Field Programmable Gate Arrays & Applications - 1 | Embedded Systems (Web) - Computer Science Engineering (CSE) PDF Download

Field Pro grammable Gate Arrays and Applications

Instructional Objectives 

After going through this lesson the student will be able to

• Define what is a field programmable gate array (FPGA)
• Distinguish between an FPGA and a stored-memory processor
• List and explain the principle of operation of the various functional units within an FPGA
• Compare the architecture and performance specifications of various commercially available FPGA
• Describe the steps in using an FPGA in an embedded system

Introduction 
An FPGA is a device that contains a matrix of reconfigurable gate array logic circuitry. When a FPGA is configured, the internal circuitry is connected in a way that creates a hardware implementation of the software application. Unlike processors, FPGAs use dedicated hardware for processing logic and do not have an operating system. FPGAs are truly parallel in nature so different processing operations do not have to compete for the same resources. As a result, the performance of one part of the application is not affected when additional processing is added. Also, multiple control loops can run on a single FPGA device at different rates. FPGA-based control systems can enforce critical interlock logic and can be designed to prevent I/O forcing by an operator. However, unlike hard-wired printed circuit board (PCB) designs which have fixed hardware resources, FPGA-based systems can literally rewire their internal circuitry to allow reconfiguration after the control system is deployed to the field. FPGA devices deliver the performance and reliability of dedicated hardware circuitry. A single FPGA can replace thousands of discrete components by incorporating millions of logic gates in a single integrated circuit (IC) chip. The internal resources of an FPGA chip consist of a matrix of configurable logic blocks (CLBs) surrounded by a periphery of I/O blocks shown in Fig. 20.1. Signals are routed within the FPGA matrix by programmable interconnect switches and wire routes.

In an FPGA logic blocks are implemented using multiple level low fan-in gates, which gives it a more compact design compared to an implementation with two-level AND-OR logic. FPGA provides its user a way to configure:
1. The intersection between the logic blocks and
2. The function of each logic block.

Logic block of an FPGA can be configured in such a way that it can provide functionality as simple as that of transistor or as complex as that of a microprocessor. It can used to implement different combinations of combinational and sequential logic functions. Logic blocks of an FPGA can be implemented by any of the following
1. Transistor pairs
2. combinational gates like basic NAND gates or XOR gates
3. n-input Lookup tables
4. Multiplexers
5. Wide fan-in And-OR structure.

Routing in FPGAs consists of wire segments of varying lengths which can be interconnected via electrically programmable switches. Density of logic block used in an FPGA depends on length and number of wire segments used for routing. Number of segments used for interconnection typically is a tradeoff between density of logic blocks used and amount of area used up for routing. Simplified version of FPGA internal architecture with routing is shown in Fig. 20.2.

Why do we need FPGAs? By the early 1980’s large scale integrated circuits (LSI) formed the back bone of most of the logic circuits in major systems. Microprocessors, bus/IO controllers, system timers etc were implemented using integrated circuit fabrication technology. Random “glue logic” or interconnects were still required to help connect the large integrated circuits in order to:

1. Generate global control signals (for resets etc.)
2. Data signals from one subsystem to another sub system.

Systems typically consisted of few large scale integrated components and large number of SSI (small scale integrated circuit) and MSI (medium scale integrated circuit) components.Intial attempt to solve this problem led to development of Custom ICs which were to replace the large amount of interconnect. This reduced system complexity and manufacturing cost, and improved performance. However, custom ICs have their own disadvantages. They are relatively very expensive to develop, and delay introduced for product to market (time to market) because of increased design time. There are two kinds of costs involved in development of custom ICs

1. Cost of development and design
2. Cost of manufacture

(A tradeoff usually exists between the two costs) Therefore the custom IC approach was only viable for products with very high volume, and which were not time to market sensitive.FPGAs were introduced as an alternative to custom ICs for implementing entire system on one chip and to provide flexibility of reporogramability to the user. Introduction of FPGAs resulted in improvement of density relative to discrete SSI/MSI components (within around 10x of custom ICs). Another advantage of FPGAs over Custom ICs is that with the help of computer aided design (CAD) tools circuits could be implemented in a short amount of time (no physical layout process, no mask making, no IC manufacturing)

