# Research on an Intelligent Test Method for Interconnect Resources in an FPGA
## 2 FPGA Interconnect Resource Structure and Fault Model Analysis  
### FPGA Interconnect Structure Analysis  
FPGAs are composed primarily of modules, such as configurable logic blocks (CLBs), input-output blocks (IOBs), interconnect resources (IRs), and block random access memory (BRAM) [9,10]. The CLB is the core resource of the entire FPGA, consisting mainly of programmable resources, such as look-up tables (LUTs), multiplexers, carry chains, and flip-flops, to implement different logical functions. The IOBs form an interface between the chip and the external circuits, providing suitable drive signals to match the differences in the electrical characteristics between the internal and external environments. Using electronic design automation (EDA) software, different electrical standards and physical information can be configured according to certain requirements, such as adjusting the size of the driving currents and changing the resistance values of the pull-up and pull-down resistors. In general, several IOBs are grouped together (banked) for ease of design and management, with the number of IOBs per group varying across different series of FPGA chips.  
Interconnect resources can be divided into metal line segments and switch matrix resources based on their connection methods. Metal line segments are distributed mainly between each tile, connecting various logic blocks and hard-core resources. Switch matrix resources include the switch matrices distributed in the wiring channels and those that guide the signals into logic blocks or hard-core resources.  
The schematic diagram in Figure 2 shows the switch matrix and CLB structure of an FPGA, which is an important research object with respect to interconnect resources. The metal interconnect lines form the outer perimeter of the switch matrix and are divided into X-fold lines based on the number of switch matrices they traverse. The most common types of metal interconnect lines are single-fold lines, double-fold lines, and long lines. These lines typically start at the BEG port and end at the END port, with a unique mapping relationship between the starting and ending points.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Abbreviation** & **Full Name** \\ \hline FPGA & Field Programmable Gate Arrays \\ \hline CLB & Configurable Logic Blocks \\ \hline RAM & Random Access Memory \\ \hline BIST & Built-in Self Tests \\ \hline DUT & Device Under Test \\ \hline ATE & Automated Test Equipment \\ \hline IOB & Input-output Blocks \\ \hline IR & Interconnect Resources \\ \hline BRAM & Block Random Access Memory \\ \hline LUT & Look Up Tables \\ \hline EDA & Electronic Design Automation \\ \hline PIP & Programmable Interconnect Points \\ \hline BEG & Begin \\ \hline SRAM & Static Random-access Memory \\ \hline NMOS & N-Metal-Oxide-Semiconductor \\ \hline \hline \end{tabular}
\end{table}
Table 1: Abbreviation table.  
Within an FPGA, the switch matrices distributed within each tile connect the metal line segments from different directions and wiring channels. The internal connections within the switch matrix are called PIPs (programmable interconnect points), which are controlled primarily by SRAM (static random-access memory) cells to implement transmission gates and buffers in different arrangements. A commonly used example is a switch matrix that uses one buffer and N-many transmission gates to achieve 1-N multiplexing, PIPs start at the END port and end at the BEG port. The mapping relationship of the PIPs is shown in Figure 3.