Making the Case for MEMS Timing

Automotive systems require around-the-clock timing robustness.

MEMS technology can provide enhanced timing accuracy and robustness for all manner of automated-driving and ADAS components. (Continental)

Today's high-performance automotive electronic systems rely on precision timing technology for accurate, stable frequency control of digital components, from applications processors to microcontrollers to FPGAs (field-programmable gate arrays). Timing devices range from passive resonators and active oscillators to integrated clock generators and buffers, each performing a different clocking function in an automotive system design.

Many onboard vehicle systems rely on extreme timing accuracy. (SiTime)

A typical vehicle uses up to 70 timing devices to keep systems operating smoothly. That number is growing as more vehicles adopt smarter technology. Clocks and oscillators provide precise, reliable timing references for a wide array of critical digital systems within electronic control units (ECUs) for advanced driver-assistance systems (ADAS), in-vehicle networks, infotainment and other subsystems.

Timing devices are the heartbeat of all electronics. They provide the precision clock references required for high-speed communication links that transfer ever-increasing amounts of data from sensors to ADAS computers. They enable vehicle-to-everything (V2X) and 5G communications. Timing also is the foundation of the Global Positioning System (GPS) and other global navigation satellite systems.

In vehicles, quartz-based timing devices are known to be a weak link that causes system issues, either when new or as they age. There is a compelling need for more-robust timing solutions that can operate reliably for decades in demanding environments. That's where precision timing comes in to solve the problem.

Looking toward MEMS timing

Traditionally, the most common clock source is a crystal oscillator, which is based on 70-year-old technology that has matured to a point where improvements are marginal. Quartz crystals have fundamental limitations such as fragility and susceptibility to mechanical stresses. Automotive electronics operate in demanding environments subject to vibration, shock and temperature extremes, which can take a toll on sensitive quartz timing devices.

As an alternative to quartz components, microelectromechanical system (MEMS) technology has emerged as a primary timing source for automotive applications, such as ADAS and EV power- and battery-management systems, which demand fail-safe reliability while operating in harsh environments. MEMS timing components are designed to meet rigorous AEC-Q100 automotive qualification requirements, unlike quartz counterparts, which are commonly qualified to the more-lenient AEC-Q200 standard. The AEC-Q100 qualification assures automakers that their chosen timing components provide the robustness, reliability and performance demanded by automotive electronic systems.

Silicon MEMS technology is widely used in current electronics systems — from cellphones to automotive and aerospace applications. MEMS devices serve as gyroscopes, accelerometers (for airbag deployment, for instance), microphones, loudspeakers, sensors, magnetometers, and many other functions. All silicon MEMS devices, including SiTime's MEMS resonators, are manufactured at scale in mainstream fabs, providing proven technology for demanding applications.

A low FIT rate

SiTime’s SiT1881 oscillator. (SiTime)
Etienne Winkelmuller, SiTime’s Director, Segment Marketing – Automotive. (SiTime)

MEMS-based precision timing components have proven advantages for automotive applications, particularly safety systems that enable automated-driving functions, including ADAS computers, gateways, cameras and radar/lidar systems.

Silicon MEMS technology is much more reliable than quartz crystals for clocking applications. This reliability is expressed in terms of failure per 109 hours of operation or FIT. Because 109 hours (billion hours) is quite a long time to measure, the industry uses statistical analysis and accelerated models to determine FIT. The FIT rate of a silicon MEMS device is < 0.5 FIT (MTTF > 2 billion hours), calculated with a 90% confidence level, which is up to 50 times better than quartz technology.

A low FIT rate is of prime value for automotive safety integrity level (ASIL)-rated automotive systems. All ASIL-rated systems must undergo functional safety certification based on the ISO 26262 standard. This certification process consists of computing hardware safety metrics relative to a given target. For example, an ASIL D target is more difficult to meet than ASIL B. The FIT rate of individual elements in a system is used in this calculation. A better FIT rate for clock devices means enhanced system-level safety metrics and greater ease in achieving higher ASIL ratings. Since MEMS-based precision timing solutions are up to 50 times better in reliability than quartz, they should be preferred in ASIL-rated applications.

Robust and resilient

MEMS resonators, which are at the heart of precision timing devices, are up to 80% smaller than quartz crystals, which allows them to be integrated into smaller, lower-mass oscillators used in space-sensitive automotive applications such as camera modules and radar/lidar sensors. The lower mass has a significant side benefit – it allows MEMS oscillators to offer up to 100 times more robustness to mechanical environmental disturbers such as shock and vibration.

On the electrical side, MEMS-based precision timing devices have 100 times better robustness to EMI disturbers. This resiliency is especially beneficial for applications with high currents and electromagnetic fields, such as battery-management systems for EVs.

Silicon MEMS also have excellent intrinsic material properties. For example, the frequency accuracy is well controlled over a high-temperature range and does not diverge exponentially at extreme temperatures (a common crystal behavior). A typical MEMS oscillator has a frequency stability of ±50 ppm over -40°C to +125°C (-40° F to 257° F) – five times better than crystal. This number includes initial accuracy, temperature effects and aging. Adding temperature compensation improves stability to as low as ±0.1ppm. This level of accuracy enables better synchronization of V2X and 5G communications over an extended temperature range.

In addition, MEMS timing is free from "cold-start issues" at the bottom of the temperature range, which often plagues systems using quartz-based oscillators. Silicon MEMS resonators also are not subject to so-called "micro-jumps.” Random, non-reproducible jumps in frequency, common with crystal oscillators, can result in a loss of signal for GNSS or V2X/5G communications, which is a significant issue in ADAS.

Etienne Winkelmuller is Director, Segment Marketing – Automotive for SiTime .