Using ORCAD Hierarchical Schematic Design to Stay Organized and Synchronized

 Hierarchical design is an essential technique for organizing complex schematics in ECAD tools like OrCAD. Top-down hierarchical methods help you break down a complicated schematic into logical pieces and work on them separately. This keeps projects structured, efficient to edit, and avoids synchronization issues.

In this article, we’ll explore some of the key benefits of schematic hierarchy in OrCAD, along with guidelines and examples for implementing it effectively in your designs.

What is Hierarchical Schematic Design?

A hierarchical schematic is one that is constructed using a tree-based topology of parent-child relationships. The top level is the root block diagram that instantiates major sub-systems as blocks. Each child block can then represent a more detailed schematic chunk. This subdivision repeats recursively to arbitrary depth.

So rather than a giant, flat schematic, the design is split up logically into interconnected pieces at different levels of abstraction. This is analogous to how software engineers use hierarchy with classes, modules and functions.

The hierarchy provides two major advantages for ECAD design in OrCAD:

1. Organisation - Logically grouping circuits into related chunks keeps complex projects clean and structured. This improves readability and makes the schematics easier to navigate.
2. Re-Use - Common sub-circuits can be saved as reusable blocks. Instancing them multiple times avoids re-drawing duplicate content.

By taking a hierarchical approach, you can break a sprawling schematic into manageable pieces that are easier to conceive, design, and manage.

OrCAD Hierarchical Design Features

OrCAD provides a full suite of features to enable hierarchical schematic design and synchronization. Some key capabilities include:

• Design resources - Save schematic sheets as reusable blocks in the design resources cache. Sheet symbols represent these blocks.
• Ports - Define interface ports on schematic sheets to connect to instances of the sheet. Ports handle cross-sheet connectivity.
• Sheet symbols - These symbols placed on parent sheets represent instances of child sub-sheets, connecting to them through ports.
• Sheet entries - Sheet entries act as cross-sheet connectors for signals that pass between sheets. They connect to sheet ports.
• Hierarchy manager - This manager dialog gives visibility into the hierarchy tree and assists navigation between sheets.
• Hierarchy navigation - Navigate up and down the sheet hierarchy using the hierarchy menu, hotkeys like ‘Goto Sheet’ and back-annotation.
• Error reporting - Electrical and ERC errors propagate up the hierarchy so they are visible on parent sheets for easy debugging.

These capabilities allow you to develop schematics in a structured, divide-and-conquer manner while keeping everything connected and synchronized.

Hierarchical Design Guidelines

To leverage the advantages of hierarchy effectively, it helps to follow some schematic organization guidelines:

• Plan hierarchy ahead of time - Think through the logical sub-systems and abstraction levels to create a purposeful hierarchy tree. Don’t let it emerge randomly.
• Top-down design - Work top-down to flesh out the block diagram first, then divide blocks into sub-sheets with increasing detail downwards.
• Divide cohesively - Group circuitry into sub-sheets based on logical relationships, not random cuts. Keep related circuitry together.
• Limit sheet complexity - Target sub-sheet sizes that are readable without excessive panning and zooming. Avoid overloaded sheets.
• Minimize global connections - Limit signals that span large portions of hierarchy, as they are harder to track. Localize connectivity on sheets when possible.
• Leverage re-use - Identify common sub-circuits to save as reusable design resources for instancing multiple times.

Following these principles will help you get the most out of OrCAD’s hierarchical design flow.

Hierarchical Design Examples

To illustrate the benefits of hierarchical design, let’s walk through some examples of implementing hierarchy in OrCAD.

Top Level Block Diagram

The root sheet should provide a high level block diagram of the key sub-systems in the design. This serves as the overall schematic skeleton that sets the organization.

For example, a top sheet for an alarm clock design might contain:

• Power supply block
• Microcontroller block
• User interface block
• Alarm block
• Speaker driver block

Each block represents a more detailed sub-sheet describing that functional section. This clean block diagram provides a clear roadmap to dive into individual circuits.

Intermediate Sub-Sheet

At the next level down, each block can be expanded into a dedicated schematic sheet with more details. Here is an example alarm sub-sheet:

• RTC clock device
• Alarm time setting circuitry
• Alarm activation logic
• Snooze button circuit
• Sheet entries to top level I/O

This sheet localizes and isolates all the alarm-related circuitry in one place for easy comprehension and editing. The sheet entries and ports connect to the upper hierarchy levels.

Low-Level Sub-Sheet

Finally, a leaf sub-sheet presents implementation details for a discrete section. For example, the alarm activation logic:

• Alarm state machine
• Comparators
• Latch
• Gate drive logic
• Sheet entries/ports to alarm block

So even intricate circuitry can be managed by diving down multiple hierarchy levels into logical chunks. This is far more efficient than a giant flat schematic.

Benefits Summary

Using a hierarchical approach with OrCAD ECAD has many benefits for your schematic design process:

Organization

• Logical structure for easy navigation
• Cleaner high-level overviews
• Related circuitry localized together

Efficiency

• Divide and conquer by sub-sections
• Re-use common circuit chunks
• Changes kept local, avoiding widespread edits

Synchronization

• Cross-hierarchy signal connectivity
• Error reporting bubbles up for easy debugging
• Top-down or bottom-up design flow

By investing some time up front to plan and create meaningful hierarchy, you can streamline the entire schematic design and development process. The result is a cleaner, better organized, and more maintainable schematic.

Frequently Asked Questions

What are some tips for deciding how to break up a schematic into hierarchical blocks?

Focus on splitting the design into logical groupings based on functionality. For example, isolate all the microcontroller circuits together separated from power supply sections. Think about abstraction levels, with increasing detail as you go down the hierarchy.

If I make a change to a sub-sheet, does it automatically propagate to parent sheets in OrCAD?

Yes, connectivity and error reporting will bubble up the hierarchy. So if you change a sub-sheet, any effects are visible when you navigate back up to parent sheets. You don't have to manually synchronize changes between sheets.

Is there a way to visualize the entire hierarchical schematic tree in OrCAD?

Yes, OrCAD includes a hierarchy manager dialog that displays the hierarchy tree. You can view the connectivity between sheets and quickly navigate to any sheet by clicking on it in the tree diagram. This provides a great top-down view.

What is the best practice for re-using the same circuitry multiple times in a hierarchical design?

You can save common sub-circuits as design resources, then add sheet symbols to instantiate these saved blocks. For example, create an op-amp design resource that can be instanced wherever needed, rather than re-drawing each time.

How do I quickly navigate up and down hierarchical levels when editing schematics?

OrCAD provides hotkeys like 'Goto Sheet' and back-annotation that let you quickly traverse hierarchy levels. The hierarchy menu also lists all sheets for navigation. So moving between abstract and detailed views is very smooth.