Four major trends are shaping the future of transportation. These are electrification, shared ownership, active safety systems and driving automation. They are having a major impact on the way vehicles are being designed, demanding more digital systems on board to handle aspects such as usage monitoring and billing, drivetrain control, autonomous braking and steering, navigation, positioning, contextual sensing and V2X communication. Accordingly, the market for automotive electronic components and software is growing at twice the rate of the automotive market overall and is predicted to reach $450bn by 2030.
Currently, a typical vehicle contains about 70-100 timing devices and this number is increasing as these trends continue to take hold.
Data integrity and safety
The rising numbers of digital systems on board are driving a huge increase in the amount of high-speed data transfers occurring throughout the vehicle, including across the car’s Ethernet backbone and over wireless networks. As more sensors are added, vehicles can generate more than 20TB of data per hour. This requires seamless communications and advanced processing capabilities with up to 100 Tflops of compute power.
Precision timing is essential in managing this amount of data. The vehicle is dependent on timing devices to synchronise high-speed, high-volume data transfers from various sensors to ADAS computers and ECUs, effectively manage multi-Gbps interfaces, and ensure seamless communications between internal and external vehicle systems.
In safety-critical systems, timing devices must be extremely reliable as well as highly accurate. As the industry moves from passive to active safety systems, precision timing is critical. In these, and autonomous driving capabilities, ultra-low jitter, exceptional stability and reliable operation are extremely important to ensure functional safety.
Accuracy and resilience
Automotive operating environments are harsh, with high ambient temperatures, high vibration and mechanical shock. Shared automotive use models are expected to keep vehicles in operation up to 90% of the time compared to single-user vehicles. This will intensify demands for highly reliable automotive systems. All systems will be subjected to frequent and repeated turn-on and turn-off cycles and increased wear and tear, requiring robust components that can withstand elevated duty cycles.
Electric vehicles (EVs) are setting the pace for more advanced and complex electronic automotive systems that pose design challenges. These dense electronic systems must cope with increasing electromagnetic interference (EMI) due to the higher currents in the vehicle, which can disrupt the operation of electronic devices. Robust timing products play a vital role in clocking and synchronising complex automotive electronics, enabling EVs to operate reliably, even in noisy and demanding environments.
Silicon timing and automotive
Historically, quartz crystal devices have provided the timing reference for electronic circuits. MEMS technology now makes it possible to take advantage of the thermal, mechanical and electrical properties of silicon to create precision timing devices that ensure safer, more reliable automotive system designs.
MEMS resonators can be built within much smaller dimensions than quartz crystals, enabling timing devices to have a smaller footprint (down to 1.0×1.2mm), which is suited to space-sensitive automotive applications such as camera modules and radar/lidar sensors. With smaller size and less mass, the devices can be up to 100-times more resilient to environmental shock and vibration.
MEMS resonators also have up to 100-times better resilience to EMI disturbances, compared to quartz. This resiliency is especially beneficial for applications with high currents and electromagnetic fields, such as EV battery management systems.
The intrinsic material properties of silicon MEMS devices permit very well controlled frequency accuracy over a wide temperature range. Unlike typical crystal behaviour, the accuracy does not diverge exponentially at extreme temperatures. A typical automotive-grade MEMS oscillator has a stability of ±20ppm over -40°C to +125°C. This number includes initial accuracy, temperature effects and ageing. Adding temperature compensation to MEMS TCXOs can improve stability to ±0.1ppm. This level of accuracy enables better synchronisation of V2X and 5G communications over extended temperature ranges.
MEMS timing is free from cold start issues at the bottom of the temperature range, which can plague systems using quartz-based oscillators. Silicon MEMS resonators are not subject to so-called micro-jumps, random, non-reproducible jumps in frequency. These are common with crystal oscillators and can result in a loss of signal for GNSS or V2X/5G communications. In fact, the frequency generated by a MEMS-based oscillator stays consistent even when the ambient temperature increases rapidly, while quartz-based oscillators output random frequencies with temperature spikes.
Reliability metrics
The reduced probability of component failure results in a lower probability metric for hardware failure and equates to greater safety in automotive systems and better system-level safety metrics. Taking advantage of the tight integration of MEMS and CMOS technology, coupled with safety mechanisms, contributes to improved single-point fault metrics and latent fault metric. These improvements are crucial for meeting safety standards such as ISO 26262 for automotive functional safety management. Switching from quartz to MEMS timing technology can enable automotive systems to achieve enhanced safety metrics while enabling a simpler overall system architecture that permits easier system-level safety assessments.
Mamera module
Sensor timing with a camera module…
An increasing number of cameras are used around the vehicle to provide visual sensing for systems such as blind-spot detection and traffic-sign recognition, as well as advanced driver assistance systems functions (for example, parking assistance and adaptive cruise control). Common design challenges are board space constraints, exposure to extreme ambient temperatures and fast temperature changes, exposure to shock, vibration and EMI. Designers also demand greater sensor resolution, which calls for higher data rates, placing increased emphasis on minimising clock jitter.
A MEMS-based oscillator can meet these challenges through its small size and stable, high accuracy over a wide temperature range, even when the temperature is changing quickly. Spread spectrum and configurable rise/fall times help to minimise EMI and low jitter and resilience to shock and vibration.
MEMS oscillators
MEMS oscillators with special features can provide specific attributes, such as the increased robustness to power supply noise provided by differential oscillators.
Where temperature stability is a priority, selected oscillators in SiTime’s Super-TCXO series leverage DualMEMS die construction and TurboCompensation to deliver temperature stability comparable to a quartz oven-controlled oscillator at lower cost and lower power.
For DualMEMS technology, two silicon MEMS resonators are co-fabricated on a single die. One is optimised to have a virtually flat frequency response over temperature, the second resonator performs as a temperature sensor whose frequency is sensitive to temperature changes.
The sensitivity is linear, with a slope of about ±7ppm/oC, so that the ratio of frequencies between the two resonators provides a fast reading of the resonator temperature (or TurboCompensation). Resolution is 30μK and bandwidth is in the order of 100s of Hz. The temperature reading is input to a temperature compensation algorithm running in the companion mixed-signal CMOS IC. Temperature compensated frequency shift is better than ±1ppm, reports SiTime.
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