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Honey, I Shrunk the FPGA

Lattice UltraLite Takes Power, Package Size to New Lows

February 23, 2015

By Linley Gwennap


Mobile phones contain dozens of tiny resistors, capacitors, and analog components crammed onto small circuit boards inside thin handheld cases. At just 1.4mm square, the Lattice UltraLite FPGA is no bigger than some of these parts, but it packs a passel of programmable logic as well as an array of analog features. At a starting price of less than 40 cents (in high volume), it won’t break the bank, either.

UltraLite is the newest member of the company’s Ice40 family of low-power FPGAs. It fits underneath the Ultra line, containing fewer programmable gates but cutting static power in half—to just 35 microamps—and also reducing the package size, as Figure 1 shows. Using these two products, Lattice now offers FPGAs ranging from 640 to 3,520 logic cells, all with static power of less than 75 microamps.

 

Figure 1. Lattice UltraLite FPGA. The product fits into a BGA package measuring just 2.5mm on a side with 36 balls, as the photo shows. The WLCSP version is even smaller at 1.4mm but has only 16 balls. (Photo courtesy of Lattice)

Lattice is the leading vendor of low-power FPGAs, having shipped more than 250 million Ice units since launching the line in 2012. These products appear in flagship smartphones such as the Galaxy S5 from Samsung, driving this impressive volume. In addition to phones, the chip targets other mobile devices, including tablets, wearables, and portable medical devices.

Blue’s Big Idea

Unlike FPGA leaders Xilinx and Altera, Lattice has focused on smaller FPGAs and PLDs; the latter still provides about two-thirds of the company’s $360 million annual revenue. These low-end devices appear mainly in communications equipment, where they often perform bridging functions using their programmable logic and integrated serdes.

In 2011, Lattice acquired Silicon Blue for $63 million. Based in Silicon Valley, the startup had developed a unique low-power FPGA suitable for mobile devices. Lattice began shipping these FPGAs in 2012 using the Ice brand and quickly won favor with several smartphone makers, including market leader Samsung. In fact, the tiny FPGAs appear in the Galaxy S4, Galaxy S5, and other Samsung flagship products. These little FPGAs generate big volume: unit shipments exceeded 100 million in 2014, and we estimate they generated about $70 million in revenue.

Today, the Ice40 line comprises three families, as Figure 2 shows. The older LP offerings provide up to 7,680 logic cells (LUTs) but have little analog or serial I/O other than a 24mA output. The Ultra products, which began shipping last year, offer up to 3,520 logic cells while keeping static power to just 71 microamps. The new UltraLite series further reduces static power to 35 microamps while restricting the cell count to 1,248. Both Ultra and UltraLite offer several hard-wired functions such as I2C, high-current drive, oscillators, and PWM. Together, these two product lines replace the LP for all but the highest-gate-count designs.

Figure 2. Lattice Ice40 FPGA families. The new UltraLite family replaces the older LP product, offering similar gate counts, much lower power, and additional integrated IP. (Data source: Lattice)

These low-power FPGAs are well suited to a variety of mobile applications, including smartphones and tablets. Wearables such as smartwatches and fitness trackers have even lower power requirements but can use an FPGA to offload always-on functions. Simple battery-operated devices such as remote controls can operate from an FPGA. Low-cost Internet of Things (IoT) devices may need a small FPGA. Lattice also has many customers for these products in industrial, scientific, and medical (ISM) applications. Such ISM devices typically have lower volume but higher margins than consumer products.

An FPGA? In a Phone?

Why put an FPGA in a phone? Vendors such as MediaTek and Qualcomm supply tightly integrated chipsets that provide all the core smartphone functions, eliminating the need for glue logic or extra chips. Makers of low-cost phones avoid adding anything extra to the bill of materials. But high-end phones are different. Companies often design their flagship phones in house, combining the best chips even though they come from different vendors. These models are also at the vanguard of adding new functions and features, which often require additional third-party chips that may not connect directly with the main chipset.

If the application processor runs out of GPIOs, an FPGA can provide I/O expansion, or it can provide voltage translation when connecting a processor built in a leading-edge node with an older or trailing-edge chip. Google Glass, for example, uses an FPGA to connect the processor to the micro-projector, which is not designed for smartphone interfaces. Alternatively, a phone designer may want to add more cameras or displays than the processor supports (the Amazon Fire phone, for example, has six cameras). An FPGA can also save power by collecting data from slow UART and I2C devices, only waking the main processor when necessary.

