How Embedded Design Teams Can Simplify I/O and Routing

Zachariah Peterson
|  Created: January 4, 2023  |  Updated: January 16, 2023
How Embedded Design Teams Can Simplify I/O and Routing

Real electronic products have been slowly getting smarter, both from implementation of an embedded application and connections back to a cloud platform or application. Embedded development teams have to work together to create these new generations of products. One area where the PCB layout engineer, embedded developer, and even the MCAD engineer can get delayed in finishing a project is in I/O selection. This happens whenever you have connectors, peripherals, and a host processor.

So, to help keep things simple for both sides, I want to share some of my experience working with embedded developers to optimize I/O selection and the overall process for completing the PCB layout. With a bit of collaboration on the front end, you can make PCB routing and embedded development easier on the backend.

Who Does What in Embedded Development?

I take the view that each member of development teams should understand what the other team members need to be successful. This is also about efficient use of resources, avoiding engineering quagmires that take days to resolve, and getting to market quickly.

 

What they do

What they need

PCB designer

  • Constraints from MCAD designer
  • Peripheral list from 
  • Desired I/O pin list from embedded developer*

MCAD designer

  • Create the device housing/enclosure
  • Define constraints (keepouts, placement, etc.)
  • Connector 3D models
  • Mechanicals 3D models
  • SI/EMI constraints from PCB designer

Embedded developer

  • Develop embedded firmware
  • Define outputs to web/cloud application (if used)
  • Use I/Os required by PCB designer
  • I/O functions from PCB designer
  • Desired I/O pin list from PCB designer*

 

I put an asterisk (*) on the pin list entries because the exact process for assigning I/Os depends on the specific component being used to run the embedded application. Is it a simple MCU, where the available I/Os are fixed in specific locations, or is it an FPGA that can have a customized pinout in certain banks? The other aspect is connectors: is the pinout standardized, is the pinout set by another board in the assembly, or can it be customized?

This is where we can start to see the challenge involved in a PCB designer and embedded developer working together on I/O selection. How can the two sides come together and nail down pinouts, I/O selection, and ultimately the PCB layout?

Embedded Developer Defines PCB Requirements

In my opinion, this works best when the pinout on the main processor is flexible, i.e., the application will be instantiated in an FPGA. In this case, the I/Os can be set by the developer in the device logic, and the result is that the PCB designer will have to work with the pin assignments they are given.

The problem is this: if pin assignments are given on an individual pin basis, you may not get optimal routing, resulting in a mess of connections crossing over each other to get to components. Instead, if the embedded developer just gives bank assignments to the PCB designer, then the PCB designer can choose pins within a bank to make optimal connections.

PCB FPGA
Screenshot showing bank assignments in an FPGA. Certain banks will support different interfaces, and the PCB designer can select some pins within these banks for pin swapping.

By assigning banks on an FPGA instead of a specific pinout, the designer can use a tool like pin swapping to move I/O assignments within a pin bank. If pin swapping is desirable, then a custom schematic symbol might be required in order to properly separate pins into their respective banks. Using a custom symbol enables much faster grouping of pins for pin swapping operations while routing the PCB.

PCB Designer Defines Embedded Requirements

In my opinion, this works best when the pinout on the main processor is fixed, i.e., an MCU and its peripherals. In an MCU, the I/O locations are basically fixed. You can have some flexibility in terms of how each I/O is used, for example when using a bank of GPIOs. However, pin or function assignments for standard interfaces may be fixed (e.g., SPI, ADC pins, PWM pins, I2C, etc.).

In this case, I think it’s best if the PCB designer selects the pins they will use to match with different components. The PCB designer has to choose within a fixed pinout, so they will have to work out how to place components in such a way that the PCB layout is solvable. Once placed and routed, it’s the PCB designer’s job to tell the embedded developer which pins are being used for certain functions.

STM32 LQFP pinout
STM32 LQFP48 pin assignment list. We can see how different pins functions on GPIOs are assigned to specific positions. This can limit what is done in the PCB layout, so the PCB designer should have the freedom to select pins they want to use, and the embedded developer will need to follow this when writing firmware.

What if a Connector Pinout is Standardized?

When the pinout on your connector is standardized, then it’s a bit of a wildcard. I say this because “standardized” can mean two different things:

  • The pinout could be part of a standard interface (Ethernet, USB, etc.)
  • The pinout could be set by another board in the assembly or system

If neither of these is the case, then your pinout is customizable. In projects where I’ve dealt with other boards in a larger assembly, we often have the freedom to set a pinout, usually because we were building both boards. In any case, the two sides have to work together to figure out an objectively “best” pinout for the connector and how this matches up with the processor. 

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About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 2000+ technical articles on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, American Physical Society, and the Printed Circuit Engineering Association (PCEA). He previously served as a voting member on the INCITS Quantum Computing Technical Advisory Committee working on technical standards for quantum electronics, and he currently serves on the IEEE P3186 Working Group focused on Port Interface Representing Photonic Signals Using SPICE-class Circuit Simulators.

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