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How Low Can You Go?

Tue, 06/05/2012 - 5:58am
Donnie Garcia

"Low power” is almost always used to describe MCUs, and it’s usually accompanied by power mode specifications and battery life calculations based on milliamp hours. While those aspects of low power design are important, there are other embedded design techniques that can lead to a significant extension of the usable lifetime of a product, which is the focus of this article.

Medical device manufacturers should consider a few system level guidelines to enhance their designs with little or no impact to battery life. For many medical devices, the microcontroller is a central component that performs most—if not all—of the application tasks. A microcontroller can be the main contributor to the device power consumption, so making use of the microcontroller’s features is essential to achieving battery life targets. Whether on the shelf, in a standby mode, or performing measurements, having certain capabilities can affect battery lifetime and thus determine the value of a product to the end customer. The examples in this article can be used as guidelines for the development phase of a medical device and can assist in microcontroller selection.

Separate Power Domain
As medical devices continue to be enhanced with supplemental connectivity and human machine interface features, the microcontroller memory and input output capabilities grow as well. Today, microcontrollers with half a megabyte of flash and greater than 100 I/O lines are becoming common in semiconductor product portfolios. This trend presents multiple challenges to semiconductor manufacturers, as power and cost targets do not change as quickly as the microcontroller memory sizes. In regards to power consumption, the problem with larger memory and I/O counts is leakage. Each memory bit cell and I/O driver has a leakage current. The greater the microcontroller memory size and I/O count, the greater the leakage will be.

One example of the impact of leakage to battery life is considering the shelf-life scenario. The storage time and temperatures can vary from device to device, but one year shelf time with extended temperature conditions is not uncommon. Due to leakage, even a minimal amount of functionality, such as a real-time clock, can lead to a 20% reduction in battery capacity of a coin-cell battery to support a shelf time. As leakage currents are impacted exponentially by temperature, there are some scenarios where the impact can lead to a very poor out-of-the-box experience for the end customer.

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Figure 1: Typical current consumption for time keeping across temperature

To address the shelf-life problem, a separate power domain is implemented on microcontrollers, like the Kinetis K-series devices. This separate power domain is used to power only a subset of the microcontroller features, specifically the crystal oscillator and real time clock registers needed to perform time keeping. The end result is that only a small subset of the microcontroller logic and I/O are powered. With less I/O and memory being powered, the RTC power domain has an excellent leakage profile over temperature (Figure 1). At 105°C the typical leakage for the RTC power domain is just 2.0 µA.

To make the most out of this microcontroller feature, the MCU power should be removed during shelf time. Removing the MCU power can be implemented by adding a two position switch to allow the user to control when the entire microcontroller is powered. Another more user friendly option could be a push button switch connected to a power transistor. After the push button is pressed, the MCU can be used to disable the MOSFET switch so that the microcontroller will remain powered. In some medical devices where switches or buttons cannot be used, an infrared or magnetic sensor could be used instead of a push button switch. The sensor will add to the power consumption during shelf mode, but this would be offset by the benefit of the separate RTC power domain.

Using the RTC power domain ensures that the medical device can be ready with time and date preset with little or no user interaction. For many devices, this is a critical requirement and utilizing this microcontroller feature not only conserves battery life, but also enriches the user experience. This type of functionality could be implemented with an external IC, but having a microcontroller with this feature optimizes for bill of materials cost and board space. When designing a medical device, corner case scenarios must be considered with storage temperature in the range of 50°C or higher. So, following this guideline will ensure that only a minimal amount of battery life is spent supporting a standard shelf time and temperature specification.

Standby Mode
Once off the shelf, leakage can continue to have a significant impact on battery life. This will depend on the amount of time the medical device spends in standby mode versus the amount of time the device is active and functioning. Leakage has the largest impact for devices that spend most of their time in standby mode, which is the case for many low power medical devices. In general, low power medical devices should be in active mode for as short a time as possible. When architecting medical device standby modes, a critical parameter to consider—in addition to standby time—is response time. If the medical device can have very long standby times (one minute or greater) and there are no special response time requirements (one millisecond or greater), then the device could be placed back into shelf mode. One additional feature of the Kinetis K-Series separate RTC power domain is an RTC_WAKEUP pin that will drive a low signal when a preset alarm occurs. Using this alarm signal and an external MOSFET, the MCU can be powered at the alarm interval and then determine if additional functions must be performed.

