Clock Tree 101​​​​

A Primer on Timing Solutions

Clocks | Buffers | Jitter Attenuators/Cleaners | VCXOs/XOs

What is a Clock Tree?

  • A clock tree distributes timing signals within a system and includes clocking circuitry and devices.
  • Since systems often have several ICs with different clock performance requirements and frequencies, a "clock tree" refers to the multiple clocks required to meet the system's needs.
  • Clock tree complexity depends on the system's requirements.
  • A single reference clock is sometimes cascaded and synthesized into many different output clocks, resulting in a diagram that looks a bit like a sideways tree.

Timing Components

Clock trees can be complex with many timing components, or very simple with a single reference and a few copies. While there are many timing component types for different applications, the most common timing components are:

  • Crystals - a piece of quartz or other material that resonates at a given frequency when used in conjunction with an on-chip oscillator circuit;
  • Crystal Oscillators (XOs) - a self-contained resonator and oscillator that outputs a given frequency and format;
  • Voltage controlled oscillators (VCXOs) - a self-contained oscillator that varies its output frequency in concert with differing voltages from a voltage reference;
  • Clock Generators - an integrated circuit that uses a reference clock or crystal to generate multiple output clocks at one or multiple frequencies;
  • Clock Buffers - an integrated circuit that creates copies or derivatives of an input/reference clock;
  • Jitter Attenuators or Jitter Cleaners - an integrated circuit that removes jitter (noise) from a reference clock.

 

Crystals and Crystal Oscillators (XOs)

Crystals and XOs are generally cost-effective unless the output requirements are unusual or stringent.

  • Crystals provide a frequency output when an electrical signal is applied. The output is a single-ended sine wave typically ranging from 32 kHz to 50 MHz. Each output frequency requires a different resonator cut and an oscillator circuit to operate.
  • XOs integrate the crystal with the oscillator circuit in a standalone package. XOs output either a LVCMOS single-ended or differential square wave. Differential signaling is used in high-speed, jitter sensitive applications. Some XOs provide multiple frequencies via I²C or pin control. XOs are generally cost effective unless the application requires a variety of clock frequencies.
  • VCXOs are XOs with a varactor diode that allows applied voltage to change or skew the output frequency.

Three Common Types of Frequency Reference Sources

 

Clock Generators

Clock generators are integrated circuits (ICs) that generate multiple output frequencies from a single input reference frequency. The input or reference frequency is supplied by a crystal, XO or other clock in the system. Clock generators may have other features that are controlled by I²C or pins including:

  • turning on/off outputs,
  • skewing frequencies, and
  • adding/removing spread specturm to frequencies to reduce noise.

The perceived challenge with clock generators is system layout. Placing a crystal adjacent to its target IC is simple and cheap. Routing a clock signal from a clock generator to its target IC might not be as easy, although it can save money. Careful design, and other techniques can ensure a centralized clock source provides equal performance. And, generally speaking, if four or more clocks are required designers can save money with a clock generator. The clock generator shown here is programmable with up to eight single-ended outputs or four differential outputs. It allows designers to replace eight single-ended crystals or four differential ones with a single IC.

Clock Buffers

Clock buffers distribute multiple copies or simple derivatives of an input/reference clock. The reference clock can be from a clock generator, XO, or a system clock. Clock buffers scale their input clock from 2 to more than 10 outputs. They may include I²C, SPI, or pin-controlled features like signal level and format translation, voltage level translation, multiplexing, and input frequency division. These features save space and cost by eliminating components, voltage dividers, and/or signal level transition circuits.

Jitter Attenuators

Jitter attenuators are clock generators with specialized circuitry for reducing jitter (noise). They may also be called clock cleaners or jitter cleaners. These highly specialized timing devices remove jitter from incoming reference clocks and minimize jitter in the system. Jitter attenuators are typically used in high-speed applications such as Synchronous Ethernet and SDI Video to ensure that all physical layer data transmission is synchronized.

Critical Clock Tree Design Criteria

When starting a clock tree design, the design team needs to carefully assess the system requirements and layout. The system's clocking requirements will determine what type of components to use, their performance levels within the system and its overall network, and will also likely indicate whether or not clock generators can provide signals or if crystals and XOs are needed. Of course, the system may require a mixture of the various timing components. The decisions to be addressed are:

  • Selecting a clock generator versus a crystal, XO or VCXO.
  • Determining if the system is free-running or synchronous.
  • Determining the system's clock jitter requirements.
  • Selecting timing components that meet the system requirements.
  • Estimating the overall clock tree jitter with the selected components, and making adjustments if needed.

 

Crystal, XO, or Clock Generator?

When to Use a Crystal vs a Clock

  • Crystals are typically used if the target IC has an integrated oscillator and on-chip phase-locked loops (PLLs) for internal timing.
  • Crystals are cost-effective components that exhibit excellent phase noise and are widely available.
  • They can also be placed in close proximity to the IC, simplifying board layout.
  • One of the drawbacks of crystals is that their frequency can vary significantly over temperature, exceeding the parts-per-million (ppm) stability requirements of some applications.
  • In many stability-sensitive high-speed applications, crystal oscillators (XOs) are a better fit because they guarantee tighter temperatre stability.
  • Clock generators and clock buffers are useful when several reference frequencies are required and the target ICs are all on the same board or in the same IC or FPGA.
  • In some applications, FPGA/ASICs have multiple time domains for the data path, control plane and memory controller interface, and as a result require multiple unique reference frequencies. This is a good place for a clock generator.
  • A clock generator or buffer is also better when the IC cannot accomodate a crystal input, when the IC must be synchronized to an external reference (source-synchronous application), or when a high-frequency reference is required.

