drain the battery quickly if operational power is not low. Also, because some of these products use a wireless protocol like ZigBee, 802.15.4, or a proprietary wireless protocol such as TI‚Äôs SimpliciTI, these systems may need to wake more often than normal to maintain the wireless network. Interestingly, even if the medical product is only turned on for a brief period each day for measurement purposes, leveraging an MCU standby current of <1.1 ¬µA, low active power is often important in maintaining low average current consumption.
Meeting the form factor requirements for portable medical products often means that a Ball Grid Array (BGA) package or Chip-Scale Package (CSP) should be used. The trade-off for the smaller size is that these packages are more difficult to manufacture than the traditional leaded package. Also, designing and debugging could require X-rays to ensure pads are soldered, and reworking a CSP or BGA is more challenging than a leaded package.
While a larger quad flat pack or plastic small outline package allows the designer to easily probe and monitor signals on pins, having the option to go directly to a smaller package for the prototyping phase will save time by letting the designer leverage code and schematics developed in the proof-of-concept phase.
SoCs aiding breakthroughs
This is an exciting time in the medical industry, and the innovations happening in the medical market are improving lives the world over. At some point, everyone will be affected by the innovation and medical advances occurring now. Health and medical designers, manufacturers, and innovators are increasingly taking advantage of the low-power, high-processing capabilities and high-performance analog integration found in today‚Äôs microcontrollers. If the marketing product design requirements for size, battery life, or accuracy seem too aggressive, engineers should take a look at what is available in microcontroller SoCs.