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Choosing the Right Memory for Your Wearables Design | Cypress Semiconductor

Choosing the Right Memory for Your Wearables Design

“Wearable” electronics and portable medical devices are among the fastest-growing IoT categories on the market, with smart watches, fitness trackers, glucose meters, and blood pressure monitors helping to keep consumers fit and healthy. These devices pack a powerful combination of compute, sense, store, and connect technologies that collect important fitness data and periodically transmit it to one’s doctor for analysis.

With an emphasis on small size, low power, performance, and reliability, the decision about which nonvolatile memory technology to choose for your wearable IoT device can be somewhat daunting because there are several types of nonvolatile memories worthy of consideration. Leading examples include Flash, EEPROM, MRAM and F-RAM. Figure 1 below illustrates the characteristics of these four nonvolatile memory types. As you will see, one type of memory may not be optimized for all IoT requirements, so designers should consider the needs of their application in order to make the right choice.

Figure 1. Comparison of nonvolatile memory technologies

 

Which Nonvolatile Memory is Best for Wearables?

Flash is ideal for some IoT applications because it is available in high densities and is well-suited for storing code and configuration data. Although it supports both byte and page writes, it requires sector erase, probably eliminating it from portable medical and fitness devices requiring heavy data logging. EEPROMs are available in small densities with reasonable endurance and byte “writeability,” which makes them marginally better-suited than Flash for data logging. That said, EEPROMs consume additional cycle time after every write, which increases power consumption.

F-RAM devices provide an alternative to both Flash and EEPROM by offering virtually infinite endurance, instant nonvolatility, higher nonvolatile write speeds, and reduced power consumption. IoT sensors periodically log a few bytes of data to memory, so the fast writes enabled by F-RAM not only reduce the power consumed by the memory, but also reduce overall system power consumption by cutting power-on time. The chart in Figure 2, below, illustrates the energy advantage of F-RAM vs. EEPROM during a one-byte data write.

 

Figure 2. Energy advantage of 64Kb F-RAM vs. 64Kb EEPROM during a 1-byte data write

 

An Ideal Solution for Medical Devices

Medical devices, including diagnostic patient monitors, glucose meters, and pulse oximeters are closing the gap between professional medical equipment and portable health-monitoring products available to consumers. Today’s portable medical devices are becoming increasingly more accurate, reliable, and secure, allowing consumers to take personal control over their health, while avoiding costly and unnecessary doctor visits. 

As system complexity increases, internal on-chip memory capacity may be insufficient, requiring additional off-chip storage. For several applications, serial Flash meets these requirements because of its low cost and large storage capacity. However, Flash memories tend to consume more energy than competing chips, substantially reducing the operating life of battery-powered devices. One way around the power problem is to partition the Flash memory by replacing a portion of it with EEPROM. However, this approach is still not optimized for power consumption and will further complicate your design.

Once again, F-RAM offers several advantages vs. other nonvolatile memories for portable medical applications, including high endurance and low power:

High-Write Cycle Endurance – The limited write endurance of EEPROM and Flash create potential issues for medical devices that need to reliably store data logs that are constantly being updated. Flash offers endurance on the order of 1E+5, while EEPROM is slightly better at 1E+6. F-RAM write cycle endurance, by comparison, is 100 trillion (1E+14)—several orders of magnitude higher than both EEPROM and Flash. This level of endurance enables designers to log more data without having to over-provision the memory and implement complex wear-leveling algorithms.

Low Power Operation – The internal architecture of F-RAM has been designed to consume lower active energy than charge-based storage devices, such as Flash or EEPROM (see Figure 3 below). This factor, combined with immediate nonvolatility, increases operating time for applications with limited power sources. The figure below provides a battery-life comparison for different nonvolatile memory technologies.

 

Figure 3. Battery life comparison for different nonvolatile memory technologies

 

The efficiency and reliability of F-RAM makes it possible for a single memory technology to serve both code and data. As a memory technology, F-RAM has the endurance to expand and support high-frequency data logging, while reducing cost, improving system efficiency, and reducing design complexity, making it an ideal choice for portable IoT device designs.

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