In p>1984, Bernie Vonderschmitt, Ross Freeman and Jim Barnett, who worked in Silicon Valley, created an idea, and they dreamed of setting up a different company. They hope to create a company to develop and introduce advanced technology in the whole new field. Moreover, they also hope to lead it in this way: people who work here love their jobs, enjoy their work and are fascinated by what they are doing.
creatively introduced the concept of "wafer-less semiconductor" company. On February 18th, 29, Ross Freeman was inducted into the 29 American Inventors Hall of Fame for his invention-Field Programmable Gate Array (FPGA). Mr. Freeman's invention is a computer chip composed entirely of "open doors", and its patent number is 4,87,32. Using this kind of chip, engineers can program according to their needs, add new functions, meet the requirements of developing standards or specifications, and modify them in the final stage of design.
Anyone familiar with PROM, EPROM and E2PROM knows that the programmable principle of these programmable devices is to change the carrier density inside the triode or MOS tube by applying high voltage or ultraviolet rays, so as to realize so-called programmability. However, these devices can only be programmed once or the programmed state is difficult to be stable. FPGA is different. It adopts a new concept of logic cell array (LCA), which includes three parts: configurable logic block (CLB), input output block (IOB) and Interconnect.
The programmability of FPGA actually changes the flip-flop states of CLB and IOB, so that repeated programming can be realized. Because FPGA needs to be burned repeatedly, the basic structure of combinational logic can't be completed by a fixed NAND gate like ASIC, but only a structure that is easy to be configured repeatedly. Look-up table can meet this requirement well. At present, mainstream FPGAs adopt look-up table structure based on SRAM technology, and some military and aerospace FPGAs adopt look-up table structure of Flash or fuse and antifuse technology. Repeated configuration of FPGA is realized by burning files to change the contents of lookup table.
according to the basic knowledge of digital circuits, it can be known that there are only 2n kinds of results for a logical operation with n inputs, whether it is NAND operation or XOR operation. Therefore, if the corresponding results are stored in a storage unit in advance, it is equivalent to realizing the function of NAND gate circuit. The same is true of the principle of FPGA, which configures the contents of the lookup table by burning files, thus realizing different logic functions under the same circuit situation.
Look-Up-Table is called LUT for short, and LUT is essentially a RAM. At present, 4-input LUTs are mostly used in FPGA, so each LUT can be regarded as a RAM with 4-bit address lines. When a user describes a logic circuit through schematic diagram or HDL language, PLD/FPGA development software will automatically calculate all possible results of the logic circuit, and write the truth table (i.e. results) into RAM in advance, so that
every time a signal is input for logic operation, it is equivalent to inputting an address to look up the table, finding out the contents corresponding to the address, and then outputting it.
As can be seen from the table, LUT has the same function as the logic circuit. In fact, LUT has faster execution speed and larger scale. Because LUT-based FPGA has a high degree of integration, its device density varies from tens of thousands to tens of millions of gates, and it can complete extremely complex sequential and logic combinational logic circuit functions, so it is suitable for high-speed and high-density high-end digital logic circuit design. Its main components are programmable I/O unit, basic programmable logic unit, embedded SRAM, rich wiring resources, embedded functional unit at the bottom, embedded special unit, etc. The main design and manufacturers are Xilinx, Altera, Lattice, Actel, Atmel and QuickLogic, among which the largest is Xilinx Company of the United States, accounting for more than 5% of the programmable market.
the working state of FPGA is set by the progRAM stored in the on-chip RAM, so it is necessary to program the on-chip ram when working. Users can adopt different programming methods according to different configuration modes. At power-on, the FPGA chip reads the data in the EPROM into the on-chip programming RAM, and after the configuration is completed, the FPGA enters the working state. After power failure, the FPGA reverts to a blank, and the internal logic relationship disappears, so the FPGA can be used repeatedly. The programming of FPGA does not need a special FPGA programmer, just use a general EPROM and PROM programmer. In this way, the same FPGA with different programming data can produce different circuit functions.
therefore, the use of FPGA is very flexible. As mentioned above, the working state of FPGA is set by the RAM stored in the chip, so it is necessary to program the on-chip RAM when working. Users can adopt different programming methods according to different configuration modes. There are five common configuration modes of Xilinx FPGA: master string mode, slave string mode, elect MAP mode, Desktop configuration and direct SPI configuration.
At present, the FPGA produced by Xilinx Company and Altera, the two largest companies with the highest market share of FPGA, are based on SRAM technology, so it is necessary to connect an off-chip memory to save programs when using. At power-on, FPGA reads the data in the external memory into the on-chip RAM, and enters the working state after configuration. After power failure, the FPGA reverts to blank, and the internal logic disappears. In this way, FPGA can not only be used repeatedly, but also need a special FPGA programmer, just a general EPROM and PROM programmer. Actel, QuickLogic and other companies also provide anti-fuse technology FPGA, which has the advantages of radiation resistance, high and low temperature resistance, low power consumption and high speed, and is widely used in military products and aerospace fields. However, this kind of FPGA can not be erased repeatedly, and it is troublesome and expensive in the initial development. Lattice is the inventor of ISP technology and has certain characteristics in small-scale PLD application. The early Xilinx products generally did not involve the military and aerospace markets, but at present, many products have entered this field.
FPGA chip structure At present, the mainstream FPGA is still based on look-up table technology, which has far exceeded the basic performance of the previous version, and has integrated hard-core (ASIC-type) modules with common functions (such as RAM, clock management and DSP). In fact, each series of FPGA has its own internal structure. The FPGA chip is mainly composed of six parts, namely: programmable input and output unit, basic programmable logic unit, complete clock management, embedded block RAM, rich wiring resources, embedded bottom functional unit and embedded special hardware module.