A Small But Mighty 32-kHz Ultra-Low Power Automotive Clock
December 06, 2022
Blog
Timing, like power, is an ever-present topic in every modern automotive electronic system. Modern cars use up to 70 timing devices, and this number is expected to grow as new automated driving features are introduced.
Passive resonators and active oscillators are well known timing components. More complex components include clock generators, synchronizers, and jitter attenuators. The main purpose of timing in automotive systems is to provide a stable reference to every digital integrated circuit, synchronize the transfer of vast amounts of data, and enable V2X and 5G communication.
In this blog, we focus on automotive 32.768 kHz clocks, and the newly introduced SiT1881 oscillator. The SiT1881 improves upon its competitors and previous generation devices in significant areas:
32.768 kHz clocks are mainly used for three functions in automotive electronic systems:
- as a standby clock,
- for time keeping,
- as reference clock for functional safety purposes.
Standby Clock
If we take a close look at automotive electronic control unit (ECU) processors, systems-on-chip (SoC) or other chipsets such as WiFi or Bluetooth®, we see that many require a 32.768 kHz clock in addition to their main clock, which is usually in the 20 MHz to 40 MHz range. Why an additional 32.768 kHz clock, when the device already has a main clock? Simple: a 32.768 kHz oscillator consumes much less power than a MHz oscillator. Saving power is the name of the game. Many automotive electronics systems (such as domain controllers or ADAS computers) are permanently powered by the vehicle’s battery, even when the engine is not running. The necessity to save battery imposes drastic power consumption limits while in standby. In standby mode, ECUs switch off any unneeded functional block, including the main MHz clock. Only a limited functional set is kept operating, driven by the 32.768 kHz clock. The 1000-times-lower clock frequency contributes to the lower power consumption of the ECU itself.
Low power requirements make the SiT1881 32.768 kHz oscillator the ideal clock source for ECUs. At 490 nA typical supply current with output active, the SiT1881 enables power consumption as low as 0.6 µW with a power supply of 1.2 V.[1,2] The SiT1881 accepts supply voltages ranging from 3.63 V to 1.14 V, making it suitable to applications powered with a backup coin cell or supercapacitor. The low power consumption of the SiT1881 maximizes the lifetime of the battery and ensures that the system standby power remains well controlled.
Low power consumption of the SiT1881 32.768 kHz oscillator make it an ideal standby clock for ECUs
Time Keeping
Time keeping is very similar to the standby clock use case. It is achieved by means of a real-time clock (RTC) that continuously operates whether in active or standby mode. Although standalone RTCs are still widely used in automotive systems, most modern ECUs or SoCs integrate an RTC function. An RTC is driven by a 32.768 kHz clock, again to save power compared to a MHz clock. It simply counts seconds, minutes, hours, and days. An alarm function can trigger events, such as waking up the ECU at a specific time or at regular intervals.
The SiT1881 allows for precise time keeping. Its frequency is stable to within ±50 ppm (4.3 seconds per day) over -40°C to +105°C. This excellent stability—which is five times better than the best crystal resonator and includes initial tolerance, aging, and temperature effects—is achieved thanks to the MEMS resonator at the heart of the SiT1881.
To ensure accurate time keeping, it is common to “wake up” a system at regular intervals and synchronize its RTC to a known reference such as GPS or a network time protocol (NTP). Because of the better stability of the SiT1881, the interval between wake up states can be extended, allowing the system to stay in standby mode longer and saving even more power!
In the recent years, OEMs increased their requirements for time keeping accuracy to within ±100 ppm, ±50 ppm, or sometimes even less. At worse than ±200 ppm total accuracy, traditional 32.768 kHz tuning-fork crystals cannot meet this requirement. To overcome this, crystal oscillator vendors sometimes revert to using a more precise AT-cut crystal that oscillate at several MHz, and then divide its output down to 32.768 kHz. Although this workaround enables a total accuracy of ±100 ppm or less, its power consumption is terrible. The SiT1881 combines stability and low power – a much better solution.
Safety Clock
Functional safety is a prominent concern for every automotive electronic system designer. Some automotive ECUs and SoCs feature a special 32.768 kHz "safety clock" input, which drives internal safety mechanisms. Even if all clocks of the system (processor clock, PCI-Express clocks, etc.) are elegantly provided by a single clock generator, the 32.768 kHz safety clock should remain independent from any other clock source to avoid common failure modes. As the safety concept of the system heavily relies on the safety clock, including monitoring of other clocks, it must be the most robust.
The silicon MEMS resonator, on which the SiT1881 relies, provides an appreciable answer to this question. Crystal resonators have always represented the weakest link of timing devices. Their large size and cantilevered construction make them sensitive to shock, vibration, and electromagnetic interference. SiTime MEMS-based timing devices are 50 time more reliable than crystal-based devices, thanks to the robust design and excellent material properties of the small (0.4 mm x 0.4 mm) resonator. The SiT1881 FIT rate is less than 0.5. SiTime's production process is geared toward quality,[3] with an observed defects per million units shipped of less than 0.1 DPPM. SiTime MEMS timing devices are guaranteed to be the most reliable timing components in your system and are backed with a lifetime warranty.
The 32.768 kHz Oscillator
The small resonator of the SiT1881 enables an ultra-small 32.768 kHz oscillator with a 1.2 mm x 1.1 mm footprint and profile of only 0.5 mm. This tiny size makes it ideal for applications where PCB space is at a premium, such as sensors, smart mirrors, or WiFi/BLE modules.
A byproduct of the small resonator size is the fast start-up of the oscillator. Crystal-based 32.768 kHz devices use a tuning fork resonator, which is notably slow to start: up to 0.5 seconds. The SiT1881 is five times faster, achieving stable oscillation within 115 ms or less. This is of prime importance for systems such as ADAS computers, Infotainment/cluster, domain controllers, driver monitoring systems or sensors such as automotive cameras, radar or lidar.[4] These systems have a very aggressive boot time requirement of only a few seconds from “key on”, i.e., vehicle start by the user, to fully operating state. The SiT1881 optimizes boot-up time by eliminating the crystal start-up delay bottleneck.
Conclusion
Timing components are critical for today’s smart, connected automotive systems. The low power, unbeatable frequency stability, excellent reliability and small size make the SiT1881 ideal for fulfilling all 32.768 kHz needs in automotive systems.
