FPGAs

What are FPGAs?

Field programmable gate arrays（FPGAs）arc digital integrated circuits（ICs）that contain  configurable（programmable）blocks of logic along with configurable interconnects between these blocks. Design engineers can configure（program）such devices to perform a tremendous variety of tasks.

Depending on the way in which they are implemented，some FPGAs may only be programmed a single time, while others may be reprogrammed over and over again. Not surprisingly, a device that can be programmed only one time is referred to as one-time programmable (OTP)．

The “field programmable" portion of the FPGA’s name refers to the fact that its programming takes place“in the field”（as opposed to devices whose internal functionality is hardwired by the manufacturer）．This may mean that FPGAs are configured in the laboratory, or it may refer to modifying the function of a device resident in an electronic system that has already been deployed in the outside world. If a device is capable of being programmed while remaining resident in a higher-level system, it is referred to as being in-system programmable (ISP)．

Why are FPGAs of interest?

There are many different types of digital ICs, including "jelly-bean logic”（small components containing a few simple，fixed logical functions）, memory devices，and microprocessors (μPs)． Of particular interest to us here，however，are programmable logic devices（PLDs），application-specific integrated circuits（ASICs）, application- specific standard parts（ASSPs)，and-of course-FPGAs.

Although ASICs offer the ultimate in size（number of transistors）, complexity，and performance；designing and building one is an extremely time-consuming and expensive process，with the added disadvantage that the final design is“frozen in silicon”and cannot be modified without creating a new version of the device.[1]

Thus, FPGAs occupy a middle ground between PLDs and ASICs because their functionality can be customized in the field like PLDs，but they can contain millions of logic gates and be used to implement extremely large and complex functions that previously could be realized only using ASICs.

The cost of an FPGA design is much lower than that of an ASIC（although the ensuing AIC components are much cheaper in large production runs）．At the same time，implementing design changes is much easier in FPGAs，and the time-to-market for such designs is much faster. Thus，FPGAs make a lot of small，innovative design companies viable because-in addition to their use by large system design houses-FPGAs facilitate “Fred-in-the-shed"-type operations. This means they allow individual engineers or small groups of engineers to realize their hardware and software concepts on an FPGA-based test platform without having to incur the enormous nonrecurring engineering（NRE）costs or purchase the expensive toolsets associated with ASIC designs.[2] Hence，there were estimated to be only 1,500 to 4,000 ASIC design starts and 5,000 ASSP design starts in 2003（these numbers are falling dramatically year by year），as opposed to an educated“guesstimate”of around 450,000 FPGA design starts in the same year.

What can FPGAs be used for?

FPGAs are often used to prototype ASIC designs or to provide a hardware platform on which to verify the physical implementation of new algorithms. However，their low development cost and short time-to-market mean that they are increasingly finding their way into final products（some of the major FPGA vendors actually have devices that they specifically market as competing directly against ASICs）．

By the early-2000s，high-performance FPGAs containing millions of gates had become available. Some of these devices feature embedded microprocessor cores，high-speed input/output (I/O) interfaces, and the like. The end result is that today’s FPGAs can be used to implement just about anything，including communications devices and software-defined radios（SDR）；radar，image, and other digital signal processing (DSP) applications；all the way up to system-on-chip（SoC）components that contain both hardware and software elements.

To be just a tad more specific，FPGAs are currently eating into four major market segments：ASIC and custom silicon，DSP，embedded microcontroller applications，and physical layer communication chips. Furthermore，FPGAs have created a new market in their own right：reconfigurable computing（RC）.

l ASIC and custom silicon：As was discussed in the previous section．Today’s         FPGAs are increasingly being used to implement a variety of designs that could         previously have been realized using only ASICs and custom silicon.

l Digital signal processing：High-speed DSP has traditionally been implemented        using specially tailored microprocessors called digital signal processors（DSPs）．However，today’s FPGAs  can  contain  embedded  multipliers，dedicated arithmetic routing, and large amounts of on-chip RAM, all of which facilitate DSP operations. When these features arc coupled with the massive parallelism provided by FPGA，the result is to outperform the fastest DSP chips by a factor of 500 or more.

l Embedded microcontrollers：Small control functions have traditionally been        handled by special-purpose embedded processors called microcontrollers. These low-cost devices contain on-chip program and instruction memories, timers, and I/O peripherals wrapped around a processor core.  FPGA prices are falling, however, and even the smallest devices now have more than enough capability to implement a soft processor core combined with a selection of custom I/O functions. The end result is that FPGAs are becoming increasingly attractive for embedded control applications

l Physical layer communications：FPGAs have long been used to implement the glue logic that interfaces between physical layer communication chips and high-level networking protocol layers. The fact that today’s high-end FPGAs can contain multiple high-speed transceivers means that communications and networking functions can be consolidated into a single device.

l Reconfigurable computing：This refers to exploiting the inherent parallelism and reconfigurability provided by FPGAs to “hardware accelerate” software algorithms. Various companies are currently building huge FPGA-based reconfigurable computing engines for tasks ranging from hardware simulation to cryptography analysis to discovering new drugs.

WORDS AND PHRASES

configurable 可配置的；结构的

interconnect            连接线，内连线，连接器产品

prototype 原型，标准，模范

peripheral  外设

Field programmable gate arrays（FPGAs）   现场可编程门阵列

one-time programmable（OTP）  一次性可编程

in-system programmable         在系统可编程

programmable logic devices（PLDs）可编程逻辑器件

application-specific integrated circuits（ASICs）专用集成电路

software-defined radios  软件无线电

system-on-chip（SoC）   系统单晶片

reconfigurable computing（RC）  可重构计算

NOTES

[1] Designing and building one is an extremely time-consuming and expensive process，with the added disadvantage that the final design is “frozen in silicon” and cannot be modified without creating a new version of the device.

设计和建造一个ASIC是一个非常费时和昂贵的过程，同时还有最终设计要“冻结在硅”、以及如果不创造一个新的版本的装置将不能进行修正的弊端。

[2] This means they allow individual engineers or small groups of engineers to realize their hardware and software concepts on an FPGA-based test platform without having to incur the enormous nonrecurring engineering（NRE）costs or purchase the expensive toolsets associated with ASIC designs.

这意味着允许个人工程师和工程师小组可以在基于可编程逻辑门阵列测试平台上实现他们的硬件和软件设计概念，而不必要承担巨大的一次性工程成本或者购买昂贵的ASIC设计相关的工具。