Space Allocation

When starting with a new schematic, you’ll face the challenge of managing available space. The approach you take will depend on various factors, including the project’s size, complexity, target audience, relevant standards, and lifecycle considerations. Additionally, whether or not you need to print the schematic will influence how you allocate space.

The first key decision is whether to use a single sheet or multiple sheets. For smaller projects, it’s practical to begin with a single A4 sheet. As your design grows, you can switch to A3, which still allows for printing and provides more space for expansion as the project evolves.

Logical Organization

Good schematic design follows best practices to ensure clarity and ease of understanding. Here are some guidelines:

Divide the schematic into logical blocks – such as power supply, microcontroller, input connectors, output connectors, etc. While there are no strict rules for partitioning, it’s important that the divisions align with your project’s structure. Hierarchical sheets can be used for modular design, or components can be spaced evenly within a single sheet.

Ensure a left-to-right flow – components and modules should be arranged so that the logical flow of your project moves from left to right. Inputs should appear on the left side of the schematic, with outputs on the right. This principle applies both inside and outside of the blocks. For example, in a power supply schematic, the power input should be on the left, and the outputs should be on the right. The power supply block itself can either be positioned on the left or placed separately, depending on whether you view it as an input or a global element.

It’s essential to maintain a clear, logical sequence throughout the schematic. For instance, if your design includes an NFC reader, the MCU, and communication bus outputs, the NFC reader should be positioned on the left, the MCU in the middle, and the output to the bus on the right. As your design evolves (e.g., adding an RTC), the component’s placement should respect the logical flow – the RTC should be placed to the left if it influences the MCU based on time, or to the right if the MCU configures it based on NFC data.

Your schematic should be easy to follow, even after time has passed or when viewed by someone else. Add notes to clarify the function of specific sections, keeping them concise but informative. Links to additional documentation or simulations are encouraged for further reference (e.g., “3V3 to 5V level shifter ”).

Consider using hierarchical sheets for complex designs. For example, you can add a note like “50Ω diff. pair” and link it to a separate sheet with detailed calculations.

Best Practices

There are several essential habits that will improve the clarity and readability of your schematic (ordered by importance):

• Avoid routing wires under components.
• Use horizontal or vertical wire routes; free-angle lines should be avoided unless necessary to indicate swapped signals (e.g., TX/RX).
• Ground connections should always be directed downward, while power connections should point upwards.
• Use net labels to simplify connections.
• Limit junctions to a maximum of three connections to reduce complexity and potential errors.
• Use graphical lines and text to create clear block descriptions, such as “Power Supply” or “MCU”.

Multi-Sheet Schematics

In larger designs, it’s beneficial to split the schematic across multiple sheets. This hierarchical approach organizes your schematic into logical blocks, with each sheet acting as a virtual component on the parent sheet. Sheets are typically numbered according to their function, such as 1-99 for the main sheet, 100-199 for the power supply, and 200-299 for the MCU.

Net Labels

Net labels allow you to connect wires without physically linking them, reducing clutter in your schematic. They also assign logical names to the connections (e.g., “5V”, “I2C_SDA”), which should be consistent across the entire design.

There are three types of net labels: local, global, and hierarchical. Local labels are confined to the sheet where they are defined, meaning you can use the same name on different sheets without causing conflicts. However, using unique names across the schematic reduces confusion. Hierarchical labels expose local nets to the parent sheet, while global labels connect nets across all sheets, facilitating cross-sheet connections.

Global labels connect nets across all sheets, ensuring that signals remain consistent regardless of the sheet they are defined on.

Consistency and Symmetry

It’s important to maintain consistency and symmetry throughout your schematic. Use a consistent style for components (e.g., resistor notation, IC symbols), and arrange similar components symmetrically. For example, if you have four capacitors with similar functions, they should be arranged in the same pattern for clarity.

Metadata

Every schematic should include a metadata section or title block. Even if you’re the only one who will view it, include basic information such as the project name, revision, and date. This helps ensure that you can easily identify the schematic version in the future.

Notes

Include the appropriate level of notes in your schematic to explain each part. Use free text notes for general descriptions (e.g., “power supply input voltage range”) and component-specific notes for detailed information like part numbers and requirements. Many design tools allow for both visible and invisible notes, which can be included in the bill of materials or project documentation.

Decoupling Capacitors

Decoupling capacitors are essential for stabilizing voltage and reducing noise in IC circuits. Typically, these capacitors are placed close to the power pins of the ICs they are decoupling. However, some designs may opt to group them in a separate block. While there is no definitive rule, the key is to remain consistent with your placement strategy and document their function clearly, especially if you place them separately from the ICs.

Summary

The guidelines presented here are more like recommendations than strict rules. Most design software won’t enforce them, but it’s still important to follow these practices for clarity and professionalism. If you choose to deviate from them, ensure you have a solid rationale for doing so.

Bonus Advanced Tip – Net Ties

Sometimes, you may need to connect two nets without combining them into one. For example, connecting a trace antenna to ground could affect the entire antenna if treated as one net. A net tie allows you to connect two nets while maintaining their distinct identities. This technique is particularly useful when designing antennas or other specialized components.

If you have questions about PCB or PCBA design, feel free to reach out via email at info@wellcircuits.com.