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Application-Driven Design: ASIC, SoC and MPU

Integration Blurs the Line between MCUs and SoCs

Time and money are major considerations when approaching a design. With today’s high scales of integration, the available devices offer a wide array of alternatives, all of which involve different combinations of time, money and other resources. Selecting the right mix can be vital to success.

BY JASON TOLLEFSON, MICROCHIP TECHNOLOGY

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The System on a Chip (SoC) represents the pinnacle of tailored designs. Expressly selected peripherals, especially analog, give the promise of a perfect fit with no waste, delivering very low component cost. Wikipedia defines the SoC as “an integrated circuit that integrates all components of a computer or other electronic system into a single chip.” With the level of integration that is commonplace today, many ICs can qualify as an SoC, especially the Microcontroller.

The investment required to develop a custom or semi-custom SoC is substantial in both time and cost. There are non-recurring engineering (NRE) costs, negotiating the design specification, design time, fabrication time, and finally, developing the application. But then there is that low component cost as the reward.

Consider now the MCU, a standard product, widely available, without NRE and with many, if not all, of the required peripherals, such as analog and communications, but it does not match the cost of the custom SoC. Should you choose future perfection (i.e., a custom SoC) or an MCU that’s available today? This is the decision designers must make when considering a custom SoC or standard MCU for their next high-volume design.

Criteria for Comparison

Let’s look deeper into this question and compare our choices amongst several key criteria. First, let’s define the boundaries of the discussion. We will consider four types of products: a standard MCU, a full custom SoC (or ASIC), a semi-custom SoC, and the FPGA with integrated CPU. 

The semi-custom SoC is different from a full custom SoC in that it is already available and was designed with an application in mind. These products can be found from vendors such as Broadcom. Toshiba and Infineon offer full custom ASIC solutions. An FPGA is well known in system design, but recently companies such as Xilinx are offering hybrid devices with an embedded CPU complemented by programmable logic. Meanwhile, the MCU has grown in complexity. Companies such as Microchip Technology are integrating advanced analog peripherals, lots of memory, and hoards of communication and timing peripherals, making the once sharp lines between MCU and SoCs blurry. 

Now let’s bring the differences back into focus by establishing some criteria for comparison. For the assessment to be valid and complete, we need to consider the total cost of ownership, not just the unit cost. This includes the three broad areas of product features, design enablement and time-to-market (Table 1).

Table 1
SoC considerations.

Product Features

When it comes to obtaining the peripherals that are an exact fit for your application, it’s hard to beat the custom SoC. You work with the vendor and include just the right peripherals to optimize your design. There is little waste and fewer compromises. If you want a 10 Msample/s Pipelined ADC, you simply specify it. 

The FPGA is similar in that you can program the logic to be what you want, but may be forced to make sacrifices with analog. For example, I can have a 1 Msample/s SAR ADC, but not a 10 Msample/s Pipelined ADC.

The semi-custom SoC offers a variety of peripherals, but they are designed with application segments in mind, such as communication processors, and may be mismatched to your application. So it has more constraints, along with some peripherals that you will not use.

There are literally thousands of different MCU configurations, each “dialed in” for an application space. It’s hard to find an application that cannot be served by the MCU. But, vendors scale cost with integration. So, getting that 10 Msample/s Pipeline ADC might also mean you get an LCD controller and USB, whether you need them or not. Advantage: Custom SoC.

Sometimes core performance matters, sometimes it does not, depending on the application. Rarely would you need a core running at 200 MHz for a home thermostat, for example. And you would not want it if the thermostat were battery powered.

With FPGAs and semi-custom SoCs, you will typically get the Ferrari. They tend to integrate a CPU so screaming fast and power hungry that it ensures high performance in almost any application. This might be overkill for your application, but it will definitely work.

The MCU, much like the custom SoC, can be scaled to fit. There are lots of choices within 8-/16-/32-bit MCUs. You can easily find one that will fit your processing load and power budget. Many vendors have put special emphasis on CPU efficiency and current consumption, which is a great combination for battery-powered applications.  But if you need a Ferrari, you can find that too. Advantage: MCU & Custom SoC.

Cost is typically the reason that people consider an SoC. The perception is that the cost of the SoC is lowest, and that is often the case. But we must be sure that the total cost of ownership is fully understood and considered before committing to the custom SoC.

