Our client required a processing board with numerous inputs and outputs for their specific product. They had selected the i.MX SoM as being appropriate to their performance requirements but needed various other inputs and outputs not commonly found on off the shelf boards. The product was designed to connect to a wide variety of other electronics via various mechanisms:
- Support for an i.MX8 SoM
- Four 5 gigabit USB ports (a mixture of USB-A and USB-C) including one 5V PD port
- M.2 NVMe SSD storage (PCI Express Gen 3 (8 GHz))
- Dual Gigabit Ethernet ports
- Multiple voltage supplies at relatively high currents (around 90W in total)
- Precise voltage and current monitoring on supplies to other devices
- SD card support
- Four UART interfaces
- Multiple GPIOs with selectable voltage level shifting
- An LCD screen
- Optional Power over Ethernet support
- Thermocouple
- Serial debug
This was a hardware only project since the client has significant in-house embedded software expertise. However, we did make use of generic Linux software, drivers, and tools on the IMX to exercise all hardware both during development and during debugging with the client later.
Development Process
Getting Started
We began with a workshop to agree what was actually required technically and to establish target unit costs and volumes for the product. The form factor was not especially important to the client although power arrangements were. Several end user use cases were discussed to clearly establish the requirements for the product.
We set up a shared slack channel to allow direct communication between our engineering team and the client’s project manager and several engineers. In addition we had weekly Google Meet calls where any issues, timelines, etc. could be agreed. Regular reviews were done with the client’s principal software engineer to agree GPIOs, UART pins, USB interfaces etc.
Design
Significant consideration was made during the development to minimise EMC risks around radiated and conducted emissions given the large number of high speed signals and the client’s preference for no enclosure on the product.
Development proceeded as usual:
1) Component selection
2) Schematic entry
3) PCB layout
Full internal peer reviews were done at each stage to verify the design. As is usual some changes were made to the schematics during layout in order to optimise track routing.
3D printed parts were created in-house to support the PCBs and to provide some mechanical features where required.
Prototype Manufacture
Prototypes were manufactured at AdaptEMS and fully tested.
Prescan EMC testing (at dB Technology) was also conducted at this stage to look for any issues. Results were outstanding with very few areas for concern as a result of the extensive care taken during the design phases.
Challenges
This board was always going to be challenging due to the numerous high speed busses on the board (two Gigabit Ethernet interfaces, 8GHz PCI Express, and 5GHz USB). The absence of a metallic enclosure compounded these challenges since such high speed signals would typically have a 360° shield around them.
The initial EMC testing of the product showed significant and unexpected noise spikes at 4GHz. With the radio expertise of one our hardware engineers this was subsequently tracked down to the SoM’s heatsink. This off-the-shelf heatsink had fins spaced ideally to resonate at 4GHz. Removing the heatsink resolved this issue entirely and we subsequently removed it entirely since it was determined not to be required in this application.
We also found that the thermocouple wildly misread during EMC immunity testing. We modified the recommended design for this thermocouple circuit with the addition of two capacitors which entirely resolved the issue.