FEDEVEL
Platform forum

Bitu , 01-19-2025, 04:41 AM

I am currently trying to learn high-speed PCB design and would like to understand some fundamental concepts better. Specifically, I am confused about:
1. Which pins/signals require length matching?
2. Which pins/signals need impedance matching?
It would be great if you could explain the criteria or share any resources that could help me determine these requirements for high-speed designs and communication protocols.

QDrives , 01-19-2025, 06:07 PM

A (overly) simplified rule of thumb is any signal where the propagation delay is more than 10% of the rise time.

Robert Feranec , 01-20-2025, 10:45 AM

2. I do that for for every digital signal + for all other signals which specifically say they need specific impedance

Robert Feranec , 01-20-2025, 10:53 AM

1. You can get some ideas here, starts at page 140: https://www.kontron.com/resources/collateral/white_papers/picmg_comdg_100.pdf

Robert Feranec , 01-20-2025, 11:00 AM

For slower interfaces (e.g. below 1MHz) the maximum required length matching is a high number, so if signals of these slower interfaces are routed together normally all the tracks would be similar length and within this maximum required length matching tolerance even if they are not specifically length matched. Unless routed by a very weird way, slower interfaces would not require a specific length matching.
Check out the document I attached above, you can see, for example PCI interface at 33MHz has length matching requirements around 200mils / 5mm

Bitu , 01-20-2025, 02:51 PM

@Robert Feranec @QDrives Thank you for sharing this information and the attached document. The explanation about length matching requirements for slower interfaces and example is very clear and helpful. I appreciate your guidance
Thanku so much