Evaluation of FPGA 
In the world of digital electronic systems, there are three basic kinds of devices: memory, microprocessors, and logic. Memory devices store random information such as the contents of a spreadsheet or database. Microprocessors execute software instructions to perform a wide variety of tasks such as running a word processing program or video game. Logic devices provide specific functions, including device-to-device interfacing, data communication, signal processing, data display, timing and control operations, and almost every other function a system must perform The first type of user-programmable chip that could implement logic circuits was the Programmable Read-Only Memory (PROM), in which address lines can be used as logic circuit inputs and data lines as outputs. Logic functions, however, rarely require more than a few product terms, and a PROM contains a full decoder for its address inputs. PROMS are thus an inefficient architecture for realizing logic circuits, and so are rarely used in practice for that purpose. The device that came as a replacement for the PROM’s are programmable logic devices or in short PLA. Logically, a PLA is a circuit that allows implementing Boolean functions in sum-of-product form. The typical implementation consists of input buffers for all inputs, the programmable AND-matrix followed by the programmable OR-matrix, and output buffers. The input buffers provide both the original and the inverted values of each PLA input. The input lines run horizontally into the AND matrix, while the so-called product-term lines run vertically. Therefore, the size of the AND matrix is twice the number of inputs times the number of product-terms.

When PLAs were introduced in the early 1970s, by Philips, their main drawbacks were that they were expensive to manufacture and offered somewhat poor speed-performance. Both disadvantages were due to the two levels of configurable logic, because programmable logic planes were difficult to manufacture and introduced significant propagation delays. To overcome these weaknesses, Programmable Array Logic (PAL) devices were developed. PALs provide only a single level of programmability, consisting of a programmable “wired” AND plane that feeds fixed OR-gates. PALs usually contain flip-flops connected to the OR-gate outputs so that sequential circuits can be realized. These are often referred to as Simple Programmable Logic Devices (SPLDs). Fig. 20.3 shows a simplified structure of PLA and PAL.

With the advancement of technology, it has become possible to produce devices with higher capacities than SPLD’s.As chip densities increased, it was natural for the PLD manufacturers to evolve their products into larger (logically, but not necessarily physically) parts called Complex Programmable Logic Devices (CPLDs). For most practical purposes, CPLDs can be thought of as multiple PLDs (plus some programmable interconnect) in a single chip. The larger size of a CPLD allows to implement either more logic equations or a more complicated design.

Fig. 20.4 contains a block diagram of a hypothetical CPLD. Each of the four logic blocks shown there is the equivalent of one PLD. However, in an actual CPLD there may be more (or less) than four logic blocks. These logic blocks are themselves comprised of macrocells and interconnect wiring, just like an ordinary PLD.

Unlike the programmable interconnect within a PLD, the switch matrix within a CPLD may or may not be fully connected. In other words, some of the theoretically possible connections between logic block outputs and inputs may not actually be supported within a given CPLD. The effect of this is most often to make 100% utilization of the macrocells very difficult to achieve. Some hardware designs simply won't fit within a given CPLD, even though there are sufficient logic gates and flip-flops available. Because CPLDs can hold larger designs than PLDs, their potential uses are more varied. They are still sometimes used for simple applications like address decoding, but more often contain high-performance control-logic or complex finite state machines. At the high-end (in terms of numbers of gates), there is also a lot of overlap in potential applications with FPGAs. Traditionally, CPLDs have been chosen over FPGAs whenever high-performance logic is required. Because of its less flexible internal architecture, the delay through a CPLD (measured in nanoseconds) is more predictable and usually shorter. The development of the FPGA was distinct from the SPLD/CPLD evolution just described.This is apparent from the architecture of FPGA shown in Fig 20.1. FPGAs offer the highest amount of logic density, the most features, and the highest performance. The largest FPGA now shipping, part of the Xilinx Virtex™ line of devices, provides eight million "system gates" (the relative density of logic). These advanced devices also offer features such as built-in hardwired processors (such as the IBM Power PC), substantial amounts of memory, clock management systems, and support for many of the latest, very fast device-to-device signaling technologies. FPGAs are used in a wide variety of applications ranging from data processing and storage, to instrumentation, telecommunications, and digital signal processing. The value of programmable logic has always been its ability to shorten development cycles for electronic equipment manufacturers and help them get their product to market faster. As PLD (Programmable Logic Device) suppliers continue to integrate more functions inside their devices, reduce costs, and increase the availability of time-saving IP cores, programmable logic is certain to expand its popularity with digital designers. 

