MEMS Clocking in Automotive Electronics
by Etienne Winkelmuller, Director, Segment Marketing, Automotive, SiTime
From IoT devices to communications infrastructure to industrial and automotive, every electronic system relies on timing technology for accurate, stable frequency control of digital components. Timing devices range from passive resonators and active oscillators to integrated clock generators and buffers, each performing a different clocking function.
To keep automotive systems operating smoothly, today’s cars use up to 70 timing devices, and the number is growing as more cars adopt “smart” technology. Clocks and oscillators provide precise, reliable timing references for a wide array of digital systems in automotive designs (Figure 1).
Timing devices are essential components in various electronic systems, including advanced driver assistance systems (ADAS), in-vehicle networks, infotainment systems, and other subsystems. These devices synchronize critical clocking functions within electronic control units (ECUs) to ensure that different components within the system operate harmoniously.
For example, in ADAS, timing devices synchronize the transfer of large amounts of data from sensors to ADAS computers, enabling the system to respond quickly and accurately to changing driving conditions. Moreover, timing is the foundation of GPS and other global navigation satellite systems. These systems rely on precise timing to determine device location, and any errors in timing can result in inaccuracies in location data. Therefore, timing devices are critical components in these systems, ensuring that they provide accurate and reliable location information.
Silicon MEMS technology is widely used in today’s electronics systems, from cell phones to aerospace applications. MEMS devices serve as gyroscopes, accelerometers (for instance, for airbag deployment), microphones, loudspeakers, sensors, magnetometers and many other functions. All silicon MEMS devices, including MEMS resonators, are manufactured at scale in mainstream fabs, making them a cost-effective solution.
The most common clock source is a crystal oscillator, a 70-year-old technology that has matured to a point where improvements are marginal. That’s, in part, because quartz crystals have fundamental limitations such as fragility and susceptibility to mechanical stresses. Automotive electronics operate in unforgiving environments subject to vibration, shock and temperature extremes that can take a toll on sensitive quartz timing devices.
MEMS timing is disrupting clocking by replacing these fragile devices with resonators as it is much more reliable than quartz crystals. MEMS resonators are much smaller than quartz crystals, enabling smaller footprint timing devices (down to 1.0 mm x 1.2 mm) for space-sensitive automotive applications such as camera modules and radar/lidar sensors (Figure 2). Smaller size and less mass also mean more resilience to environmental shock and vibration.
Their reliability (Failure in Time or FIT) would take a long time to measure, so the industry uses statistical analysis and accelerated models to determine FIT. The FIT rate of a silicon MEMS device is <0.5 FIT that translates to an MTTF greater than 2 billion hr. calculated with a 90% confidence level, which is 50 times better than crystal 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. Part of 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 better system-level safety metrics and greater ease in achieving higher ASIL ratings.
Compared to quartz, MEMS resonators have 100 times better resilience to EMI disturbances, which is beneficial for applications with high currents and electromagnetic fields, such as battery management systems for electric vehicles. MEMS devices also have excellent intrinsic material properties. For example, frequency accuracy is very 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 stability of ±50 ppm over a temperature range of -40°C to +125°C, which includes initial accuracy, temperature effects, and aging. Adding temperature compensation increases stability up to ±0.1 ppm. 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 plague systems using quartz-based oscillators. Silicon MEMS resonators also are not subject to so-called “micro-jumps.” These random, non-reproducible jumps in frequency, common with crystal oscillators, can result in signal loss for GNSS or V2X/5G communications.
MEMS timing devices offer several benefits for automotive applications, including high accuracy, enabling precise timing synchronization between different components within the vehicle. They also are very small, making them well suited for automotive applications where space is limited. In addition, MEMS timing devices consume low power, are highly resistant to vibrations and shocks, and can operate over a wide temperature range.