Architecture of Electronics

Architecture of Electronics:

Logic blocks:

Architecture of Electronics Simplified example illustration of a logic cell, The most common FPGA architecture consists of an array of logic blocks (called configurable logic block, CLB, or logic array block, LAB, depending on vendor), I/O pads, and routing channels. Generally, all the routing channels have the same width (number of wires). Multiple I/O pads may fit into the height of one row or the width of one column in the array.

An application circuit must be mapped into an FPGA with adequate resources. While the number of CLBs/LABs and I/Os required is easily determined from the design, the number of routing tracks needed may vary considerably even among designs with the same amount of logic. For example, a crossbar switch requires much more routing than a systolic array with the same gate count. Since unused routing tracks increase the cost (and decrease the performance) of the part without providing any benefit, FPGA manufacturers try to provide just enough tracks so that most designs that will fit in terms of lookup tables (LUTs) and I/Os can be routed. This is determined by estimates such as those derived from Rent’s ruleor by experiments with existing designs.

In general, a logic block (CLB or LAB) consists of a few logical cells (called ALM, LE, slice etc.). A typical cell consists of a 4-input LUT, a full adder (FA) and a D-type flip-flop, as shown below. The LUTs are in this figure split into two 3-input LUTs. In normal mode those are combined into a 4-input LUT through the left mux. In arithmetic mode, their outputs are fed to the FA. The selection of mode is programmed into the middle multiplexer. The output can be either synchronous or asynchronous, depending on the programming of the mux to the right, in the figure example. In practice, entire or parts of the FA are put as functions into the LUTs in order to save space.

Hard blocks:

Modern FPGA families expand upon the above capabilities to include higher level functionality fixed into the silicon. Having these common functions embedded into the silicon reduces the area required and gives those functions increased speed compared to building them from primitives. Examples of these include multipliers, generic DSP blocks, embedded processors, high speed I/O logic and embedded memories.

Higher-end FPGAs can contain high speed multi-gigabit transceivers and hard IP cores such as processor cores, Ethernet MACs, PCI/PCI Express controllers, and external memory controllers. These cores exist alongside the programmable fabric, but they are built out of transistors instead of LUTs so they have ASIC level performance and power consumption while not consuming a significant amount of fabric resources, leaving more of the fabric free for the application-specific logic. The multi-gigabit transceivers also contain high performance analog input and output circuitry along with high-speed serializers and deserializers, components which cannot be built out of LUTs. Higher-level PHY layer functionality such as line coding may or may not be implemented alongside the serializers and deserializers in hard logic, depending on the FPGA.

Clocking:

Most of the circuitry built inside of an FPGA is synchronous circuitry that requires a clock signal. FPGAs contain dedicated global and regional routing networks for clock and reset so they can be delivered with minimal skew. Also, FPGAs generally contain analog PLL and/or DLL components to synthesize new clock frequencies as well as attenuate jitter. Complex designs can use multiple clocks with different frequency and phase relationships, each forming separate clock domains. These clock signals can be generated locally by an oscillator or they can be recovered from a high speed serial data stream. Care must be taken when building clock domain crossing circuitry to avoid metastability. FPGAs generally contain block RAMs that are capable of working as dual port RAMs with different clocks, aiding in the construction of building FIFOs and dual port buffers that connect differing clock domains.

3D architectures:

To shrink Architecture of Electronics, the size and power consumption of FPGAs, vendors such as Tabula and Xilinx have introduced new 3D or stacked architectures. Following the introduction of its 28 nm 7-series FPGAs, Xilinx revealed that several of the highest-density parts in those FPGA product lines will be constructed using multiple dies in one package, employing technology developed for 3D construction and stacked-die assemblies.

Xilinx’s approach stacks several (three or four) active FPGA die side-by-side on a silicon interposer – a single piece of silicon that carries passive interconnect. The multi-die construction also allows different parts of the FPGA to be created with different process technologies, as the process requirements are different between the FPGA fabric itself and the very high speed 28 Gbit/s serial transceivers. An FPGA built in this way is called a heterogeneous FPGA.

Altera’s heterogeneous approach involves using a single monolithic FPGA die and connecting other die/technologies to the FPGA using Intel’s embedded multi-die interconnect bridge (EMIB) technology.

Design and programming:

To define the Architecture of Electronics behavior of the FPGA, the user provides a design in a hardware description language (HDL) or as a schematic design. The HDL form is more suited to work with large structures because it’s possible to just specify them numerically rather than having to draw every piece by hand. However, schematic entry can allow for easier visualisation of a design.

