Clock In Digital Electronics: Theory and Fundamentals

Key Takeaways

• The clock is the heart of digital electronics, providing a stable, high-speed signal for logic updates.

• Due to manufacturing, design, and performance effects, the clock signal can encounter issues; the designer must be mindful of these limitations.

• Given its central role, board design rules must incorporate the constraints of the clock.

The oblong-shaped clock in digital electronics rises above smaller components like a clock tower.

The clock in digital electronics is the device's heartbeat, providing the rhythm for system state updates. The clock produces a constant and consistent periodic oscillation (provided by none other than an oscillator, like a quartz crystal oscillator) that rapidly toggles between high and low states to provide memory and logic a simultaneous update to prevent racing conditions, or unstable circuit behavior due to ambiguous logic states. For most, synchronous circuit functionality is synonymous with the performance they expect from their electronic devices, making its implementation vital to modern circuit design.

Synchronous vs. Asynchronous (Clockless) Digital Design

The Central Role of the Clock in Digital Electronics

Logic elements like latches and flip-flops require a periodic waveform to establish the rate at which the logic updates; all synchronous logic depends on the signal integrity of the clock to perform as expected. Depending on the components, status updates may occur on the low-to-high voltage transition (rising edge) or vice versa (falling edge). Selection of the clock speed depends on the slowest possible propagation path across the circuit, resulting from both material selection and the efficiency of the board design.

However, it’s impossible to predict the accurate speed of a waveform traveling through a circuit due to PVT:

• Fabrication process variance - No two items are completely identical, even with the exacting quality control of modern component manufacturing. While manufacturing information provides nominal values and tests to ensure compliance, there is an acceptable level of deviation in all manufacturing, especially one as complex and multi-staged as IC manufacturing. Therefore, the final product (or prototype) is the only accurate performance measure.

• Supply voltage variance - Most power signals require significant conditioning to reach a consistent voltage level, and transients owing to off- or on-board interference can also disrupt power supply voltage. Regardless of the source, changes to the supply voltage affect the saturation current through the device, inducing an exponential decrease in the propagation delay over a wide voltage operating range.

• Operating temperature variance - Temperature variance is a normal part of operation owing to heat dissipation from Ohmic heating. As temperature increases, holes and electrons encounter more resistance, reducing their speed and increasing the propagation delay. 

Additional factors, such as parasitics arising from conductive pathways and differences in computing logic operations, also play a role in a clock signal’s propagation speed through a network. Hence, synchronous circuit design revolves around a staging of the clock signal in steps: first, the signal reaches all the appropriate pins in the network, then the entire synchronous circuit waits for all of the combinational logic to finish, and finally, the results of the clock cycle propagate through the circuit. While this may sound like a lengthy process, the travel time of the clock signal and any consecutive logic gateways along its propagation path can occur on the order of nanoseconds.

The function of the clock will also depend strongly on the shape of its waveform. For digital circuits, an ideal square wave with instantaneous changes to the voltages at the high-to-low or low-to-high voltage transitions might be the goal, but this form includes high-frequency harmonics for mixed-signal circuits. A square wave becomes more trapezoidal even for purely digital circuits due to the non-instantaneous rise and fall time, with a longer rise time alleviating harmonic distortion. It’s worthwhile to design around an even higher clock frequency for appreciably high clock speeds for worst-case harmonics, assuring signal integrity for associated harmonics up to that frequency.

The Clock’s Effects on Layout Rules and Design

Generally, the rules for high-speed design do not discern between clocks and other circuit nets, but a further level of precaution is worth considering. Designers and engineers should begin by assessing a few fundamental aspects of the clock signal:

• Given that inductive coupling increases with frequency, the board layout should provide ample spacing between the clock signal and other traces, especially for low-frequency traces highly sensitive to neighboring high-frequency signals.

• Carefully consider the interconnection between functional blocks of the system, both physically and logically. Divvying up the clock signal between these sections will require a clock driver and equal-length traces to eliminate timing differences and prevent reflections.

• Is the design better suited for stripline than microstrip fabrication? Furthermore, what constraints for trace parameters (i.e., gaps, widths, heights, etc.) are most suitable for performance? 

To the last point, simulation is vital to establish the design rules that constrain various board aspects to improve manufacturability and performance. This effort requires collaboration between designers and manufacturers to ensure the design intent aligns with the manufacturer's capabilities. At the earliest stage possible in development, designers establish communication, especially for board design that pushes the limits of manufacturability or for non-standard requests.

Cadence Solutions Save Development Time

It’s difficult to understate the importance of the clock in digital electronics during board layout and routing: this is arguably the most critical single signal on the board, both in terms of immediate simulation and long-term reliability. When planning the layout, designers must provide short, direct paths that minimize any nonessential length of the trace and ample space from other signals, especially lower-speed traces susceptible to inductive coupling. Perfecting the balance between performance and reliability can be a difficult needle to thread, but simulation tools can greatly assist as a pre-production check of layout. Cadence’s PCB Design and Analysis Software suite provides development teams with a comprehensive toolset of DFM tools that accelerate production without sacrificing accuracy. With Cadence, users can seamlessly integrate simulation results into OrCAD PCB Designer under a unified ECAD ecosystem.

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