Feature Story

Low-Density Serial Flash Is Back—and IoT’s the Reason

By Paul Hill, Adesto Technologies

Low power and enhanced system performance fuel a re-emergence.

For the past decade, we have seen an abundance of systems employing off-the-shelf MCU devices with embedded flash. However, as designers now try to future proof their applications by enhancing performance, reducing power consumption, and extending battery life, several new challenges have emerged.

Amazon Echo, Google Home, and many other home hubs, media controllers, and building/home automation controllers are beginning to have a significant impact on our lives. These technologies bring with them new protocols and standards. Designers want to add this new functionality to their applications in the form of Over-the-Air (OTA) updates. This requires larger firmware images for these new applications to retain compatibility, functionality, and interoperability. These enhanced applications need more memory space and often exceed the flash and RAM resources on the embedded MCU. OTA capabilities require extra memory to store the additional firmware code and must support up to three of four complete firmware images: factory default image, current image and the new downloaded image ready to be shadowed to the internal MCU flash. OTA updates must happen easily, reliably, securely, safely, and autonomously.

Figure 1: New edge nodes need OTA update capability, local data storage, and system flexibility. (Courtesy Adesto Technologies)

Problems

So what about the MCUs? As MCU vendors scale to smaller geometries, the size and cost of embedded flash pose problems of their own. Larger density flash embedded inside the MCU can often increase cost while reducing performance and/or increasing power consumption. Even with the latest MCUs, engineers are wondering when they will again run out of memory. We have already seen the first of a new generation of flash-less MCU devices this year: MCU vendors abandoned embedded flash and moved to a more cost-effective, higher-performance MCU-only solution.

Modern day IoT application challenges have brought about an increased demand for low-density serial flash. This is all happening while the market enters a phase of supply shortages, capacity restrictions, and a rash of portfolio-wide end-of-life notifications. An MCU used inside a smart sensor, smart door lock, or other IoT edge node device with 256Kbit to 8Mbit of embedded flash will typically need between 1Mbit and 32Mbit of external flash to provide external OTA capability.

Standard serial flash devices have been available for decades, but evolution has focused on achieving higher density, higher performance, and lower cost. Designers often select the external memory device as a last resort when it is evident they need a larger MCU with more flash; this last-minute decision comes at higher cost, with a new PCB layout and complete new code image. Alternatively, designers choose to include an external memory to allow expansion of the current system to support the new features and requirements.

At Odds

It can be argued that the current range of serial flash solutions are often at odds with application demands. The serial flash devices today have architectures suited to high read performance and lower cost, while power consumption and programming flexibility are sacrificed. Adding these memory devices substantially increases power consumption, reduces the battery life, increases MCU overheads, reduces system performance, and often exceeds the embedded SRAM resources required to temporarily support the external flash during OTA programming and updates.

Figure 2: New protocols and standards are arriving in the wake of new home hubs, media controllers, and building/home automation controllers. (Courtesy Adesto Technologies)

Factors such as system security are also impacted. This is due to the need to erase and reprogram large blocks of data, the inability to permanently protect factory code images and the generic un-personalized nature of the commodity memory devices—all of which can lead to vulnerabilities in the device trust chain. In addition, components such as voltage regulators may be required to ensure correct operation.

The latest generation of low-density memory devices have architectures and features optimized to address many IoT device and edge node application demands.

Wide Voltage Range Operation
- Devices that support a wide operating voltage (e.g., 1.65V to 4.4V) that matches the VCC range of the host MCU to eliminate the need for additional voltage regulation and maximize battery life.
Optimized Operating Power
- Read and programming current is optimized to improve energy consumption and to further maximize battery life.
Ultra-Low-Power Standby Power Consumption
- Devices that have a user command driven by ultra-low standby power consumption at under 100 Nano amps negate the need to switch the power supply to the memory device and conserve power when the memory is not being used, saving on further external components and MCU GPIO pins.
OTA optimized and Data Storage Small Page Program and Erase Structure
- Small page erase capability allows code updates as small as 256Bytes to be erased and programmed with minimal MCU over heads, while improving performance and reducing update time.
Serial EEPROM Emulation Capabilities
- A byte write/byte erase capability that does not require large block erase and considerable MCU resources to manage it allows simple data and system configuration to be updated a few bytes at a time.
One Time Programmable Lockable Secure Sectors
- One-time-programmable sector locking capability to protect critical code and factory code images.
Intelligent MCU-Friendly Active Memory interface
- Features that allow the MCU to sleep and reduce system power whilst the memory is programming or erasing. The memory device becomes an active peripheral device that generates its own interrupt signal when a process or operation is completed, negating the need for the memory to wake up, test, or poll the memory for status, or run an inefficient fixed delay loop counter for each programming or erase operation.
Enhanced Command Rich Serial Memory Interface
- A rich command set reducing the MCU overheads, such as a single command that acts as both an erase and buffer program in a single operation, versus having to issue an erase command, wait and monitor the erase operation, before issuing and monitoring the programming command. This saves on MCU overheads and execution time and improves system performance and battery life.
On-Chip User Accessible Security Numbers
- Memory devices that contain user accessible unique identity numbers to allow the device to be integrated into a system trust chain of components for enhanced security.
Flexible SRAM Buffers
- Flexible SRAM buffers inside the memory device that can be read from as well as used in the traditional programming-only mode, allowing data and code storage to be loaded directly to the SRAM buffer and read back or modified later. These R/W SRAM memory buffers can even be used as extended scratch pad space to supplement the embedded SRAM and stack space on the MCU.

Low Density, High Interest

The rise of new IoT applications is introducing new demands and driving a resurgence of interest in low-density memory. Many of these devices are battery powered and need to be Amazon Alexa or Google Home compatible. To keep pace with evolving and growing standards all of these new edge nodes require OTA update capability, local data storage, and system flexibility. Having a low-power memory that forces the MCU to burn more energy to manage the memory is a false economy. The latest Adesto Serial Memory product options provide intelligent, MCU-friendly features to reduce overheads, improve security options, increase device flexibility, improve performance, enhance energy consumption, and extend battery life. The memory device is a critical component in any modern system, and can now be treated as an intelligent system peripheral that works autonomously with the MCU rather than just a simple memory storage solution that is a slave to the host MCU.

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