Feature: Automotive Design
water residues are leſt in the package. Eliminating these residues, which are known to cause mass loading of the resonator and frequency driſt , has proved pivotal in producing silicon-based devices that display high frequency stability. By using standard CMOS foundry tools and materials, MEMS
oscillators can be produced cost-eff ectively with excellent quality and reliability. CMOS manufacturing also off ers the opportunity for fast production ramping up to high volumes. T e oscillator chips can be packaged using standard IC backend processes such as plastic molding, fl ip chip, chip stack and chip-scale packaging. T ese familiar and widely understood package styles simplify the task for board designers to take advantage of silicon-based oscillators as direct replacements for legacy quartz devices.
MEMS Oscillators in Zonal Architectures As vehicle electrical/electronic architectures now migrate towards the zonal model, the performance of high-speed interfaces becomes even more critical to ensure correct functioning of all the vehicle subsystems. While adopting this architecture will greatly lighten and simplify vehicle wiring, data speeds are expected to rise signifi cantly. Precision timing will become critical in data exchanges between zone controllers and the central computing cluster. T e requirements this architecture places on timing will be even more stringent than with the outgoing domain architecture. Zonal architectures will demand clocks with even lower jitter, higher timing accuracy, and better stability. MEMS-based oscillators consistently meet and exceed the demanding specifi cations established for automotive applications such as central and zonal computers, PCIe and SerDes high-speed communications, cameras and sensors, playing a pivotal role at the heart of these systems. Reliability and robustness are critical for achieving performance functional safety targets in every system on-board the vehicle, from critical systems such as advanced driver-assistance systems (ADAS) to infotainment systems that consumers expect to deliver a faultless user experience.
Compared to quartz-based timing components, silicon-based devices can offer several advantages that are particularly important for automotive applications. These include superior frequency accuracy, 10x lower aging, and reduced temperature drift. In addition, they can deliver up to 50-times better reliability. Apart from reducing the amount of field failures, better reliability translates into a lower FIT rate. MEMS oscillators in the market now have demonstrated about two orders of magnitude lower FIT rates than quartz. This provides better hardware safety metrics in an FMEDA, which is the quantitative analysis required as part of a functional safety assessment.
Mean time between failure (MTBF), which is the inverse of FIT rate, is another key reliability metric for semiconductor components. SiTime oscillators provide up to 50 times better reliability than same-class quartz devices.
Enhanced Temperature Stability The automotive environment is known to be unrelentingly hard on electronic components. Parts can be subjected to shock and vibration, prolonged high temperatures in locations close to the engine, high and low atmospheric temperatures and thermal cycling. Frequency stability, defined as the variation of frequency
over temperature, is a key metric for oscillators. While increasingly important in zonal architectures, this stability is critical to proper function of safety systems and timing- dependent systems such as GNSS receivers. After selecting the appropriate timing device to meet the
basic frequency stability requirements, system designers must study its specification in depth to understand the frequency vs. temperature slope (ΔF/ΔT), short-term frequency stability, also known as the Allan deviation (ADEV), and the operating conditions. This is important knowledge, needed to help make informed decisions. However, achieving the required stability by selecting
The zonal architecture simplifi es vehicle wiring and is dependent on timely data exchange to maintain overall control and coordination of vehicle functionality.
devices such as temperature-controlled oscillators (TCXO) has typically compromised other important parameters such as reliability, size, power consumption and warm-up time. Moreover, quartz-based oscillators can also suffer from activity dips and micro jumps as environmental stressors such as high temperatures, thermal transients, and vibration affect the crystal structure. Silicon-based oscillators are inherently
www.electronicsworld.co.uk October 2024 19
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