Clock and Timing

Clock and timing devices are a wide variety of components used in creating, managing, counting and distributing digital clocks. Digital clocks are periodic waveforms, most typically square waves. All digital circuitry with memory require the edge transitions of clocks to update and store the next state of the processes. This is called synchronization, and processes that use clocks are referred to as synchronous processes. Applications that require clocks are everywhere – almost every digital device mankind has constructed since the first primitive computation devices have relied upon them.
A timing reference oscillator typically generates clocks. This is a resonant circuit that is tuned or tunable to a particular frequency. When power is applied, a positive feedback loop circulates associated noise energy in the resonant section energy. As the resonant section is a band pass filter, energy at the resonant frequency is amplified, whilst all else is suppressed until the point where the oscillator continues to resonate at a single frequency. If this energy hits amplitude limits it becomes a clipped sine wave, and ultimately a square wave.
At its simplest, a clock circuit can be constructed from discrete components in the form of a Hartley or a Colpittes oscillator circuit. These are typically not very accurate. It is very common to find crystal resonators (for example 32.768kHz watch crystals) placed from input to output across an inverting CMOS buffer to produce a clock (as used in many microcontrollers). Quartz crystal oscillators (XO) are the most common devices used today. Ceramic, surface acoustic wave (SAW) resonator based oscillators are also common. They are available in a number of different forms to provide additional accuracy and control.
Many resonators are based upon materials that change characteristics with age, temperature and voltage. These characteristics affect accuracy, frequency and pullability (how far the frequency can be tuned either way from the center resonant frequency). Temperature controlled crystal oscillators (TCXO) have inbuilt compensation networks to reduce the drift caused by temperature changes. Oven controlled crystal oscillators (OCXO) have inbuilt heaters and temperature control loops that improve accuracy further through controlling the materials environment. Voltage controlled crystal oscillators (VCXO) provide a control port that allows the frequency to be tweaked higher or lower dependent upon voltage.
Clock references can be used to synthesize other frequencies using clock generators, clock multipliers, phase locked loops and frequency synthesizers. They can be buffered and distributed around a PCB and backplane using clock buffers and clock fan-out distributors. Clocks can be dithered in the frequency domain to reduce EMI (spread spectrum clock generator). Clock noise can be reduced with jitter attenuators. Many communications protocols embed the clock in the data transitions for recovery in the receiver with a phase locked loop (PLL). Data is then recovered using a clock data recovery (CDR) block. The recovered data can be retransmitted. synchronized to the regenerated clock using a Reclocker. This provides for repeated cable extensions and allowing high speed data to be communicated over extremely long distances.
Real-time clocks incorporate a counter that accumulates oscillations to create timing information. They can be updated with the current time and maintain a measure of time as accurate as the oscillator technology used will provide. Real-time clocks typically have a backup power supply capability – internal and external options are available. GPS modules are a popular source of highly accurate timing reference for designs. A GPS module will typically provide absolute time information and may generate 1-second timing events (epochs).