Using their programmable gates, FPGAs can perform simple functions, offloading the main processor. For example, QuickLogic provides IP that enables its FPGAs to act as a sensor hub, aggregating and preprocessing data from accelerometers and similar devices (see MCR 5/12/14, “QuickLogic Powers Down Sensor Hub”). Lattice FPGAs can also perform these tasks, but in this case, customers must create their own IP. FPGAs can also preprocess image data (e.g., rotate, scale, and overlay) to simplify connection of image sensors and displays. The Ultra models even include some DSP hardware that can offload filtering and other signal processing.

Samsung has not disclosed the function of the FPGA in its Galaxy phones, but an unusual feature of these phones is that they can transmit and receive infrared (IR) signals to control televisions and other audio/video products. Lattice highlights a use case in which its FPGA directly drives an IR LED (using its 25mA high-current sink) while the programmable logic controls the timing of the flashes. This design allows the processor to send a code to the FPGA and then enter sleep mode while the FPGA transmits the code, which may require several dozen milliseconds.

Mobile designers, even of high-end devices, have typically shied away from FPGAs, which are usually large, expensive, and power hungry—three red flags in the mobile world. But Lattice’s technology has overcome these objections. Having packages as small as 1.4mm, prices below 40 cents, and active power consumption well below 1mA, the Ice40 products are well suited to mobile designs.

UltraLite Packs an Analog Punch

The new UltraLite comes in two flavors: one with 640 logic cells and one with 1,248, but they are otherwise identical in their specifications. Like most FPGAs, these gates can instantiate customer-defined logic. Lattice provides tools to convert Verilog or VHDL into the necessary configuration file.

FPGAs store their logic configuration using internal SRAM, but this method typically requires the FPGA to reload the configuration each time it powers on. The UltraLite devices include a one-time-programmable nonvolatile configuration memory (NVCM), so the FPGA can configure itself on power-up. The NVCM eliminates the need for an external configuration memory, and preloading the configuration at the factory is more secure than downloading the configuration from the host processor. If the NVCM is left unprogrammed, the FPGA will automatically load its configuration from an external flash-memory chip through a dedicated serial (SPI) port; this mode is useful for debugging and prototyping.

UltraLite includes 56Kbits (7KB) of embedded block RAM. Designers can store values in these SRAM elements and route them through the logic cells to compute new values. The chip also includes SRAM to hold the logic configuration, but the 7KB excludes that SRAM.

Using memory and logic gates, a designer can synthesize almost any function. To simplify designs, however, UltraLite includes two hard IP cores that provide I2C interfaces, which allow the design to send and receive serial data without taking any of the user-defined logic gates. Whereas Ultra also provides two higher-speed SPI ports for customer use, UltraLite omits these ports, limiting the chip’s performance. UltraLite also lacks 16x16-bit multiply-accumulate units, whereas Ultra provides up to four, as Table 1 shows.

Table 1. Lattice Ultra and UltraLite specifications. The new UltraLite products provide a feature set similar to that of the Ultra versions, but they reduce the gate count, power, and package size. *Accessible by programmable logic. (Source: Lattice)

Both families provide a range of analog IP as well, including one programmable PLL circuit, two oscillators (a 10kHz one for low power and a second that operates up to 48MHz), and three outputs that can drive 24mA. Ultra includes a 500mA output, whereas UltraLite provides a 400mA output and a separate 100mA output that can be combined for 500mA of total drive current. This high current might be needed for a vibration motor or high-intensity white LED (e.g., camera flash).

UltraLite is available in two packages. The smaller WLCSP with only 16 balls measures 1.4mm square, but it limits the number of I/Os available to the designer. (Lattice supports I/O multiplexing to various pins, so the 16-ball package can support any feature, just not all of them.) For designs that require more I/O signals, the company offers a 2.5mm package with 36 balls. This package also uses slightly larger ball spacing (0.4mm instead of 0.35mm), as some PCB manufacturers cannot handle the smaller pitch. In contrast, Ultra has a single package option: 2.1mm for 36 balls (0.35mm pitch).

Cooler Than PolarPro

Because of their focus on the high end, neither Altera nor Xilinx offers FPGAs that compete with the Ice40 products. The closest competitor (literally, since it is located in the same town as the former Silicon Blue office) is QuickLogic, which offers a mobile-FPGA line called PolarPro 3. (The older PolarPro 2 parts are not field programmable and thus don’t compete directly against FPGAs.)