FPGA Structural 
Classification Basic structure of an FPGA includes logic elements, programmable interconnects and memory. Arrangement of these blocks is specific to particular manufacturer. On the basis of internal arrangement of blocks FPGAs can be divided into three classes:

Symmetrical arrays 
This architecture consists of logic elements (called CLBs) arranged in rows and columns of a matrix and interconnect laid out between them shown in Fig 20.2. This symmetrical matrix is surrounded by I/O blocks which connect it to outside world. Each CLB consists of n-input Lookup table and a pair of programmable flip flops. I/O blocks also control functions such as tristate control, output transition speed. Interconnects provide routing path. Direct interconnects between adjacent logic elements have smaller delay compared to general purpose interconnect

Row based architecture 
Row based architecture shown in Fig 20.5 consists of alternating rows of logic modules and programmable interconnect tracks. Input output blocks is located in the periphery of the rows. One row may be connected to adjacent rows via vertical interconnect. Logic modules can be implemented in various combinations. Combinatorial modules contain only combinational elements which Sequential modules contain both combinational elements along with flip flops. This sequential module can implement complex combinatorial-sequential functions. Routing tracks are divided into smaller segments connected by anti-fuse elements between them.

Hierarchical PLDs 
This architecture is designed in hierarchical manner with top level containing only logic blocks and interconnects. Each logic block contains number of logic modules. And each logic module has combinatorial as well as sequential functional elements. Each of these functional elements is controlled by the programmed memory. Communication between logic blocks is achieved by programmable interconnect arrays. Input output blocks surround this scheme of logic blocks and interconnects. This type of architecture is shown in Fig 20.6.

FPGA Classificationon user programmable switch technologies
FPGAs are based on an array of logic modules and a supply of uncommitted wires to route signals. In gate arrays these wires are connected by a mask design during manufacture. In FPGAs, however, these wires are connected by the user and therefore must use an electronic device to connect them. Three types of devices have been commonly used to do this, pass transistors controlled by an SRAM cell, a flash or EEPROM cell to pass the signal, or a direct connect using antifuses. Each of these interconnect devices have their own advantages and disadvantages. This has a major affect on the design, architecture, and performance of the FPGA. Classification of FPGAs on user programmable switch technology is given in Fig. 20.7 shown below.

SRAM Based 
The major advantage of SRAM based device is that they are infinitely re-programmable and can be soldered into the system and have their function changed quickly by merely changing the contents of a PROM. They therefore have simple development mechanics. They can also be changed in the field by uploading new application code, a feature attractive to designers. It does however come with a price as the interconnect element has high impedance and capacitance as well as consuming much more area than other technologies. Hence wires are very expensive and slow. The FPGA architect is therefore forced to make large inefficient logic modules (typically a look up table or LUT).The other disadvantages are: They needs to be reprogrammed each time when power is applied, needs an external memory to store program and require large area. Fig. 20.8 shows two applications of SRAM cells: for controlling the gate nodes of pass-transistor switches and to control the select lines of multiplexers that drive logic block inputs. The figures gives an example of the connection of one logic block (represented by the AND-gate in the upper left corner) to another through two pass-transistor switches, and then a multiplexer, all controlled by SRAM cells . Whether an FPGA uses pass-transistors or multiplexers or both depends on the particular product.

Fig. 20.8 SRAM-controlled Programmable Switches.