Clock Signal In Digital Circuits

Timing components are one of the most ubiquitous components in electronics. They are needed in nearly every complex design and all our electronics wouldn’t work without them. 

In this article, I will be explaining the different kinds of timing components available and just why clocks are so important in digital electrical circuits.

What is a clock signal?

We can define a clock signal as a particular type of signal that oscillates between a high and a low state. The signal acts like a metronome, which the digital circuit follows in time to coordinate its sequence of actions. Digital circuits rely on clock signals to know when and how to execute the functions that are programmed.

If the clock in design is like the heart of an animal, then clock signals are the heartbeats that keep the system in motion.

How are the clock signals produced?

There are different ways a clock signal can to be produced, but they all start off with the crystal resonator. A crystal resonator is commonly referred to as a crystal. In order to operate, crystals are combined with an amplifier circuit to apply voltage to an electrode near or on the crystal.

The quartz crystal is a tiny slit of quartz with each of the two surfaces metalized and attached with an electrical connection. It’s important the physical size and shape of the quartz crystal are precisely cut because this determines the frequency of oscillations produced from the crystal. Once the crystal is cut and shaped, it cannot be used at any other frequency. Quartz crystals are more commonly used since the frequency generated from quartz crystals are more resistant to changes in temperature. If an internal RC resonator was used instead, changes in temperature would affect the behavior of the oscillator, leading to changes in the output frequency.

Crystal Oscillator

Crystals have a sinusoidal output and are typically used if the target IC has an integrated oscillator and on-chip phase-locked loops (PLLs) for internal timing. When a crystal and oscillation circuit are combined in the same package, it is commonly referred to as a crystal oscillator. This quartz piezo-electric oscillator outputs a usable oscillating signal, most commonly a square wave with a 50% duty cycle. Usually, this clock signal is fixed at a constant frequency and synchronization may become active at either the rising or falling edge of each clock cycle.

 Clock Generator

A clock generator combines an oscillator with one or more PLLs, output dividers, and output buffers. Clock generators and clock buffers are useful when several frequencies are required and the target ICs are all on the same board or in the same 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. In most cases, the oscillator is external to the clock generator, although it is becoming more common that oscillators are combined into the same package as the clock generator in an effort to consolidate the bill of material cost and complexity, alongside other advantages. There are many different types of clock generators and each is optimized for different performance and cost targets depending on the application.

Synchronous and free-running designs:

Systems and their combination of various subsystems may require a timing architecture that is free-running or synchronous.

If a system is free-running, independent clocks can be used without any special phase-lock or synchronization requirements. Examples include standard processors, memory controllers, SoCs, and peripheral components (e.g., USB, PCI Express switches).

An example of a complex IC everyone is familiar with would be the microcontroller. Microcontrollers rely on a clock from a crystal oscillator to function with an exception for when used in asynchronous circuits, like in the case of asynchronous CPUs. Most common microcontrollers contain an internal R-C oscillator that is good enough for things like UART communication, although external crystal oscillators are necessary for other types of communication like USB or ethernet.

Conversely, synchronous timing 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.

There are various applications that require accurate frequency or timing other than communications, though. Some applications require long-term synchronization between two subsystems that are not connected to one another. If an oscillator used as the basis for a real-time clock was off just 0.1%, a week later the clock would be almost 10 minutes off. Long-term accuracy might also be needed without having to know real-time.

For example, suppose you want several Bluetooth modules to wake up once every hour to exchange data for a few seconds and then go back to sleep, in order to preserve battery power. A standard 20ppm oscillator would be off just fractions of a second per hour, whereas a 1% RC resonator could be off by half a minute. If the RC resonator is used, the Bluetooth modules would have to remain on for longer periods of time in order to communicate with one another, thus wasting battery power.

Internal and external oscillators:

Internal oscillators are commonly used to provide timing for MCUs that don’t require accurate timing. Internal oscillators are good enough for low-baud UART communication, although external crystals and oscillators are required for communication protocols such as CAN, USB or Ethernet which have stricter timing accuracy requirements.

Using an external oscillator allows a wider range of frequencies where the internal oscillator(s) are typically one frequency with a handful of clock pre-scaler options. In electronics, time is a property that can be measured accurately and cheaply, so oftentimes a problem is transformed into one of measuring time or producing pulses with accurate timing.