Recently, I've been deeply involved in embedded systems and schematic design. After completing the initial layout, I spent nearly two weeks carefully reviewing the schematic to ensure its accuracy. In this post, I’d like to share my personal experience and best practices for checking circuit schematics. I welcome any feedback or corrections you may have—after all, we're all learning together.
Once a schematic is drawn, it's common knowledge that it needs to be reviewed. But where do you start? What are the key things to look out for during the review process? Let me walk you through my own approach based on real-world experience.
1. Check All Chip Package Pins for Errors
This refers specifically to the package symbols I’ve created. In one of my projects, I accidentally swapped two pins of a chip, which caused a problem during PCB manufacturing and made debugging very challenging. Always double-check the pin assignments before finalizing the schematic.
2. Use ERC (Electrical Rules Check) in Protel (or Equivalent Tools)
Protel’s ERC tool is invaluable. It can catch many errors that might otherwise go unnoticed. Running an ERC check after drawing your schematic can reveal issues such as unconnected pins, missing labels, or incorrect net connections. It’s surprising how many mistakes you can find even after careful work.
3. Verify All Network Connections (Net Labels)
Common issues include mismatched net names, forgotten connections, or accidental multiple connections. For example, I once used “DDR_CLK†for one clock signal and “DDRCLK†for another, thinking they were different when in fact they should have been connected. Always make sure net labels are consistent and properly assigned.
4. Check Function Pin Connections and Missing Pins
Each chip has specific function pins that must be correctly connected. For instance, an audio processor I used had three clock inputs—LCLK, BCLK, and MCLK—that needed to match exactly with the main chip’s corresponding pins. Also, always cross-reference each pin with the datasheet to ensure none are left unconnected or mislabeled.
5. Ensure All Passive Components Have Valid Values
Resistors, capacitors, and inductors shouldn’t be chosen randomly. Most datasheets provide typical values or reference designs. If not, look up similar designs online or consult experienced engineers. Using arbitrary values can lead to unexpected behavior or even damage to components.
6. Confirm Power Supply Filtering
Every chip should have decoupling capacitors at its power supply input to reduce noise and stabilize voltage. Typically, a 0.1µF ceramic capacitor is placed close to the power pin. During schematic review, make sure these capacitors are included and correctly placed near the respective chips.
7. Review All Interface Circuits
Interfaces like USB, Ethernet, or serial ports require proper protection and driving capability. Input circuits should include overvoltage protection, optocouplers, or clamping diodes. Output circuits may need pull-up resistors or level shifters to ensure reliable signal transmission.
8. Check Power-On and Reset Sequences
Some chips require a specific power-on sequence. For example, the DM6467 chip I worked with needed 1.2V first, then 1.8V, and finally 3.3V. To implement this, we used a delay IC to control the order of power delivery. Always check the datasheet for such requirements and design accordingly.
9. Verify Grounding Strategy
Analog and digital grounds should be separated if the system involves both types of signals. Make sure that analog chips are connected to the analog ground, and digital chips to the digital ground. Proper grounding is crucial for minimizing noise and ensuring stable operation.
10. Optimize and Improve Module Design (Optional but Recommended)
During the initial design phase, it's easy to overlook certain optimizations. However, after the whole system is built, you often find better solutions. For instance, our power module went through four revisions before reaching a stable and efficient design. Continuous improvement leads to better performance and reliability.
Schematic checking is not just about finding mistakes—it's also about improving the overall design. Whether you're a beginner or an experienced engineer, taking the time to thoroughly review your work can save you a lot of headaches down the line. Happy designing!
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