Field Programmable Gate Arrays (FPGAs) An Enabling Technology

1. Field Programmable Gate Arrays (FPGAs) An Enabling Technology

2. Introduction • Field Programmable Gate Array or FPGA -a type of programmable device. • Programmable devices - a class of general-purpose chips that can be configured for a wide variety of applications (e.g. PROM- Programmable Read-Only Memory )

3. A gate array where the logic network can be programmed into the device after its manufacture. An FPGA consists of an array of logic elements, either gates or lookup table RAMs, flip-flops and programmable interconnect wiring. • FPGAs are similar in principle to, but have vastly wider potential application than, programmable read-only memory (PROM) chips.

5. Two basic PROM versions: • 1) Mask-Programmable Chip - programmed only by the manufacturer • 2) Field-Programmable Chip - programmed by the end-user ; two types: the Erasable Programmable Read-Only Memory (EPROM) and the Electrically Erasable Programmable Read-Only Memory (EEPROM).

6. Programmable Logic Device (PLD) - constructed to implement logic circuits; included an array AND gates connected to an array of OR gates. • Programmable Array Logic (PAL) - commonly used PLD consisting of a programmable AND-plane followed by a fixed OR-plane; come in both mask and field versions; for small logic circuits .

7. Mask-Programmable Gate Array (MPGA) - developed to handled larger logic circuits • usually consists of rows of transistors that can be interconnected to implement desired logic circuits; • User specified connects are available both within the rows and between the rows which enable implementation of basic logic gates and the ability to interconnect the gates.

8. Overview of the FPGA • Four main categories of FPGAs commerically available: • symmetrical array • row-based • hierarchical PLD • sea-of-gates (Figure 1). • Currently used technologies: • static RAM cells • anti-fuse • EPROM transistors, • EEPROM transistors.

9. Figure 1. Classes of FPGAs

10. Static RAM Technology --programmable connections are made using pass=transistors, transmission gates, or multiplexers that are controlled by SRAM cells; it allows fast in-circuit reconfiguration. • Anti-Fuse Technology - resides in a high-impedance state; and can be programmed into low impedance or "fused" state; less expensive than the RAM technology, this device is a program once device. • EPROM / EEPROM Technology --same as used in the EPROM memories; it can be reprogrammed without external storage of configuration; EPROM transistors cannot be re-programmed in-circuit.

11. Fuse

12. Anti- fuse

13. Characteristics of FPGA technology

14. The following table shows some of the commercially available FPGAs

15. The SRAM based FPGA • Field-Programmable Gate Arrays (FPGAs) : • provide the benefits of custom CMOS VLSI, while avoiding the initial cost, time delay, and inherent risk of a conventional masked gate array. • customized by loading configuration data into the internal memory cells. • can either actively read its configuration data out of external serial or byte-parallel PROM (master mode), or the configuration data can be written into the FPGA (slave and peripheral mode). • can be programmed an unlimited number of times and supports system clock rates of up to 50 MHz.

16. FPGA Elements • configurable logic blocks (CLBs) - provide the functional elements for constructing user's logic (Figure 2). • input/output blocks - provide the interface between the package pins and internal signal lines • Interconnects - programmable interconnect resources provide routing paths to connect the inputs and outputs of the CLBs and IOBs onto the appropriate networks. Customized configuration is established by programming internal static memory cells that determine the logic functions and internal connections implemented in the FPGA.

18. Figure 3. CLBs, Interconnects • Figure 3 depicts a FPGA with a two-dimensional array of logic blocks that can be interconnected by interconnect wires. • All internal connections are composed of metal segments with programmable switching points to implement the desired routing. • An abundance of different routing resources is provided to achieve efficient automated routing. • There are four main types of interconnect, three are distinguished by the relative length of their segments: single-length lines, double-length lines and Longlines. • Eight global buffers drive fast, low-skew nets most often used for clocks or global control signals.

19. Figure 4. Configurable Logic Blocks • The principle CLB (Configurable Logic Block) elements are shown in Figure 4. • Each CLB contains a pair of flip-flops and two independent 4-input function generators. These function generators have a good deal of flexibility as most combinatorial logic functions need less than four inputs. Configurable Logic Blocks implement most of the logic in an FPGA. The flexability and symmetry of the CLB architecture facilitates the placement and routing of a given application.

20. Programming the FPGA The standard digital design flow for Hardware Object implementation.

21. Digital Design Stage • the digital design is created with a schematic digital design editor or a Hardware Description Language (HDL). The schematic entry program utilizes graphic symbols of the circuitry. • As the output of these programs produce netlists, one must be sure the library sets of the targeted FPGA are available in the tool you have selected.

22. Design Implementation Stage • the netlist produced by the design entry program is converted into the bitstream file which configures the FPGA. The first step Maps the design onto the FPGA resources;. The second step places or assigns logic blocks created in the mapping process in specific locations in the FPGA. The third step Routes the interconnect paths between the logic blocks. The output is a Logic Cell Array File (LCA) for the particular FPGA. This LCA file is then converted into a bitstream file for configuring the FPGA.

23. Design Verification Step • tests the design's logic and timing using input stimuli. Various software packages provide verification/simulation tools. These tools are designed to perform detailed characterization of the design, by performing both functional and timing simulations. In-circuit verification tests the circuit under typical operating conditions, e.g. The Virtual Computer tm , reconfigurable computer.

24. FPGA Configuration • process in which the circuit design (bitstream file) is downloaded into the FPGA. The method of configuring the FPGA determines the type of bitstream file. FPGAs can be configured by a PROM. The serial PROM is the most common. The FPGA can either actively read its configuration data out of external serial or byte-parallel PROM (master mode), or the configuration data can be written into the FPGA (slave and peripheral mode). Where the FPGA is used in a Reconfigurable Computing Platform, the bitstream file is converted into a High Level Language (i.e. 'C" ) function