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Figure 2: Analog to Digital Converter Compare Mode functionality

For cases when response time and or standby times are shorter (standby time in the one second range, response time in 100 µs range), a different strategy that uses a microcontroller low power mode must be employed. The Kinetis K-Series microcontroller implements a number of very low leakage stop modes that internally power down different amounts of volatile RAM memory. The amount of RAM powered can vary from 64 bytes up to 128 Kbytes. To ensure the best leakage profile, application software should be written to require a minimum amount of RAM memory, so the optimal power mode can be used.

For use cases that have short standby times and require a very fast response, a microcontroller low power mode with active logic must be used. Recovery from a peripheral interrupt will allow the device to wake up and quickly perform the desired function and return to the standby state to conserve power. The guideline for this use case is to make the most out of the active logic on the microcontroller. This can be done by ensuring that peripherals are preconfigured before entering standby and software variables are ready to be processed and the application firmware is optimized for execution time.

Table 1 provides general guidelines for different scenarios that would lead to the lowest average power consumption for a microcontroller. In many applications, the standby time and response time may change between any number of the highlighted cases, so a combination of the guidelines could be utilized.

When actively performing measurements or other application requirements, the strategy for a low power design is to perform the task with the least amount of energy. This requirement is addressed by microcontroller functionality in low power modes. For example, continuing on the subject of standby modes, the Kinetis K-series MCU contains an integrated touch sense peripheral that can be active in very low leakage stop modes. Having this functionality in the lowest power modes allows a device to implement a touch interface with minimal power and BOM cost. Alternative solutions require a microcontroller to actively poll I/O pins.

Standby time

Recovery time

Key Microcontroller feature

Guidelines

1 minute or greater

1millisecond or greater

Separate power domain for RTC

RTC WAKEUP signal

Use RTC WAKEUP signal to power the MCU at preset alarm rates

1 second or greater

100microseconds or greater

Multiple MCU low leakage modes that allow different amounts of memory to remain powered

Optimize application software to use a low leakage mode with less logic powered

<1 second

<10microseconds

Low leakage mode with fast recovery time

Optimize application software for fastest execution time by configuring peripherals and software variables

Table 1: General guidelines for utilizing microcontroller features to extend battery life

Analog to Digital Converter
Concerning analog measurements, the analog to digital converter (ADC) for the Kinetis devices contains an independent clock source that can be used to perform measurements in standby modes. In addition, the ADC has a comparison function that can compare the measured ADC reading to preset ranges and check for a number of conditions, including less than threshold, greater than threshold, inside range, and outside range. The digital values placed in the ADCCV1 and ADCCV2 registers are compared against analog to digital conversions (Figure 2).

For applications that require a certain measurement threshold to be met before data is collected, this functionality could be used to reduce power consumption as the Kinetis MCU ADC STOP mode functionality allows the majority of the MCU logic to be halted while only the ADC peripheral performs the data collection and automatic compare. The STOP mode is not exited until the desired threshold has been met, conserving battery energy.

Peripheral functionality in low power modes is an important component of a microcontroller’s low power benefit. In future platforms, such as the recently announced Kinetis L-Series devices, Freescale has added a number of enhancements in order to support peripheral low power functionality. Some of these enhancements include UART functionality in STOP modes, timer input capture, output compare, and PWM in STOP modes and DMA asynchronous operation in STOP modes. Working together along with the ADC functionality previously described, a device built from the Kinetis L-Series platform could support use cases where ADC data is collected and then transmitted through the UART peripheral using asynchronous DMA operation. This will result in use cases that have even shorter periods in full active mode. The Kinetis L-Series also includes the recently announced ARM Cortex-M0+ core and will also support very efficient processing capability with a single cycle 32-bit multiply and a new low latency I/O port.

Conclusion
Supporting the microcontroller features such as the separate RTC power domain, multiple power modes with configurable RAM retention, and peripheral functionality in STOP modes presents unique design and cost challenges to the semiconductor manufacturer. But these challenges are surpassed by the value that can be gained by making use of the features. Longer battery life could lead to customer loyalty for a medical device. In some cases, lower average power could support smaller, more comfortable form factors for patients. User friendly features such as not having to set the time and date for the device could be a differentiator from the competition. Considering some design guidelines, the full value of the right microcontroller can be extracted and utilized in an upcoming design.

Donnie Garcia is a Systems Engineer in the Microcontroller Solutions Group at Freescale.He is a regular contributor to the Embedded Beat Medical By Design blog Donnie is responsible for new product introduction encompassing product definition, development tool requirements, and microcontroller product cost and feasibility studies.

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