FPGA Fundamentals 0 (lookup table Lut and programmatic)

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Part I: Lookup table Lut

FPGA is the product of the further development on the basis of the PAL, GAL, EPLD, CPLD and other programmable devices. It is a semi-custom circuit in the field of ASIC, which solves the shortcomings of the custom circuit, and overcomes the limitation of the original programmable device gate circuit.

Since the FPGA needs to be burned repeatedly, the basic structure of the combinational logic can not be done by fixed and non-gate as ASIC, but only by a structure which is easy to be configured repeatedly. Look-up table can meet this requirement, the mainstream FPGA is now using the SRAM-based lookup table structure, there are some military and aerospace-grade FPGA using flash or fuse and reverse fuse process lookup table structure. The repeated configuration of the FPGA is accomplished by burning the file to change the contents of the lookup table.

Based on the basic knowledge of the digital circuit, it is possible to know that for a logical operation of an n input, there can be at most only 2n results, whether it is a non-operation or an XOR operation. Therefore, if the corresponding results are stored in a storage unit, it is equivalent to the realization of the function of the non-gate circuit. The principle of FPGA is also the same, it by burning write files to configure the contents of the lookup table, so that in the same circuit, the implementation of different logic functions.

The lookup table (look-up-table), abbreviated as Lut,lut, is essentially a ram. Currently more than 4 input lut is used in FPGAs, so each lut can be considered as a 4-bit address line of RAM. When a user describes a logic circuit through a schematic or HDL language, the PLD/FPGA development software automatically calculates all possible results of the logic circuit and writes the truth table (that is, the result) to RAM beforehand, so that each input signal is logically calculated to be equal to the input of an address to look up the list, Find out what the address corresponds to and then output it.

An example of a 4 and a gate circuit is given below to illustrate how the LUT implements the logic function.

Example 1-1: A truth table for 4 input and gate circuits using a lut is given.

As you can see, the LUT has the same function as the logic circuit. In fact, the LUT has a faster execution speed and a larger scale.

Part Two: Programming methods

Because of the high integration of the LUT-based FPGA, its device density ranges from tens of thousands of gates to tens of millions of gates, it can complete the complex logic circuit function of timing and logic, so it is suitable for high-speed and high-density high- end digital logic circuit design field. Its main components are programmable input/output unit, basic programmable logic unit, embedded SRAM, rich cabling resources, bottom embedded function unit, embedded special Unit, etc., the main design and manufacturers have Xilinx, Altera, Lattice, Actel,  Companies such as Atmel and QuickLogic, the largest of which are Xilinx, Altera, and lattice three.

FPGAs are stored on-chip RAM to set their working state, so it is necessary to program the on-chip RAM when working. Users can use different programming modes depending on the configuration mode. There are several configuration modes for FPGAs:
1, parallel mode: Parallel prom, flash configuration FPGA;

2, Master-slave mode: A piece of the prom configuration multi-chip FPGA;

3, serial mode: Serial prom configuration FPGA;
4. Peripheral mode: The FPGA is used as the peripheral of the microprocessor, which is programmed by the microprocessor.

At present, the FPGA market share of the two largest companies Xilinx and Altera-produced FPGA are based on the SRAM process, need to add an off-chip memory to save the program when used. When power is on, the FPGA reads the data from the external memory into the on-chip RAM, completes the configuration, enters the working state, the FPGA reverts to the white chip after the power-down, and the internal logic disappears. In this way, the FPGA can not only be reused, there is no need for a dedicated FPGA programmer, just a universal eprom, prom programmer. Actel, QuickLogic and other companies also provide anti-fuse technology FPGA, can only download once, with anti-radiation, high and low temperature, low power and speed, and other advantages in the field of military and aerospace applications more, but this FPGA can not be repeated erase, the initial development of trouble, the cost is more expensive. Lattice is the inventor of ISP technology and has certain characteristics in the application of small-scale pld. Early Xilinx products generally do not involve military and aerospace-grade markets, but there are many products such as Q Pro-r that have entered the field.

FPGA Fundamentals 0 (lookup table Lut and programmatic)