EMS clocking keeps communication wheels turning

Timing devices provide a mission-critical function in automotive systems. Even if a component or system goes awry, EMS clocking signals must continue operating without fail, says Etienne Winkelmuller, of SiTime.

Vehicles are designed to work reliably for many years while operating in harsh conditions. If a component or system malfunctions, the vehicle is still expected to operate safely and reliably at all times.

Automotive systems in today’s cars use up to 70 timing devices. That number is growing as more cars adopt smarter technology. Clocks and oscillators provide precise, reliable timing references for a wide array of digital systems in automotive designs. They synchronise critical clocking functions within electronic control units for advanced driver assistance systems (ADAS), in-vehicle networks, infotainment and other subsystems.

Timing devices are also used to synchronise the transfer of huge amounts of data from sensors to ADAS computers. They enable vehicle-to-everything (V2X) and 5G communications. Timing is also the foundation of global positioning systems and other global navigation satellite systems.

Gearing up with MEMS timing

Traditionally, the most common clock source is a crystal oscillator, a 70-year-old technology that has matured to a point in which improvements are only marginal. 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, all of which can take a toll on sensitive quartz timing devices.

Silicon MEMS resonators can replace fragile crystal devices, including crystal oscillators in automotive designs.

Silicon MEMS technology is used in electronics systems from mobile phones to automotive and aerospace applications. MEMS devices serve as gyroscopes, accelerometers (for example, for airbag deployment), microphones, loudspeakers, sensors and magnetometers. All silicon MEMS devices, including MEMS resonators, are manufactured at scale in mainstream fabs, providing proven, cost-effective technology for demanding applications.

Automated vehicles

MEMS-based timing components’ reliability is expressed in terms of failure per 109 hours of operation or failure in time (FIT). This is quite 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 (MTTF > 2 billion hours), calculated with 90% confidence level, which is 50x better than crystal technology.

Automotive safety levels

A low FIT rate is a key value for automotive safety integrity level (ASIL) rated automotive systems. All ASIL-rated systems must go through 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.

MEMS resonators are smaller than quartz crystals, enabling smaller footprint timing devices (down to 1.0×1.2mm) for space-sensitive automotive applications such as camera modules and radar/lidar sensors. Smaller size and less mass also mean more resilience to environmental shock and vibration.

Compared to quartz, MEMS resonators have 100x better resilience to EMI disturbances. This resiliency is especially beneficial for applications with high currents and electromagnetic fields, such as battery management systems for electric vehicles.

Silicon MEMS 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 behaviour). A typical MEMS oscillator has a stability of ±50 ppm over -40°C to +125°C which is five times better than crystal. This number includes initial accuracy, temperature effects and ageing. Adding temperature compensation increases stability up to ±0.1ppm. This level of accuracy enables better synchronisation of V2X and 5G communications over extended temperature range.

MEMS timing is also free from cold start issues at the bottom of the temperature range, which can occur in systems using quartz-based oscillators. Silicon MEMS resonators are also not subject to ‘micro-jumps’, the random, non-reproducible jumps in frequency that can result in a loss of signal for GNSS or V2X or 5G communications.