Using ferroelectric memory to enhance reliability of automotive systems

Author : Rich Miron | Applications Engineer | Digi-Key Electronics

01 October 2019

Using ferroelectric memory to enhance reliability of automotive systems
Using ferroelectric memory to enhance reliability of automotive systems

Non-volatile memory (NVM) plays a key role in nearly every embedded system design – but many designs have increasingly stringent NVM requirements in terms of data write & access speed, data retention & low power. This is particularly the case in automotive applications, where design engineers are looking to build in more advanced features, such as mission critical advanced driver assistance systems (ADAS).

This tutorial was originally featured in the Digi-Key Article Library and in the October 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.

But as Rich Miron, Applications Engineer at electronic component distributor, Digi-Key suggests, to ensure safe and reliable operation of these systems, designers need to take a closer look at advanced ferroelectric random access memory (F-RAM) as an automotive grade NVM option that is reliable, low power and faster than current NVM solutions.

This tutorial discusses key characteristics and advantages of F-RAM NVM technology, describing how developers can use F-RAM solutions, such as two examples referenced from Cypress Semiconductor, to enhance the reliability of ADAS – as well as use ADAS as a proxy for potential F-RAM use in other mission-critical applications.

Automotive NVM requirements
Automotive safety applications epitomise the industry trend for the integration of more advanced sensors with higher resolution and faster update rates. Automotive subsystems such as ADAS, electronic control units (ECUs) and event data recorders (EDRs) continue to evolve, relying on deep pools of data collected from a wide array of sensors. Any loss of data – or even slow access to the data – can compromise system safety, the vehicle and its passengers.

In ADAS designs, for example, the time needed to write to EEPROM (electrically erasable programmable read-only memory) can inject a potentially disastrous lag in the automatic manoeuvers designed to avoid sensed hazards. And in EDR designs, slow write performance can cause critical sensor data to be lost if power fails during a vehicular accident, likely eliminating the very data needed to understand the root cause of the accident.

F-RAM NVM characteristics
Memory devices built with F-RAM technology provide an effective NVM alternative that can meet the increasing demand and performance requirements for reliable data storage and high-speed access. The devices are fabricated from lead-zirconate-titanate (Pb[Zr x Ti1-x]O3), otherwise known simply as PZT. PZT possesses the unique characteristic where  the metal vacancy (cation) embedded within the PZT crystal will attain one of two possible polarisation states, up or down, following the direction of the applied electric field (Figure 1).

Because both are equal low-energy states,  the cation will remain fixed in its most recent polarisation state when the electric field is removed (Figure 2). Upon application of a positive or negative electric field, the cation will once again quickly transition to the appropriate polarisation state, following a characteristic hysteresis loop similar to that found in ferromagnetic materials.

The characteristics of F-RAM technology translate directly into a number of advantages for NVM devices fabricated with this technology. Because both PZT energy states are equally stable, the cation will remain in its last position for decades or possibly centuries, resulting  in unprecedented data retention rates in PZT-based F-RAM NVM devices. Also, because this technology is based on cation position rather than the charge-storage mechanisms of other NVM technologies, F-RAM devices are inherently radiation tolerant and are immune to single event upsets from ionizing radiation.

Beyond its advantages for long-term storage, F-RAM technology enhances the dynamic performance of NVM devices. The state transition is very fast and requires little energy, overcoming a fundamental limitation associated with the use of EEPROM or flash memory in mission-critical applications. EEPROM and flash devices require a significant ‘soak time’ associated with data buffering during their relatively slow write cycles. This extra delay in the write cycle results in a period where data can be at risk and completely lost if power fails before the operation completes with the final read status check (Figure 3).

To account for slower write cycles in EEPROM or flash memory, developers hoping to mitigate the effects of power failures have needed to add large capacitors or batteries, along with appropriate voltage regulators, to maintain NVM supply voltage long enough to complete write operations. In contrast, F-RAMs such  as the Cypress Semiconductor Excelon-Auto devices operate at bus speed during write operations, greatly reducing the loss of critical data and eliminating the need for supplemental power sources in the design.

Automotive grade F-RAM devices
Functionally similar to serial EEPROMs and serial flash memories, Excelon-Auto F-RAM devices are designed to meet requirements of mission-critical applications for reliable, high-performance NVM. Automotive systems designers can use these AEC-Q100-qualified devices to replace other memory types, choosing from the CY15V102QN for 1.71 to 1.89 volt supplies, or the CY15B102QN for 1.8 to 3.6 volt supplies. Both are 2 megabit (Mbit) devices, logically organised as 256 Kbits x 8.

To read the full version of this tutorial, visit the Digi-Key Article Library or read EPDT magazine's October 2019 digital issue.

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