Driver Monitoring System

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.

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SiTime MEMS Timing Benefits

Complete MEMS clock tree

Low jitter differential oscillators

32.768-kHz XOs and TCXOs

Precision TCXOs

Spread spectrum oscillators

Most robust in real world conditions

150 fs rms jitter, excellent PSNR

Resistant to shock and vibration

Stable over wide temperature

>2 billion hours MTBF

Integrated MEMS, easy to use

No quartz reliability issues

Reliable startup in cold temp

No cover or shielding needed

Short lead time for any frequency

Driver Monitoring System Block Diagram

Driver Monitoring System block diagramsitime.comSingle Ended XO SiT8924Single Ended XO SiT9025XO SiT8924XO SiT8924Differential XO SiT9396Differential XO SiT9396

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.

MEMS Timing Solutions for Driver Monitoring System

Devices Key Features Key Values
Single-ended Oscillator
SiT8924  1 to 110 MHz
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  • Up to -55°C to +125°C
  • ±20 ppm stability
  • 2016, 2520, 3225 packages
  • High reliability
  • Extended temperature range
  • Small footprint
Single-ended Oscillator
SiT9025  1 to 110 MHz
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  • Up to -55°C to +125°C
  • Spread spectrum
  • Configurable rise / fall times
  • 2016, 2520, 3225 packages
  • High reliability
  • Extended temperature range
  • EMI Reduction
Differential Oscillators
SiT9396  1 to 220 MHz
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SiT9397  220 to 920 MHz
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  • Low jitter: < 150 fs RMS [1]
  • ±30, ±50 ppm stability over -40 to +125°C
  • LVPECL, LVDS, HCSL, Low-power HCSL, FlexSwing™
  • 2016, 2520, 3225 packages
  • High reliability
  • Low jitter
  • Enables interfaces with demanding jitter requirements, such as PCI-Express and 10 GB Ethernet
Super-TCXOs
SiT5386  1 to 60 MHz
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SiT5387  60 to 220 MHz
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  • ±0.1, ±0.2, ±0.25 ppm stability over -40 to +125°C
  • ±1 ppb/°C frequency slope
  • Low jitter: 0.31 ps RMS [1]
  • Optional voltage or digital frequency control
  • High accuracy
  • Excellent frequency stability with fast temp gradients
  • No GNSS signal loss or V2X disconnect due to micro-jumps
32.768 kHz Oscillator
  • ±20, ±50, ±100 ppm stability over -40 to +125°C
  • 1.14 to 3.63 V supply
  • < 490 nA consumption
  • 1.2 x 1.1 mm
  • < 115 ms startup time
  • Excellent stability
  • Low power
  • Small footprint
  • Faster start-up time 32.768 kHz tuning-fork crystals
  • High reliability for functional safety applications

1 12 kHz to 20 MHz integration range

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.

MEMS Timing Outperforms Quartz

Higher Quality

Higher Reliability

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 SiTime – Higher Quality
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SiTime timing devices are up to 50x more reliable than legacy quartz

Tighter Stability

Better EMI Reduction

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SiTime – Tighter Stability
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SiTime – Better EMI Reduction

Immune to Vibration

Better Noise Rejection

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SiTime – Immune to Vibration
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SiTime – Better Noise Rejection
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