The mid-density FPGAs support Gigabit Ethernet links by making interfaces available on GPIO pins

BY
TED MARENA, Director of FPGA/SoC Marketing
Microsemi
Corp.
www.microsemi.com

Communications,
industrial, and even embedded designs increasingly require higher-performance
interfaces. Protocols such as DDR4 memory, 10 Gigabit Ethernet, PCIe, serial
ADC and DAC interfaces, and others are becoming commonplace. While these higher-speed
interfaces are usually found on high-end FPGAs, this is often an overkill
approach and cost-prohibitive for most applications. And using low-end FPGAs is
not an option because they do not provide the necessary performance.

Now
the cost of deploying these interfaces can be dramatically reduced using a new
class of energy-efficient, mid-range-density FPGAs that are cost-optimized,
consume lower power, and offer smaller form factors than more expensive FPGAs
while still delivering generous high-speed interface options.

Addressing new market dynamics
Although
individual market segments are unique, they also share several characteristics.
The requirements for better value and lower cost are growing drivers for all
applications and market segments. In addition, faster and numerous networking
interfaces are now more common given the emergence of the Internet of Things
(IoT). Finally, high-speed processing in most embedded designs is a new norm.

These
factors require architectures that incorporate interfaces such as Gigabit
Ethernet, along with transceivers fast enough for 10 GbE, JESD204B ADC/DAC,
PCIe interfaces, HDMI 2.0b and memory buses such as DDR4, to name a few.

Now
that these types of interfaces are available in cost-optimized, low-power
mid-range FPGAs, design engineers have important new options for addressing the
latest market dynamics for their products. Among the most valuable of these
options is the ability to use Serial Gigabit Media Independent Interface
(SGMII) on the FPGA’s generic general-purpose input-output (GPIO) pins rather
than having to use high-speed transceivers.

Leveraging SGMII on GPIOs
The
most common interface being leveraged in many communications and industrial
designs is Gigabit Ethernet (GbE). Most frequently, an FPGA connects to a PHY
via a serial SGMII interface. In the past, using FPGAs to implement an SGMII
interface also required using high-speed transceivers. This all changes with
the availability of new cost-optimized mid-range FPGAs, which make SGMII
interfaces available on generic GPIO pins for the lowest-power implementation
possible for GbE links to an FPGA.

Many
embedded product developers use GbE for an increasing number of connections. No
longer only for data payloads, these links are becoming ubiquitous for control,
management, status, and more. While both low-end and traditional mid-range
FPGAs can support these 1-Gbps speeds, they require transceivers to implement
the 1.25-Gbps SGMII interfaces as well as other high-speed interfaces. Ideally,
a device would have generic I/O pins assigned specifically to support SGMII (Fig. 1).

0318_Feature_FPGA_Fig-1

Fig. 1: Traditional and mid-range FPGAs
can support 1-Gbps speeds, but they require transceivers to implement the 1.25-Gbps
SGMII interfaces.

Low-end
FPGAs and traditional mid-range FPGAs do not have this feature, so they must
rely on transceivers. Implementing interfaces in this way can be challenging
because these transceiver interfaces are precious and frequently scarce. To
have enough of them to meet higher-performance interface requirements,
designers previously had to select a more expensive, higher-density FPGA
fabric.

These
large FPGA fabrics are often not required in industrial designs, but designers
were forced to choose them anyway just to have enough additional transceivers.
In addition, these larger devices mean that larger package form factors are
required. The result is higher power consumption and costs in industrial
markets that need just the opposite.

Today’s
lower-power, cost-optimized mid-range density FPGAs buck these trends, offering
a new way to meet the requirement for numerous Gigabit Ethernet links via SGMII
on GPIOs. Some solutions go a step further to incorporate a clock and data
recovery (CDR) circuit into high-speed LVDS I/Os that can support 1.25 Gbps.
These features allow the device to support SGMII interfaces on numerous GPIO
pins. Using this architecture, designers can reduce the cost, size, and power
of their designs as compared to those created using traditional mid-range or
high-end FPGAs.

Balancing speed, cost, and power
Not
every design with communication interfaces requires extremely high performance,
but across the markets, processing needs are increasing while interfaces are
getting faster. These factors require FPGAs to support serial interfaces of up
to 12.5 Gbps to implement the following increasingly common options:

  • PCIe Gen2
    requiring 5 Gbps
  • HDMI 2.0b, which
    needs 6 Gbps
  • 10 GbE requiring
    10 Gbps
  • JESD204B, which
    can run up to 12.5 Gbps

These
high-speed serial interfaces require transceivers to operate up to the speeds
listed above, but power consumption should also be optimized. The performance
for these rates is trivial for high-end FPGAs or mid-range FPGAs built off of high-end
architectures. Unfortunately, these devices also consume very high power,
require large form factors, and are often beyond the budget for many designs.

On
the other hand, low-density FPGAs often do not have transceivers, and those
that incorporate them do not support the performance rates listed. The answer
to balancing transceiver speed, low cost, and power efficiency is to use
mid-range density FPGAs with the right mix of logic elements (LEs) and
transceivers to support the required data rates.

With
these kinds of options, industrial architects can support the latest high-speed
serial interfaces while also having adequate LEs on-chip to implement necessary
board functions. In addition, the ability to implement SGMII on GPIOs as well
as transceivers often enables designers to select smaller package sizes and
densities. This lowers system costs and reduces the power needed for FPGA
functionality.

One
example where serial interfaces are required is for a wireless microcell. These
designs typically use a processor to handle most of the signal processing while
leveraging an FPGA for custom signal conditioning and to connect to the ADC and
DAC, as shown in Fig. 2.

0318_Feature_FPGA_Fig-2

Fig. 2: Example of a serial interface
design.

In
this example, the FPGA uses transceivers to implement either CPRI or a
proprietary serial digital link (SDL) to the Cavium processor and a serial link
to Analog Devices’ ADC/DAC via JESD204B. In addition to performing the serial
bridging functions, the FPGA’s fabric can implement signal conditioning
algorithms such as crest factor reduction (CFR) and other functions. This
implementation provides a lower-power solution and a smaller footprint for
small cell and microcell applications.

A new way to implement DDR4 interfaces
The
most common memory that engineers tend to connect to an FPGA is DDR DRAM-based
devices. There are several generations to choose from, and the best choice is
generally to use memory that has been shipping for some time and is not the
absolute newest standard.

For
DRAMs, DDR4 offers the best cost per bit, and its architecture will be
supported for numerous years. Although DDR3 is still a viable choice for
designs, DDR4 is being selected for the majority of new designs because it will
offer reduced pricing in the future, faster performance, and wider single-chip
data buses.

There
were no low-density FPGAs that supported DDR4 memory interfaces, so designers
had to use mid-range density FPGAs that were built off of high-end
architectures to implement these interfaces. In contrast, today’s latest
mid-range FPGAs are purpose-built for these interfaces, offering the required
DDR4 performance at significantly lower cost and power while enabling the much
smaller form factors required for embedded designs (Fig. 3).
Package sizes for these devices range from 16 x 16 mm down to 11 x 11 mm.

0318_Feature_FPGA_Fig-3

Fig. 3: FPGAs in multiple densities and
form factors.

With growing demand for higher-performance
interfaces, more connectivity, and lower costs for communications, industrial,
and embedded designs, system architects and engineers need to look for new
solutions. Today’s lower-power, mid-range density FPGAs solve these design
challenges, combining greater value with lower power consumption while still
providing the interface options and other capabilities demanded by modern
designs.