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20 1. INTRODUCTION
1.4.3 Dedicated elements, heterogenous structure
In advanced FPGAs specialized blocks were also available. These blocks (e.g.: embedded
memory, arithmetic unit, or embedded microprocessor) are implemented
because they are commonly used elements, therefore the specialized blocks are
using less general resources from the FPGA. The result is a highly heterogenous
structure.
The memory is a basic building block of the digital system. Flip-flops can be
used as a memory, but it will not be efficient in case of storing large amount of
data. Firstly in case of Xilinx XC4000 FPGA were the LUTs enough flexible to
use it as an asynchronous 16×1 bit RAM. Later it evolved to use as a dual-ported
RAM or as a shift register. These clusters can be arranged in a flexible way to
implement larger bit-width or deeper memory. E.g.: In case of a 4Kb on-chip
RAM on Xilinx Virtex FPGA can be defined in the following hierarchical way:
4096 × 1, 2048 × 2, 1024 × 4, 512 × 8, 256 × 16.
Among the adders, which builds up from logical elements, in FPGA we can
use embedded multipliers, or Digital Signal Processing (DSP) blocks like separate,
dedicated resources. The DSP block can make addition, subtraction, multiplication
or multiply-accumulate (MAC) operation. The solving of a MAC operation
in one clock cycle can be useful for finite-impulse response (FIR) filtering (which
occurs in many DSP applications).
The FPGA manufacturers are integrating complete dedicated microprocessors
into their devices in order to implement low bandwidth and/or complex
controlling functions. With this capability (nearly) fully embedded systems can
be implemented on the FPGA. The embedded processor can be found on Xilinx
Virtex-II Pro, on Xilinx Virtex-4 and on Xilinx Virtex-5 FPGA. The Xilinx
integrates dedicated hard processor cores (e.g.: IBM PowerPC405) into their devices
(see Figure 1.11). These embedded processors are connected with on-chip
SRAMs. That means without the configuration of the FPGA it can not make
any useful work.