The fully custom SoC is intentionally a perfect fit for the application, with little to no extraneous features. This generally leads to the lowest unit cost. But there are other considerations. There will be the design and test charges (NRE) that need to be added total cost. Once the chip is out of the fab, if issues are found, it will need to be fixed. This is an additional NRE cost. A trip back to the fab for a mask revision is an additional cost and can wipe out the unit cost savings in a hurry. A re-spin also takes time. A fab cycle can be as long as 90 days, leaving you without product to develop your application; an opportunity cost. Another consideration is development tools. Tools for developing application code and testing hardware will need to be custom designed, developed and purchased. These costs can vary widely. However, if your volumes are significant and your product lifetime long, the custom SoC unit savings may just overcome these additional costs.

The FPGA has a high unit cost in the tens of dollars, due in part to the advanced process geometries that enable its flexibility. But other costs include support chips, such as boot memory and numerous voltage regulators. Development tools for FPGAs start around $1,000, depending on how many tool seats are needed. These costs might be absorbed if the application has a high price. But, typically, there are better choices if system cost is a primary concern.

The MCU fitting the application might have a higher unit cost, but can still represent a lower total cost of ownership. For one thing, there are no startup costs (NRE). You simply order your chip online, and get it a few days later. The MCU has its own flash memory and regulator built in, so no supporting chips are necessary. Finally, most MCU vendors offer free software tools and low-cost hardware starting at $20. So, in essence, the total cost of ownership is the product cost (Figure 1). Advantage: MCU.

Figure 1
Total Cost of Ownership (where it comes from).

With a custom SoC, all of the flexibility is at the beginning of the design. You can select peripherals, core and I/O to match your exact application needs. But, after the SoC design becomes a chip, flexibility is lost. The same is true for a semi-custom SoC, where you can select the one that fits your application, but you cannot change the features after that—you are locked in.

Contrast that with the flexibility of the MCU and FPGA. Both offer scalability in memory, peripherals and I/O. However, they accomplish this differently—the FPGA through programmability, and the MCU through proliferation of product families—but the end result is the same. Changes can be made throughout the design cycle, even after the product is launched.  Advantage: MCU & FPGA.

Design Enablement

Where do I go for information? Who do I talk to when I’m stuck or have a problem? How do I integrate the chip into my application? These are three critical questions that you will encounter after you have selected your chip, whether it is an SoC, FPGA or MCU. How these questions are answered by your vendor is crucial to the success of your application.

For the fully custom SoC, you have a face-to-face relationship with the vendor. All information flows through your contact at the company. Sounds great, but what if you live across the globe, with 12 time zones separating you and your contact? Because the SoC is custom, you must seek out your vendor who is the expert to get information. The situation is similar for the semi-custom SoC, in that information comes from the vendor and is not widely available. A relationship is required to get information.

Contrast that with the MCU and the FPGA. Look on the vendor website for information and you will find a plethora of free-flowing information about your product. Videos, code examples, data sheets, errata documents, peripheral user manuals, package information and reference designs, all available 24 hours a day, 7 days a week. A relationship with a person is not required to gain access to information. But if you want to establish relationships with people in the know, there are community user forums, distribution partners, and even 24/7 online engineering support available. Advantage: MCU & FPGA.

Time-to-Market

The famous American entrepreneur and statesman Benjamin Franklin once said, “Time is money.” This quote can be interpreted two ways when marketing your product. Franklin’s meaning was to not waste time. In other words, the faster you are to market, the more money your product can make. Another meaning could be to take the time to get your product right and you will make more money. These two approaches are illustrative of designing with an MCU/FPGA vs. the SoC. 

The MCU and FPGA are your chips of choice if you want to get to market fast (Figure 1). With myriad design resources and information, combined with overnight availability of product from sources such as Digi-Key, there is little in the way of getting the product to market. The trade-off, of course, is unit cost. This can be higher, as noted above in system cost. But, if you have calculated the total cost of ownership, considered the risk of re-spin and know that unit cost is your long-term issue, taking the time to design a custom SoC might be your best choice. Advantage: MCU & FPGA.

Figure 1
Total Cost of Ownership (where it comes from).

We’ve learned that time can be as important as features and design resources when reviewing the SoC options. In the end, we as engineers have to study the trade-offs and make decisions. But good decisions will include considerations beyond unit cost, and will consider the total cost of ownership for the application. By looking at the advantages that the MCU, FPGA and SoC have relative to each other in their entirety, we will make a great choice. Table 2 shows a parting summary of the considerations we’ve made. Good luck with your design!

Table 2
Advantages by Scoring Criteria.

Microchip Technology
Chandler, AZ.
(480) 792-7200
www.microchip.com