Chapter 19 - Configuration Bitstream Generation

Abstract

The bitstream spatially represents the configuration data of a large collection of small, relatively simple hardware components. This chapter identifies these components and discusses the ways in which the bitstream is used to produce a working digital circuit in a reconfigurable logic device. Although there is really no limit to the types of units possible in a reconfigurable logic device, two basic structures make up the microarchitecture of most modern field-programmable gate arrays (FPGAs). These are the lookup table (LUT) and the switch box. While the generation of bitstream data to configure an FPGA device is a very common activity, there is very little information available on the details of either the configuration bitstream or the underlying FPGA architecture. Thus, the FPGA can best be viewed as a collection of microarchitecture components, chiefly LUTs and switch boxes. These components are configured by writing data to the LUT values and to control memories associated with the switch boxes. Setting these bits to various values results in custom digital circuits. A variety of tools and techniques are used to program reconfigurable logic devices, but all must eventually produce the relatively small configuration “bitstream” data the devices require. This data is in as rigid a format as any binary execution data for a microprocessor, but this format is typically proprietary and unpublished. While direct examination of actual commercial bitstream data is largely impossible, the general structure and the microarchitecture components configured by this data can be examined, at least in the abstract.