Common Types of Low Power Clocks and the Use of Simulation Software

October 29, 2019 Cadence PCB Solutions

Wall of hanging analog clocks on a wall

 

The brain is considered the most important organ found in the human body with the heart coming in as a close second. This, of course, is due to its critical role in the proper functionality of the human body’s circulatory system. Then when you consider the fact that a normal resting heart beats (adults 18 and older) between 86,400 and 144,000 times per day, it puts the importance of your heart into perspective.

Furthermore, the human body itself is considered to be the one machine that we as humans have yet to fully understand. However, if our bodies are machines, then the brain would be our processor and our heart would be the clock.

In the field of electronics and design, the clock is the one component that is necessary to keep all of the internal functions of any device synchronized and this includes PCBs.

The Low Power Clock Circuits

The clock is an intricate part of any circuit design that requires synchronization or the aligning of operational frequencies. The clock circuit is quite critical to the overall functional design and as I eluded to earlier, its function is like that of the (human) heart.

In general, if the clock of the circuit system is wrong or malfunctions, the system will be disordered. Therefore, it is imperative that you take great care in properly designing your low power clock circuit before incorporating it into your PCB design.

Which brings me to the other critical and integral part of the manufacturing process, simulation. Today’s high demands for speed and accuracy in the PCBs we produce and the subsequent products they are used in is undeniable. This, of course, includes the low power clocks used in PCB designs. Well, the most efficient and accurate way to achieve proper low power clock circuit design is through the use of circuit design and simulation software. However, first, I will discuss the most common types of low power clock circuits before elaborating on the simulation aspect.

Types of Low Power Clock Circuits

The more commonly used clock circuits are crystal, crystal oscillator, and clock distributor. Below, I’ll go into each of these circuits a bit more in-depth. 

The Crystal Clock Circuit 

The basic and commonly used crystal packages are the 2-pin DIP package and the 4-pin SMD package.

Even though the crystals have different specifications, they share a basic circuit design.  Therefore, the layout and routing rules of the PCB are shared as well.

In addition, if we were to visualize the actual circuit schematic, we would see that the circuit consists of a crystal plus 2 capacitors (gain capacitor and the phase capacitor).

Also, when we build or simulate the crystal circuit layout, the two capacitors should be placed close to the crystal and the line should be as short as possible and thickened and packaged.

This, of course, is the most basic and most common crystal circuit (clock) design. However, there are also some deformation designs, such as adding series resistance, and test points.

The layout should pay attention to the following.

  • The crystal and IC layout are on the same level so that fewer holes can be punched.

  • The layout should be compact. Also, the capacitor should be located between the crystal and the IC, as well as placed close to the crystal. This is to ensure that the clock line to the IC is as short as possible.

  • For cases with test points, try to avoid stubs or make stubs as short as possible;

  • Do not place amplifying power devices nearby, such as power chips, MOS transistors or inductors.

When wiring, you should pay close attention to the following.

  • The crystal and IC are laid out in the same layer. The same layer is routed and the hole is drilled as little as possible. Also, if punching, it is necessary to add a reflow hole nearby.

  • The class differential traces.

  • The trace should be bold, usually, 8~12mil since the crystal clock waveform is a sine wave.

  • The signal line should be covered by the ground and the grounding wire or copper skin needs to be shielded.

  • The crystal circuit module area is equivalent to the analog area. Finally, try not to pass other signals.

The Crystal Oscillator Clock Circuit 

In comparison to the crystal circuit, the crystal oscillator clock circuit is an active circuit. It is mainly composed of three parts: crystal oscillator, power supply filter circuit, and a source matching resistor.

The Clock Distributor Circuit

There are several types of clock distributor circuits. When approaching design considerations, ensure that the distance between the clock distributor and each IC is as short as possible (usually symmetrical).

Summary of Clock Distributor Circuit layout and wiring.

  •  The more common clock generation circuit is a crystal or crystal oscillator circuit and it should be placed close to the clock distributor.

  • The clock distribution circuit is also placed in a symmetrical position to ensure that the clock signal lines to each IC are as short as possible.

  • Do not place amplification power devices nearby. For example, power chips, MOS tubes or inductors.

  • Also, whenever the clock signal line is too long, it can go inside the inner layer. There should also be a reflow hole in the 200 mil range of the layer-changing hole as well.

  • Other signals and clock signals maintain a recommended spacing.

  •  Pack the ground and add the shield hole.

Picture of a clock placed on a circuit board

There are pitfalls to be wary of in each of the common clock circuit designs

 

The Importance of Simulation of Low Power Clock Circuits

As I am sure you are aware, simulation of electronic circuits uses mathematical models to get the actual behavior of the printed circuit board (PCB) or electronic device. Overall, simulation software also allows for the modeling of electronic circuit operation. In addition, simulating a circuit’s behavior before actually manufacturing it improves the overall design, design efficiency, and lowers manufacturing costs.

In terms of lower power clock circuits, the benefits of simulation are somewhat amplified. This coincides with the clock circuit's importance to the overall functionality of the design as a whole. We all know that manufacturing cost is very high and it can be very difficult to know whether or not a circuit (low power clock) will perform as intended before implementing it into your design. Therefore, simulation is crucial.

Simulation is an essential process of the design of any component, circuit or electronic device. Furthermore, it also increases the efficiency of manufacturing by providing a fast preview of how the printed circuit board is going to look and perform in the application. This, of course, includes the low power clock circuit.

PSpice component vulnerability simulation

PSpice is a valuable simulator capable of not only modeling designs but also finding tolerance and vulnerability probabilities.

 

Simulation tools for low power clocks have a variety of uses: voltage controlled oscillation measurements, low frequency and frequency spectrum modeling, pulse width modulation, and signal delay are all more than possible to be accounted for within a simulation tool. 

You can determine optimal frequencies or layouts to achieve desired frequencies, match and determine proper voltage through your circuit (while also cross-checking this voltage through your power supply and heat dissipation maps to ensure component reliability and optimal power layout), and any other potential circuit needs you might find.

In conclusion, if a PCB has any kind of functional property involving speed, frequency (clock), signals or lower current, then it is essential to get the best schematic layout before actually building the system. The best way to that is, of course, through the use of simulation tools like PSpice.

Whether it is power delivery, signal integrity, mixed-signal simulation or any of the various parameters, they can all be analyzed and simulated before actually building your design.

The best simulation plans for all of your low power clock circuit designs require tools like Cadence’s suite of design and analysis tools. OrCAD PSpice Simulator is the modeling and simulation tool you’ve been looking for to facilitate the best simulation software for all of your electronic circuit simulation needs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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