Skip to Main Content
United States - Flag United States

Please confirm your currency selection:

  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
  • Loading...
Applications & Technologies
Timing Technology Overview

Timers, counters, and clocks are a critical component in most embedded systems. Cell phones, computers, radios, watches, and many other devices rely for their success on an electronic oscillator that produces an output with a precise frequency to generate timing pulses and synchronize events.

Timing is a crucial aspect of digital systems, ensuring data smoothly progresses through processor pipelines and interconnected systems that can talk with each other. Many simple, low speed applications may simply rely on a microcontroller’s internal RC oscillator to provide the necessary clock signal. However, internal oscillators may be too slow and noisy for some designs, or you may need multiple devices to share a single clock. Whatever the case, there are instances where an external timing solution is necessary. In general, a designer can trade off accuracy for run time.

There are many devices that deal with timing, and some work better than others for specific applications. Devices used in timing are:

  • Crystals and oscillators that oscillate or pulse with a regularity that is used to synchronize chips or functions implemented in circuits.
  • Clock generators, frequency synthesizers and other integrated chips that are a more complete solution than oscillators.
  • Application specific clocks or timers:
    • Timing devices tuned to the exact needs of a specific technology.
    • Real-time clocks or calendars that tell time in synchronization with the outside world.
    • Counters.
  • Accessory chips for design implementation that perform functions related to timing signals, like redrivers, buffers, dividers, jitter cleaners, counters, and so forth.
High Voltage Semiconductors
Simple Oscillator Circuit

Oscillators are used in timing, and are comprised of a resonator circuit and a driver circuit that detects and amplifies the resonant signal used for timing. The resonator is usually a mechanical or piezoelectric device made of crystal, ceramic, or an integrated MEMS (micro-mechanical) structure with resonating properties.

A crystal oscillator circuit is the one of the simplest external timing solutions to implement. The resonating device is built using a piezoelectric material, commonly quartz, sandwiched between two metal plates. The crystal (quartz or ceramic) translates the mechanical resonance vibration into an electrical signal at a set frequency. There are crystals that can produce pulses ranging in frequency from a few kilohertz to hundreds of megahertz. Crystals are two-terminal devices that rely on additional circuitry such as a capacitor in parallel to get the crystal to generate a set frequency. The circuitry can be located in a microcontroller chip, or the crystal can be integrated with driver circuitry into a module or chip package.

With respect to different applications, there are different types of oscillators:

Standard clock oscillators typically offer a single frequency that is set at the factory. Also referred to as simple packaged crystal oscillators (SPXO), these are the most basic oscillators that combine a quartz crystal with an oscillating circuit. Applications: <100MHz - replacements, consumer electronics. >100MHz – SONET/SDH, 10GB Ethernet, Fibre channel.

Programmable oscillators are integrated chip oscillators offering commonly used reference clocks. The desired clock frequency is typically configured via pin selection as directed in the datasheet.

TCXO (also TCO) stands for “temperature compensated crystal oscillator;” is a highly stable combination of a crystal oscillator with a temperature-compensated circuit. TCXO is excellent for applications requiring precision timing. Often used for RF applications such as tactical radios, wireless devices, small cell access applications and nearly all smartphones.

TCVCXO stands for “temperature compensated voltage controlled crystal oscillator.” This is a crystal oscillator solution that is both temperature compensated and frequency-controlled via voltage. (Also abbreviated as VCTCXO, or VC/TCXO) Due to its precision and variable frequency control aspects, applications include cell and cordless phones, mobile and radio communication equipment.

VCSO stands for “SAW Voltage Controlled Oscillators.” VCSO are used for applications requiring low jitter and low noise at fundamental frequencies up to 1GHz. On the down-side, VCSOs have very low vibration sensitivity such that their frequency changes with physical vibration; but this problem is deterministic, or predictable, such that it may be compensated for. The rugged VCSO is used in various commercial, telecomm, and military applications.