 

Free-Running or Synchronous System?

Free-running clock trees

Once the clock inventory has been completed, the next step is to determine if the required timing architecture is free-running or synchronous. Free-running applications require independent clocks without any special phase-lock or synchronization requirements. Examples include standard processors, memory controllers, SoCs and peripheral components (e.g., USB, PCI Express switches).

Synchronous clock trees

Synchronous systems require continuous communication and network-level synchronization across all associated systems. In these applications, low-bandwidth PLL-based clocks provide jitter filtering to ensure that network-level synchronization is maintained. For example, synchronizing all SerDes (serialization-deserialization) reference clocks to a highly accurate network reference clock (e.g., Stratum 3 or GPS) guarantees synchronization across all system nodes.

Synchronous clock trees examples

  • Optical Transport Networking (OTN)
  • SONET/SDHMobile backhaul
  • Synchronous Ethernet
  • HD SDI video transmission

 

Clock Jitter Requirements

Clock jitter is a critical specification for timing components since clock jitter can compromise system performance. There are three common types of clock jitter, and depending on the application, one type of jitter will be more important than another.

  • Cycle-to-cycle jitter measures the maximum change in the clock period between any two adjacent clock cycles, typically measured over 1,000 cycles.
  • Period Jitter is the maximum deviation in clock period with respect to an ideal period over a large number of cycles (10,000 is typical).
  • Phase jitter is the figure of merit for demanding, high-speed SerDes applications. It is a ratio of noise power to signal power calculated by integrating the clock single sideband phase noise across a range of frequencies offset from a carrier signal.

Silicon Labs provides a detailed investigation of timing jitter in the Timing Jitter Dictionary and Technical Guide.

Jitter performance varies across a wide range of conditions including

  • Device configuration
  • Operating frequency,
  • Signal format,
  • Input clock slew rate and jitter,
  • Power supply, and Power supply noise, and so on.

Select devices with complete jitter specifications (TYP + MAX). "TYP" alone is not complete.

Selecting Components

  • Evaluate devices on maximum (MAX) jitter performance.
  • Typical (TYP) data sheet specifications do not guarantee device performance over all conditions.
  • Device performance can change across manufacturing process, supply voltage, temperature and frequency variation.
  • Look for comprehensive "Test Conditions" and MAX in data sheets, as shown below.

Estimating Total Clock Tree Jitter, End-to-End

The total clock tree jitter should be estimated to determine if there is sufficient system-level design margin before the clock tree is committed. A component with poor performance can compromise the whole system's performance.

IMPORTANT:

  • Total jitter is not the sum of the MAX RMS specifications of each component.
  • Total jitter is the root of the sum of the squares of each device's MAX RMS jitter.

Phase Noise to Jitter Calculator

Silicon Labs offers a free, on-line tool to convert phase noise to jitter requirements or performance.

The table below summarizes many other selection criteria used for both free-running and synchronous clock trees. For more infomation visit the main TIming page.

Optimizing Clock Trees - Example One

Clock trees provide a fundamentally important part of the system and must be optimized for performance, power, and cost. Silicon Labs' comprehensive portfolio applies to all ranges of applications, from the most demanding to the most cost conscious.

Optimizing Clock Trees - Example Two

Clock trees provide a fundamentally important part of the system and must be optimized for performance, power, and cost. Silicon Labs' comprehensive portfolio applies to all ranges of applications, from the most demanding to the most cost conscious.

Conclusion

Silicon Labs’ comprehensive timing portfolio provides optimized clock trees for any application, from the most demanding to the most cost-­conscious. Our solutions are easy to configure and customize, with most samples available immediately or within less than two days.  Our free tools assist you in creating the right clock tree for your application. And our experienced customer service experts are happy to help. Contact us for your timing needs. We make timing easy.

Clock Tree Terminology

  • Fan out - Fan out is a term that defines the maximum number of digital inputs that the output of a single logic gate can feed. Most transistor-transistor logic (TTL) gates can feed up to 10 other digital gates or devices. Thus, a typical TTL gate has a fan-out of 10.
  • LVPECL - LVPECL stands for Low-Voltage Positive Emitter-Coupled Logic, and it is a power optimized version of PECL or Positive Emitter-Coupled Logic. It uses a positive 3.3 V power supply.
  • LVDS - LVDS is Low-Voltage Differential Signaling, and it is only a physical layer specification, but a data link layer is often added by communication standards and applications.
  • CML - Current Mode Logic transmits data at speeds between 312.5 Mbit/s and 3.125 Gbit/s across standard circuit boards.
  • HCSL - High-Speed Current Steering Logic is differential logic with two output pins that switch between 0 and 14 mA.
  • LVCMOS - LVCMOS stands for Low Voltage Complementary Metal Oxide Semiconductor, and its goal is to reduce the device geometries of integrated circuits, with resulting reduction in operating voltage.

 

About the Author

Linda Lua is the Silicon Labs product manager for datacenter timing products, managing the datacenter clock generators and clock buffers portfolio, new product launches, new product initiatives and marketing promotions. Prior to joining Silicon Labs, Ms. Lua was at ISSI, responsible for High Speed Memory products, and at IDT Inc., responsible for timing products business development and product management in networking and the communications market. Ms. Lua holds a BS in Electrical Engineering from Iowa State University and MBA from the University of Texas at Dallas.