Driver Monitoring System

Driver monitoring systems (DMS) are increasingly designed into vehicles to help offset the rise of distracted driving. Advances in driver monitoring tech and new safety regulations are accelerating this trend. These systems require low latency and high bandwidth as they monitor drivers in real time.

Clocking plays a key role in DMS as data is transferred from the camera to the SoC/processor and out to vehicle systems. SiTime MEMS timing technology delivers the reliability, robustness and stability over temperature needed. Our low-jitter differential oscillators keep clock jitter within spec, even in the toughest environments.

Download Application Brief

A camera, either infrared or visible wavelength, monitors the driver's face. Data is sent to an image processing system based on an SoC and a machine-learning vision processor, which can be either internal to the SoC or external as shown here. In the latter case, data transfer between the two is usually done via a PCI-Express interface, which offers both the required low latency and high bandwidth.

The DMS is connected to other vehicle systems (Instrument cluster, drivetrain electronics, etc.) through CAN, Flexray and/or Ethernet interfaces.

Driver Monitoring System

The clock requirements of the driver monitoring system shown in the block diagram above are as follows:

• PHY: either a single-ended clock for interfaces such as FPD Link (TI), GMSL (Analog Devices), or a differential clock for interfaces such as MIPI A-Phy (Valens)
• SoC, Vision Processor: single-ended clock
• PCI-Express interface: 100 MHz HCSL or LP-HCSL clock with jitter level compatible with the considered PCI-Express generation. Clock jitter outside specifications create transmission errors on the bus!
• As of 2022, PCI-Express Gen 4 is common in automotive applications. The corresponding jitter requirement is 0.5 fs RMS phase noise, integrated over 12 kHz to 20 MHz. SiTime SiT9396 oscillator meets this requirement with margin to spare.

Compared to other automotive applications, driver monitoring systems are less demanding in terms of clocking. Still, a few points remain important:

• System level functional safety and SOTIF needs to be guaranteed
• Temperature range of -40°C to +105°C or more is mandatory
• Clock jitter needs to be within spec

SiTime Advantages

All SiTime devices offer the following advantages over quartz crystals, which are particularly important for automotive applications:

• 50x better reliability. Apart from reducing the amount of field failures, better reliability translates into a lower FIT rate. This provides better hardware safety metrics in an FMEDA, the quantitative analysis required as part of a functional safety assessment.
• With crystal-based oscillators, shock and vibration usually translate into increased clock jitter. This is due to mechanical coupling of vibrations into the crystal.
• Silicon MEMS oscillators are 100x more resilient to shock, vibration and electromagnetic interference than crystal-based devices, due to the smaller size (0.4 x 0.4 mm) and lower mass of MEMS resonators compared to crystal resonators. Consequently, jitter and bit error rate of data busses remain controlled even under harsh conditions. SOTIF (Safety of the Intended Functionality) is maintained regardless of the operating conditions. Physical non-reversible damage of the oscillator is also prevented.

Additional Resources

• Timing Solutions for Automotive Systems
• Increase automotive reliability and performance with ultra-robust MEMS oscillators
• AEC-Q100 Automotive Oscillators for ADAS Sensors, in-Vehicle Networking & Self-Driving Computer
• EMI Reduction Oscillators for Industrial, Automotive and Consumer Applications