VCXO (also VCO) is a voltage-controlled crystal oscillator; an oscillator whose frequency is tuned via a variable voltage input. This offers the option of a stable crystal oscillator with the ability to fine-tune the frequency within a finite range. These are often used in industrial equipment, communication relays, digital TV, digital audio, set top boxes and many other applications.

OCXO is an “oven controlled crystal oscillator,” and is the most precise and frequency-stable solution available. The quartz crystal, with a zero temperature gradient at high temperatures, operates in a miniature temperature controlled environment and is available in surface mount packages measured by the millimeter. OCXOs find applications in base stations for mobile phones, broadcasting equipment, navigation system and clock frequency standard, radar, and test & measuring instruments.

In addition to piezoelectric devices such as crystal or ceramic resonators, there is a newer addition to this fundamental basis for timing technology. A MEMS resonator is a small structure (0.1mm or less) that is designed to vibrate at high frequencies under electrostatic excitation. For precision timing applications, MEMS oscillators often include temperature compensation by using an on-chip temperature sensor. MEMS oscillators have superior resistance to shock, are thin, and are well-suited to co-fabrication in CMOS integration.

Quartz, ceramic and MEMS resonators are one option for the resonating device in a timing circuit. Another low-cost timing option is a simple RC oscillator, which uses a resistor and a capacitor with an amplifier to produce an oscillating signal using positive feedback. Most common output signal is either a sine wave, a CMOS-compatible output, a TTL-compatible output, or an ECL-compatible output.

However, for precision applications, the traditional solution has been a circuit based on a vibrating resonator. Crystals and ceramic resonators have well-known oscillating properties. MEMS resonators are micro-mechanical and are inherently integrated, resulting in a much smaller footprint, but worse noise performance than quartz. Quartz has a temperature dependency that is almost zero, whereas a MEMS oscillator (if etched in silicon) significant temperature dependency and must be compensated with additional circuitry. Nevertheless, MEMS appears to be well-suited as a real time clock (RTC) since they have very low power and size requirements, are reliable at low frequencies, and noise/jitter is not as much of a concern with an RTC. (See Application Specific Clocks for more information on RTCs.) Any oscillator and circuitry on an integrated single chip, etched in the same silicon or similar substrate, has all of the components on a single die, giving more precise operation and improved performance over temperature.

» View Standard Clock Oscillators

» View Programmable Oscillators

» View Temperature Compensated Crystal Oscillators (TCXO)

» View Temperature Compensated Voltage Controlled Crystal Oscillators (TCVCXO)

» View Oven Controlled Crystal Oscillators (OCXO)

» View Voltage-Controlled Crystal Oscillators (VCO or VCXO)

» View SAW Voltage Controlled Oscillators (VCSO)

Clock Generators and Frequency Synthesizers

Many consumer applications use simple crystal-based clock generators; other applications can have very complex timing requirements with combinations of clocks for generation, synchronization, and distribution. Fortunately, timing solutions exist for almost every circuit need. These integrated circuits (ICs) reduce total component count, allow a smaller footprint, and make designing clock systems an easier task for engineers in general. The case for using timing solutions is lower cost with lower system component costs, smaller PCBs, and hopefully lower design-labor costs with a faster time-to-market.

Clock generators (or frequency generators) are used for timing circuits, synchronizing events within a system, and producing output signals at several different frequencies. Typically, the resident microcontroller (MCU) can dynamically change output frequency of the clock generator by programming it via an I2C or SPI interface.

Clock generators and frequency synthesizers are examples of timing solutions offered in integrated circuits (ICs) that, as a rule of thumb, can save cost in a system that requires four or more discrete clocks, crystals, or oscillators. The challenge is then in locating the right solution for your application. Mouser has a large selection of timing solutions, organized by function, feature, manufacturer, and/or specification.

Clock synthesizers with jitter cleaners are ICs that produce one or more clock frequencies from one input signal and also reduce phase noise and jitter. A theoretical, ideal clock source would be a pure sine wave, but in reality all clocks have some noise. Jitter is defined as deviation from a pure periodic pulsed signal, and in this context, occurs in high-frequency digital systems. Jitter attenuation is the act of reducing this deviation with specialized timing devices or circuitry. High-speed applications requiring synchronization would need jitter attenuating clock generators.

