DESIGN AND CHARACTERIZATION OF PARALLEL PREFIX ADDERS USING FPGAS PDF

Nirmala Devi. Tech, Ph. AbstractThe binary adder is that the essential part in most digital circuit styles as well as digital signal processors DSP and microchip knowledge path units. As such, in depth analysis continues to be centered on raising the ability delay performance of the adder. The Ripple carry adder and carry skip adders output of every stage depends on the previous carry. But once coming back to hold tree adders Parallel prefix adders , It generates the carry signals in O log n time.

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Nirmala Devi. Tech, Ph. AbstractThe binary adder is that the essential part in most digital circuit styles as well as digital signal processors DSP and microchip knowledge path units. As such, in depth analysis continues to be centered on raising the ability delay performance of the adder. The Ripple carry adder and carry skip adders output of every stage depends on the previous carry. But once coming back to hold tree adders Parallel prefix adders , It generates the carry signals in O log n time.

Due to the presence of a quick carry-chain, the RCA styles exhibit higher delay performance up to bits. There are so many adders in the digital design. The selection of adder depends on its performance parameters. Adders are important elements in microprocessors, digital signal processors. ALU and in floating point arithmetic units.

But they became pragmatic if binary numbers are given. Therefore binary addition is essential any improvement in binary addition can improve the performance of system. The fast and accuracy of system depends mainly on adder performance. In this paper designing and implementation of various parallel prefix adders on FPGA are described.

Parallel prefix adders are designed from carry look ahead adder as a base. Parallel prefix adders consist of three stages similar to CLA. Figure 1 shows the PPA structure. Figure 1. Pi indicates whether Carry is propagated from that bit. In carry generation stage of PPA, prefix graphs can be used to describe the tree structure.

Here the tree structure consists of grey cells, black cells, and buffers. In carry generation stage when two pairs of generate and propagate signals Gm, Pm , Gn, Pn are given as inputs to the carry generation stage. Pn The black cell computes both generate and propagate signals as output. It uses two and gates and or gate. The grey cell computes the generate signal only.

It uses only and gate, or gate. These signals are combined using fundamental carry operator fco. So the arrangement of the prefix network gives rise to various families of adders. Here we designate black cell as BC and grey cell as GC. Kogge-Stone adder Kogge-Stone adder is one among the parallel prefix adders.

This has regular layout which makes them favoured adder in electronic technology. It has the minimum fan-out. A 16 bit Kogge stone adder is shown in the figure 2. The maximum fan-out is 2 in all the logic levels for all width Kogge-stone prefix trees.

The key of building any prefix tree is to implement the equation according to the specific features and apply the rules above described in the previous section.

The number of stages for a Kogge stone adder is calculated by log2 power N. It consists of 34 BCs and 15 GCs and buffers are given.

Sparse Kogge-Stone adder The Sparse Kogge stone adder consists of several small ripple carry adders on its lower part, a carry tree is on its upper part. It terminates with ripple carry adders. Number of carries generated is less in this adder compared to Kogge stone adder. The function of grey cells and black cells is same as discussed in previous sections.

Figure 2. Like the sparse kogge- Stone adder, this design terminates with a 4- bit RCA. As the FPGA uses a fast carry-chain for the RCA, it is interesting to compare the performance of this adder with the sparse kogge- Stone and regular kogge-Stone adders.

It also uses the black cells and gray cells and full adder blocks like sparse kogge stone adders but the difference is the interconnection between them.

But it takes larger area.. Here we design different carry tree adders and compared with Ripple carry adder in terms of delay. The verification of the adders was verfied by using Model-sim Simulator. The Xilinx ISE It is found that the Kogge Stone Prefix trees provide better delay performance for higher order bits. We seen area is high. We noticed that parallel prefix adders are faster than the ripple carry adder. The results of different parallel prefix adders are as given below.

In the Hierarchy pane. In the Processes pane, double-click Synthesize. This device utilization includes the following. Kogge Stone Adder Figure 5. No flip flops utilized in this adder. Sparse Kogge Stone Adder Figure 5. Spanning tree Adder Figure 5. HDL simulation now can be an even more fundamental step within your design flow with the tight integration of the ISim within your design environment.

All simulation commands that prepare the ISim simulation are generated by ISE Project navigator and automatically run in the background when simulating a design using thisflow Kogge Stone Adder Figure 5.

FPGA contains a two dimensional arrays of logic blocks and interconnections between logic blocks. Both the logic blocks and interconnects are programmable. Logic blocks are programmed to implement a desired function and the interconnections are programmed using the switch boxes to connect the logic blocks. To be more clear, if we want to implement a complex design CPU for instance , then the design is divided into small sub functions and each sub function is implemented using one logic block.

FPGAs contain programmable logic components called "logic blocks", and a hierarchy of reconfigurable interconnects that allow the blocks to be "wired together"somewhat like many changeable logic gates that can be inter-wired in many different configurations. Logic blocks can be configured to perform complex combinational functions, or merely simple logic gates like AND and XOR.

In most FPGAs, the logic blocks also include memory elements, which may be simple flip- flops or more complete blocks of memory. Table 5.

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DESIGN AND CHARACTERIZATION OF PARALLEL PREFIX ADDERS USING FPGAS PDF

Tygozragore In VLSI implementations, parallel-prefix adders also known as carry-tree adders are known to have the best performance. Remember me on this computer. For look ahead adder, the carry combination equation can be example 4-bit adder can be constructed by cascading four expressed as, full adders together prefux shown in Figure. Hoe Proceedings of the 44th Southeastern…. Showing of 10 extracted citations.

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