PolarPro 3 competes against Lattice’s UltraLite. It includes 1,019 logic cells, but QuickLogic uses a unique design that allows each cell to act as a single four-input LUT or dual three-input LUTs. In a typical design, many logic cells have one or more unused inputs, so in most cases, these 1,019 cells will support more gates than the 1,248 less flexible cells in the Lattice part. But most designs that fit in one will fit in the other. PolarPro 3 also has slightly more embedded memory, as Table 2 shows. QuickLogic also offers (but has not announced) a newer version called PolarPro 3E that triples the amount of embedded RAM; this version is useful for applications that require large data buffers (e.g., sensor hubs).

Table 2. Comparison of mobile FPGAs. Lattice’s UltraLite offers more analog functions, and its 40nm process enables lower power and die size. *Can be configured as up to 2,038 three-input LUTs; †accessible by programmable logic. (Source: vendors)

One big difference between the competitors is Lattice’s analog integration. As noted above, UltraLite provides a PLL, oscillators, PWM outputs, and high-current outputs; PolarPro 3 offers none of these features, except for a low-speed oscillator limited to 256kHz. In many applications, PolarPro requires an external analog chip, increasing cost and board area. Lattice also has a 30% static-power advantage, which comes largely from its use of 40nm technology; PolarPro 3 uses an older 65nm process. With its larger memory arrays, Polar Pro 3E is even more power hungry. Active power consumption depends greatly on the number of LUTs used, but Lattice’s 40nm advantage should also help on this metric.

Finally, UltraLite fits into a package that requires 63% less area that PolarPro 3’s smallest package. The 40nm technology again helps, reducing the die area and enabling a smaller WLCSP. The small package, however, comes at the cost of fewer I/Os: only 10 compared with 18 for PolarPro 3. UltraLite’s 2.1mm package option, however, provides up to 26 I/Os and is still 18% smaller than the PolarPro 3 package.

How Low Can You Go?

Lattice has built a business around niche markets too small for the top two FPGA vendors to pursue. The Silicon Blue acquisition thrust the company into the mobile market, boosting Lattice’s revenue even though its mainstream products are in decline. These tiny FPGAs have found their way into many mobile designs; like their larger brethren, they perform a variety of functions using their programmable logic.

The success of Ice40, however, comes largely from a few design wins at Samsung, which alone provides more than 20% of Lattice’s total revenue. A pessimist would say that these wins could quickly disappear should Samsung and/or Qualcomm decide to integrate this function into their application processors. An optimist would counter that less than 15% of all smartphones contain an FPGA, leaving plenty of room for growth.

Even at prices as low as 40 cents, we don’t expect FPGAs to appear in mainstream smartphones, which rely on standard chipsets and focus on minimizing system cost. That leaves the high-end smartphone market, which comprises about 350 million units, 80% of which come from Apple or Samsung. Lattice has already captured one of these giants; a design win at Apple would obviously be huge, but other smartphone vendors have modest high-end volumes. Premium tablets add another 150 million units to UltraLite’s potential market, with Apple and Samsung again taking 80% of this share. Wearables and IoT devices provide initial opportunities in the millions of units; once their feature set settles down, standard chips will probably push out FPGAs.

Off-the-shelf chipsets, however, are inadequate for leading-edge devices, which require differentiation and top-of-the-line capabilities. Low-power FPGAs are ideal for tying together chips that were not designed to work together. They can also quickly implement new functions while using much less power than implementing the same functions in software on the main CPU. Products such as Lattice’s help designers solve these problems, and high-end systems have room for a small, inexpensive chip such as UltraLite.

Lattice has also integrated analog functions into its FPGAs, so in some cases, the designer can replace an analog chip and get the programmable logic at little or no incremental board area and cost. To gain similar benefits, QuickLogic has instead integrated the FPGA into the sensor hub. In any case, squeezing an FPGA into your next mobile design is now easier than ever. In the massive mobile-phone market, a tiny bit of differentiation can go a long way.

Price and Availability

Lattice’s Ice40 UltraLite FPGA is available in two models, the UL640 and UL1K, as Table 1 shows. Both are currently sampling and will be production certified in March. High-volume pricing for the UL640 is less than $0.40. For more information, access Lattice’s web site at www.latticesemi.com/Products/FPGAandCPLD/iCE40Ultra.aspx.

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