» View Clock Generators

» View Clock Synthesizers with Jitter Cleaner

Application Specific Clocks

Most oscillators and timing circuits are general purpose, for example, most 25MHz oscillators can be used in any application that requires a 25MHz clock. However, some applications have specific timing requirements and are best served with an application-specific clock chip.

These applications often have specialized timing requirements such as managing multiple synchronous clocks, clocking a network were signal delays can be significant, or providing clock signals other than a regular frequency.

PCI Clocks

PCI and PCI Express is a very common bus interface standard that is found almost everywhere. Reliable data transfers over this interface demands a highly stable clock reference. For PCI Express the standard specifies an external reference clock, Refclk, of 100MHz with a frequency stability of ±300ppm or better. Data rates of 8GBits/sec and faster are possible.

The clock scheme for PCI bus systems is very precise. Deviations may result in bit errors, reduced throughput, or the system may fail to train, which means the clocks did not synchronize between boards on the PCI bus.

JESD204B Clocks

JESD204B defines a serial interface standard between a high speed data converter and the host processor. In these systems the data converter clock is also used to generate the clock for the JESD204B output of an ADC, or the JESD204B input of a DAC. With a data rate of up to 3.125Gbits/sec, it is important that the JESD204B clock be low jitter and also have deterministic latency.

Real-Time Clocks

A real-time clock, commonly abbreviated RTC, is an IC that provides an accurate timebase when the main power is disconnected from the system. An RTC has its own dedicated 32.768kHz oscillator and is powered by a small battery that is enabled in the absence of system power. An RTC can maintain an accurate calendar including month, day, year, and exact time down to the second. An RTC can even have a programmable alarm or count-down timer. While RTCs can be a peripheral inside the main microcontroller, a separate RTC chip can also be used.

» View PCI Clocks

» View JESD204B Clocks

» View Real Time Clocks

Clock System Design

Clock buffers, redrivers, and other clock support chips are necessary when a system requires many reference frequencies to be delivered to the system. Some devices, like FPGAs, ASICs, and some digital processors, require multiple clock frequencies. If a single clock is distributed to many different destinations, it is critical that the system be designed so that the clock signal does not arrive at its destination at different times. This is called skew, or clock skew. Skew is caused by non-ideal conditions such as crosstalk, circuit board routing, and capacitive coupling, as well as circuit loads on the clock signal. It can be especially troublesome on a large network.


Jitter is another issue in designing clock systems. Jitter is when a clock edge does not occur precisely at the ideal time, but may occur plus or minus a fraction of a second before or after the ideal edge time. Jitter can be randomly caused by unwanted electronic interference, or predictable as in deterministic jitter which is caused by known circuit physics. As long as clock jitter in a circuit is understood and minimized, it can often be compensated for in the clock system design.

Clock Buffers

Clock buffers allow a clock source to be distributed to many destinations at the proper time with little loss, adding stability to the clock distribution system. Clock buffer chips need to have a high input impedance so they will not load the clock source. Buffers must also minimize skew and jitter. Clock buffers can also be used when a delay needs to be added in order to synchronize the clock across a network.


Redrivers, also called repeaters, are used to improve the signal quality of high speed clocks. These devices clean up the clock signal, which includes minimizing jitter and boosting the strength of the signal.

Frequency Multipliers

Frequency multiplier/dividers are used to increase or decrease a clock signal by a multiple of its frequency. This is useful for circuits that need to generate another clock that is a multiple of the main system clock.

Clock Development Tools

Clock and timing circuit development tools can be invaluable in the design of complex clock circuits. These tools allow modifying the operating characteristics of one or more programmable clock chips. With an interface to a host PC, the developer can change on-the-fly clock system parameters while allowing easy measurement of signal characteristics like jitter and frequency.

» View Clock Buffers

» View Clock Redrivers

» View Frequency Multipliers & Dividers

» View Development Tools