Increasing design complexity, coupled with compressed time to market, has been an engineering reality for some time. A system-on-module (SOM) addresses both of these real engineering issues. An engineer who chooses to use a SOM trades a very complex, chip-down circuit design for a much simpler SOM connector interface.

Avnet’s MicroZed is a Xilinx Zynq-based embeddable SOM.
Modular design is a commonly accepted practice in electronics. A commonly used circuit is designed onto its own PCB, which can then be used as a pre-packaged, convenient module. Anyone who has built up a personal computer has experienced the convenience and efficiency of modular design. Consider the motherboard, the memory DIMMs, and the ATX power supply. All of these circuits could possibly be designed into one “super” PCB that combined the processor, core chipset, memory, and AC/DC power conversion. In equivalent high volumes, the all-on-one-PCB personal computer would likely be less expensive than the modular PC. However, the modular design offers superior flexibility, rapid system design, and ultimate re-use. Experts in each electronic specialty were called upon to design each subsystem, creating a superior product.

Similar to the PC, full custom electronic design could use a little pre-packaged convenience. In electronic design, a system-on-module (SOM) is a predesigned and assembled product that typically makes a lot of sense for many designers. A SOM provides a consistent and reliable subset of the common circuits surrounding a processor, field programmable gate array (FPGA), or system-on-chip (SoC). Designing with a SOM may come at a slight premium, but for many designers, the savings in engineering time and quicker time to market makes it a smart design method in many situations.

The Xilinx Zynq-7000 All Programmable System-on-Chip is a complex and powerful device now available in SOM format. A typical SOM incorporates all of those elements that are most common in a majority of designs using the primary device of interest. This is no different, whether the SOM uses a SoC like Zynq or a processor. More than likely, all the benefits hold true for other processor SOMs as well. For example, most Zynq designs, regardless of the application, will include DDR3 SDRAM, Ethernet, USB, UART, and power regulation. A pre-designed Zynq SOM that bundles working versions of these circuits saves an engineer time and money in multiple ways.

Saving Time and Money

Avnet’s MicroZed SOM, connected to a prototype carrier card.
The first way in which a SOM saves time is in the PCB design. The alternative to using a SOM is to design the circuits onto your own PCB, referred to as “chip-down.” A chip-down design using Zynq must account for a large, fine-pitch BGA package. Similarly, all of the other associated components must be selected. Once selected, an engineer creates the schematic symbols, connects the circuits, and lays out the board.

When an engineer chooses to use a SOM, they bypass the time and effort to design the core pieces, adding a SOM interface instead. In addition to the PCB design effort, a SOM saves a significant amount of time in the testing and development areas. This is the second area of focus that showcases a SOM’s potential advantage.

With a SOM, all interfaces are fully validated. New PCB validation typically requires at least one or two iterations, all of which take debug time and additional money for board revisions and prototype builds. Debugging a memory interface layout issue or an Ethernet signal integrity problem is extremely tedious and time-consuming, lasting weeks or months. An additional board revision and prototype build can cost anywhere from $10,000 to $50,000, depending on the board complexity. A SOM’s circuits are pre-verified, eliminating additional debug time and additional board revisions.

A SOM will likely come with various certifications — ROHS, CISPR (EN55022:2010 & 55024:2010), and FCC Class A for EMI radiated and conducted emissions. Your complete product with an integrated SOM will still need to complete the product-level certifications as well, but it is nice to know that a key piece with high-speed signaling meets the requirements.

SOM providers will typically provide a multitude of design help and guidelines. Detailed documentation is just the start. Example or reference designs may be provided. Detailed tutorials and online training are invaluable starting points when coming up to speed on the technology. Full board support packages (BSP) may be provided as well.

The third SOM advantage area is purely financial. With a SOM, an engineer designs a simpler, custom host or carrier board rather than designing everything onto a single, complex board. This makes for a less expensive carrier board. Additionally, if analog isolation is critical in product functionality, having all the high-peed digital signals on a separate board could also be a huge advantage.

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