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.
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
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.
As an example of what might be achievable, consider the MicroZed™ module and a few of the associated prototype carriers. The MicroZed SOM is a Zynq-based SOM. It has 10 layers, which include two extra ground layers, ensuring an ideal reference for better signal integrity. In contrast, the three prototype carriers have six, four, and two layers for the FMC, I/O expansion, and breakout carriers. Rather than requiring a combined PCB where everything must have 10 or more layers, the complex, multi-layer circuitry is on the SOM, allowing the carriers to have potentially fewer (and analog isolated) layers.
Getting to Market
The last point to consider as a SOM advantage may be the most important of all: the ability to get to market faster with a higher-quality product. Two common-sense advantages are obvious. First, a large chunk of circuitry does not have to be designed. This circuitry alone represents a design, debug, and manufacturing investment from the SOM developer that likely translates into many man-months of engineering effort — possibly a year or more. This investment likewise speeds a new engineering project that does not have to go through the same design effort for the same circuits.
Second, since engineers are not focused on the central processing unit and core circuitry, they have more time to invest in the technology that will differentiate their product. A company’s core competency becomes the true design focus, rather than figuring out all the complex and very important details around the processing unit. This allows engineers to focus their skills rather than attempting to become a power, communications, and memory interface expert at the same time.
To add up all these savings is a challenge, but a conservative estimate is likely 12 extra man-months of engineering time, and at least one PCB revision. Each company must evaluate what this is worth. Using an industry estimate of $125 per hour engineering figure1 and $25,000 PCB revision puts the value of using the SOM at about $285,000. Adjust this figure based on your own locale, and then use it to determine the cross-over point at which you might want to consider a chipdown design.
For example, if a module costs $175 in volume, but an engineer could build a chipdown equivalent for $125, the SOM premium is $50. Taking the savings amount ($285,000) and dividing by the SOM premium ($50), engineers will find the crossover point is 5,700 modules. In this analysis, if a project required fewer than 5,700 units, then using a SOM provides a real cost advantage in addition to all the other advantages, including time to market. At more than 5,700 units, it may still be worth considering a SOM for the other advantages, but the chip-down design’s raw cost is less. A complete analysis would need to include the additional sales revenue potential that the time-to-market advantage provides.
While a full custom electronic design will never be as modular as a PC, integrating a SOM offers significant advantages over designing a complex circuit that is possibly outside your specialty. The next time you upgrade your PC memory or plug in nearly any piece of electronics, take a close look at the DIMM or the AC/DC brick power supply and consider the convenience achieved by someone else expending the effort to provide a complex, tested, and certified module. By integrating a SOM into your next design, you will find similar pre-packaged convenience and enjoy the great advantages of modular design.
This article was written by Bryan Fletcher, Technical Marketing Director at Avnet Electronics Marketing, Phoenix, AZ. For more information, Click Here .
1 IEEE-USA Consultants Fee Survey Report, IEEE-USA E-Books, 2011