Non-volatile solutions to repetitive event and transaction logging

Smart power meters, automotive engine and brake controls, robotic axis controls, solar inverters, valve controls, and a slew of other consumer and industrial applications have one common memory need: a memory that can store the detail related to an ongoing operation in a small time window, capture readings until that window of time completes, and transfer the captured results onward, or, once the operation completes successfully, reset the memory and begins capturing the next set of data in the next repeating time window.

This article compares memory technologies and products that are used to perform this function. First, to align our thinking, we’ll describe several repetitive event examples in this memory class.

New energy meters on the side of homes and businesses offer the user the ability to adjust power usage throughout the day and to take advantage of lower kW/hr billing rates in non-peak hours. To do this the meter must continuously make power readings, log those readings into a small serial memory, then every few minutes upload this data to a local area consolidator for long term tracking, and then reset and begin a new collection of readings. Lost readings of just a few minutes across a neighborhood due to a power issue can cost the power company thousands of dollars. So, non-volatility is crucial.

Another example is a factory floor robotic movement, consisting of a series of small step movements that are logged until the movement is completed. Once completed, the repetitive motion and step logging begins again. If power is disturbed, it is crucial to the operation that the last completed step is remembered so the movement can continue from that position on power restore. Again, non-volatility is critical.

Most electro-mechanical systems such as HVAC control, solar dish tracking for power inverters and the like, create algorithm based data that learns and adjusts operating parameters to achieve maximum efficiencies. Captured data needs to be retained across power glitches or disturbs.

Serial memory choices
The ideal memory for repetitive event applications will have a range of densities from as small as 16kbits up to 4Mbits (lowest cost), a serial SPI interface (cost, size, switching noise), a perfect non-volatility (no readings lost), an infinite or near infinite NV endurance (number of Writes or Stores per lifetime) and it will run at the highest allowed SPI clock speeds for both Read and Write. The ideal memory will offer both random access Read/Write and sequential Read/Write, including a rollover Read/Write from highest address back to zero (simplicity). Block, page, sector, and chip erase requirements should not impact performance, and the device should rely on a high yielding technology with a good manufacturing history.

Over the last decade SPI interfaces have proliferated. Serial SRAM (Microchip, On), DataFlash (Atmel), serial Flash (Micron), and serial EEPROM (Atmel) memory products have seen growing adoption in small memory density applications where they are used to capture calibration and parametric data, user data and identification details, and hold updatable program code.

Separately, a few suppliers have introduced serial memory products specifically targeted at repetitive event applications.

Cypress Semiconductor offers a family of serial nvSRAM products (non-volatile SRAM); Ramtron created a product line based on FRAM (ferroelectric RAM); and startup Ever-spin is working to introduce a serial solution based on an MRAM (Magnetoresistive RAM) technology. These focused technologies offer the best match to the repetitive event requirements listed above, particularly the at-speed Read/Writes and non infinite NV endurance needs, albeit at a slight value based unit cost premium over serial flash and serial EEPROM solutions. Let’s compare the key features of these memories in repetitive event applications.

Serial SRAM: meets all speed and density requirement; uses CMOS process; has excellent manufacturing history; lacks non-volatility, and use of battery backup to create non-volatility is cost and area inhibitive DataFlash: meets speed and density requirement; uses CMOS with non-volatile process module; excellent manufacturing history; not fully non-volatile, will lose data in SRAM buffers on power glitch; endurance is typically only 100k STOREs; chip erase takes multiple seconds and this erase time increases with density.

Serial Flash and Serial EEPROM: meets speed and density requirements except for long Write on block or page erase times; NV Stores all data; CMOS with non-volatile process module; excellent manufacturing history; lowest market price; endurance is typically only 100k STOREs; chip erase takes multiple seconds and erase time increases with density.

nvSRAM: designed specifically as a repetitive event memory; meets all speed and density requirements, CMOS with nonvolatile process module, excellent manufacturing history, near infinite endurance (NV Store count is only consumed on a power down – device operates as a serial SRAM with infinite endurance during power up); fully random access on Read and Write with all sequential Read/Write capabilities FRAM and MRAM: designed specifically as a repetitive event memory; meets all speed and density requirement; unique process; custom fabrication; near infinite endurance; fully random access on Read and Write with all sequential Read/Write capabilities

In practice, we find the designer weighs the above criteria, but may apply extra weight to specific criteria or design preferences. For example, if having an endurance count above 1 million is a hard requirement, only nvSRAM, FRAM, or MRAM can be selected. If the designer wants to further limit his selection to CMOS processes and established suppliers, then nvS-RAM rises to the top. If low endurance and long chip erase times can be tolerated, market price is likely the key criteria, and a serial EEPROM or serial Flash may be the winning solution.

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