Bypass Capacitors

Guidelines for setting bypass capacitor constraints, including automatic detection criteria, recommended schematic practices, and required configuration details.

This constraint guarantees that bypass and decoupling capacitors are placed close to the correct pins.

Automatic Detection

When bypass capacitors are connected between a component and ground, they're automatically detected.

They're assigned to a parent pin using this priority order:

1. Explicit connection by wire in the schematic

2. Parent pins with voltage-type names (e.g. Vin)

3. If a capacitor is connected to multiple pins of the same name, it's split equally across them.

Recommended Patterns

For a capacitor to be assigned correctly to a parent pin, the best approach is to connect it directly to the pin in the schematic with a wire. Other patterns, such as blocks of capacitors linked by a net label or power port, will still be interpreted using our rules, but with lower priority.

You can also manually edit the assignment in the interface.

Constraint Details

Configure bypass capacitor constraints in the "Bypass Capacitors" section of Circuit Comprehension.

To set a bypass capacitor constraint, fill in the following fields:

• Capacitor (reference designator)

• Component being bypassed

• Pin on that component being bypassed

• Capacitance

Quilter automatically places capacitors with smaller capacitance values closer to power pins for a low-latency, low-impedance path. It can also position a single capacitor to bypass multiple pins on the same IC.

Physics Rule Checks

The PRCs that validate this constraint after compilation are:

Pin Distance calculates the Euclidian distance between the closest edges of two pins.

Layer Switch Count finds the number of times there is a layer switch in a trace path between two pins

Ground Plane Overlap determines if the ground plane below net traces overlaps at all relevant points.

Trace Path Length calculates the trace length between two pins of interest.

FAQs

Can Quilter handle multiple bypass capacitors for a single IC?

Yes, Quilter can identify and optimize one-to-many relationships between ICs and their bypass capacitors, ensuring effective decoupling across different frequency ranges.

How does Quilter decide where to place bypass capacitors?

Quilter strategically places smaller capacitors near high-current voltage sinks to minimize impedance and stabilize power delivery. The algorithm reduces trace lengths and via counts to lower parasitic effects in the decoupling network.

Can I override Quilter’s automatic placements?

Yes, you can manually adjust or pre-place bypass capacitors in your input file if you have specific design requirements.