How to Design a Connector Pinout For Your PCB
The DVI connector shown above has a very specific pinout that must be implemented when routing a connector if you want to use this interface on your PCB. The same applies to any other connector that is used to provide power, ground, and signals through a standardized interface. Ethernet, USB, HDMI, and many other protocols have a specific pinout that should be used to ensure interoperability between different devices.
Although pinouts for common protocols are standardized, there are times where you have plenty of freedom to design a custom pinout. Suppose you’re designing a board-to-board connection or a custom interface to another board; you have freedom to design the connection as you see fit. So how exactly should you do this, and what should you watch for in a connector pinout design? We’ll walk through some of the important design points around connectors in this article.
Getting Started on Your Connector Pinout Design
There are several important points to consider in your connector pinout design, such as:
- What’s the length of the connection you need to make? Is it a short board-to-board connection, or are you driving signals over a long cable?
- What frequencies are you working with (for analog signals), or what is the signal rise time (for digital signals)?
- Are you routing differential pairs or single-ended signals?
- Do you need to hit a specific impedance target, or is the connection short enough that impedance doesn’t matter?
What’s interesting is that routing over a connector is not much different from routing over a PCB. You can have the same signal integrity problems, impedance mismatch problems, and signal loss problems as you might see in a high speed PCB.
If you’ve never designed a pinout, or you’re wondering why your favorite development board used a particular pinout, it helps to break down some specific cases. For now, I’ll look at what happens with different frequencies and signal types, and we can determine some good design practices for a connector pinout..
If you’re just routing DC between two boards or over a cable, the main consideration is the total current you need to carry. Pin headers and similar light-duty connectors can accommodate a small maximum current per pin (~1 A is typical). If you need to carry more current for a given voltage, then that voltage needs to bridge across multiple pins. Another thing you can do is route multiple voltages across a single connector, which is the same approach used in desktop computer power supplies.
Connector pinouts for DC systems should carry a ground signal along the interconnection. Be mindful of this ground signal as this is the on-board reference plane, and it will need to carry a return current, so you should size the wiring and the number of wires accordingly. It is recommended that you not try to use a GND connection to bridge two different earth grounds on two different boards, especially if they are on different grid circuits and separated over some distance. You risk creating a short circuit between two points that carries so much current it will melt the cable. This occurs due to the DC ground offset that naturally exists between different points in an electrical grid.
Low Frequency/Low Speed
Hopefully by now, you’ve figured out that low frequency and low speed are all relative: what matters is the length of the connection and whether impedance is required. For a low-speed digital bus, something in the 5-10 ns range, you might not need to worry about things like crosstalk or reflections as long as the connection is short enough and you include at least 1 GND line in the connector pinout. Make sure if you’re pulling power into the connector pinout that you follow the same rules as for DC connectors.
On high pin-count pin headers or other connectors with a long line of pins, some of the signals will be a source of EMI when they are a long distance from a ground pin. Similarly, those signals can more easily receive crosstalk, especially if you’re using a ribbon cable or other flat cable. The example below uses a 14-pin connector that has ground interleaved between some IOs. By placing GND between groups of pins, the GND will provide shielding against noise and help block EMI. This example could be used with a long connector if needed. For a board-to-board connection, you could definitely remove some of the GND pins and you should still be okay from a noise perspective simply because the distance is so short.
High Frequency/High Speed
With high speed/high frequency signals, something similar to the above pinout is still okay, but you’re typically working with differential pairs. In this case, it’s best to provide pairs of ground pins to prevent crosstalk between differential pairs. In any case, more ground pins are your friend as they provide more shielding and help minimize any impedance mismatches that might arise. For high frequencies, such as in the GHz range, you won’t (or at least you shouldn’t) use a simple pin header anymore. A coaxial (U.FL) connector would be the best choice for the RF signal, while other signals and power could be routed over their own connector.
If you need to find a connector that can handle your required current, frequency/band width, specific signaling standard, or all of the above, there are plenty of connector options on the market. Make sure to check datasheets for the important specifications; you can also read a guide on Octopart from this link. If you’re unsure which connector you should use, head over to a connector manufacturer’s website; they will label their products by application (high current, RF/microwave, etc.) so that you can narrow down to the best component for your design.
Last but not least, pay attention to any shroud and pin 1 when you arrange the connector pinout for your components! You would be surprised how often a custom pinout gets reversed between two shrouded connectors, and there’s no way to correct it on a finished board; you’ll have to re-build the cable instead. This is one of those things where it might help to have the connectors in front of you to make sure you’re defining the connector pinout correctly.
Once you’ve selected a connector for your PCB and designed your connector pinout, you can start building schematics using a PCB design program like CircuitMaker. Users can create custom schematic symbols for their connector pinouts, or you can find standard connectors from the built-in parts databases. All CircuitMaker users also have access to a personal workspace on the Altium 365 platform, where they can upload and store design data in the cloud, and easily view projects via a web browser in a secure platform.