Then, using an electronic design automation tool, a technology-mapped netlist is generated. The netlist can then be fit to the actual FPGA architecture using a process called place-and-route, usually performed by the FPGA company’s proprietary place-and-route software. The user will validate the map, place and route results via timing analysis, simulation, and other verification methodologies. Once the design and validation process is complete, the binary file generated (also using the FPGA company’s proprietary software) is used to (re)configure the FPGA. This file is transferred to the FPGA/CPLD via a serial interface(JTAG) or to an external memory device like an EEPROM.

The most common HDLs are VHDL and Verilog, although in an attempt to reduce the complexity of designing in HDLs, which have been compared to the equivalent of assembly languages, there are moves to raise the abstraction level through the introduction of alternative languages. National Instruments’ LabVIEW graphical programming language (sometimes referred to as “G”) has an FPGA add-in module available to target and program FPGA hardware.

To simplify the design of complex systems in FPGAs, there exist libraries of predefined complex functions and circuits that have been tested and optimized to speed up the design process. These predefined circuits are commonly called IP cores, and are available from FPGA vendors and third-party IP suppliers (rarely free, and typically released under proprietary licenses). Other predefined circuits are available from developer communities such as OpenCores (typically released under free and open source licenses such as the GPL, BSD or similar license), and other sources.

In a typical design flow, an FPGA application developer will simulate the design at multiple stages throughout the design process. Initially the RTL description in VHDL or Verilog is simulated by creating test benches to simulate the system and observe results. Then, after the synthesis engine has mapped the design to a netlist, the netlist is translated to a gate level description where simulation is repeated to confirm the synthesis proceeded without errors. Finally the design is laid out in the FPGA at which point propagation delays can be added and the simulation run again with these values back-annotated onto the netlist.

More recently, OpenCL is being used by programmers to take advantage of the performance and power efficiencies that FPGAs provide. OpenCL allows programmers to develop code in the C programming language and target FPGA functions as OpenCL kernels using OpenCL constructs.

Basic process technology types:

  • SRAM – based on static memory technology. In-system programmable and re-programmable. Requires external boot devices. CMOS. Currently in use. It is worth noting that flash or EEPROM devices may often load contents into internal SRAM that controls routing and logic.
  • Fuse – One-time programmable. Bipolar. Obsolete of  Electronics
  • Antifuse – One-time programmable. CMOS in design section of Architecture of Electronics
  • PROM – Programmable Read-Only Memory technology. One-time programmable because of plastic packaging. Obsolete.
  • EPROM – Erasable Programmable Read-Only Memory technology. One-time programmable but with window, can be erased with ultraviolet (UV) light. CMOS. Obsolete.
  • EEPROM – Electrically Erasable Programmable Read-Only Memory technology. Can be erased, even in plastic packages. Some but not all EEPROM devices can be in-system programmed. CMOS.
  • Flash – Flash-erase EPROM technology. Can be erased, even in plastic packages. Some but not all flash devices can be in-system programmed. Usually, a flash cell is smaller than an equivalent EEPROM cell and is therefore less expensive to manufacture. CMOS.

Major manufacturers:

By 2017, long-time industry rivals Xilinx and Altera (now Intel) were the FPGA market leaders. At that time, they controlled nearly 90 percent of the market.

Both Xilinx and Altera provide proprietary Windows and Linux design software (ISE/Vivado and Quartus) which enables engineers to design, analyse, simulate, and synthesize (compile) their designs.

Other manufacturers include:

  • Microsemi (previously Actel, antifuse, flash-based, mixed-signal)
  • Lattice Semiconductor (SRAM based with integrated configuration flash, instant-on, low power, live reconfiguration)
    • SiliconBlue Technologies (extremely low power SRAM-based FPGAs with optional integrated nonvolatile configuration memory; acquired by Lattice in 2011)
  • QuickLogic (Ultra Low Power Sensor Hubs, extremely low powered, low density SRAM-based FPGAs, Display bridges MIPI & RGB inputs, MIPI, RGB and LVDS outputs)
  • Atmel (second source of some Altera-compatible devices; also FPSLIC mentioned above; acquired by Microchip in 2016))
  • Achronix (SRAM based, 1.5 GHz fabric speed),

In March 2010, Tabula announced their FPGA technology that uses time-multiplexed logic and interconnect that claims potential cost savings for high-density applications. On March 24, 2015, Tabula officially shut down.

On June 1, 2015, Intel announced it would acquire Altera for approximately $16.7 billion and completed the acquisition on December 30, 2015.

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