Design Verification

Design verification is must in designing any system. Design verification and testing is the most tedious job in implementing any complex system. Several foundries are specialized in doing verification and testing. Each and every step of VLSI design needs verification. If there is a fault in any step, one has to go to the early steps to correct it. There are two types of verifications generally carried out in making VLSI design. They are

  1. Functional verification
  2. Static Timing Analysis

The functional verification step verifies the functionality of the design or the behavior of the system. On the other hand static timing analysis (STA), concentrates on verifying the timing constraints. Both the verification steps must be carried out after every step such as translation, placing and routing.

In this post, functional verification of FPGA implementations is discussed. Smaller designs can be verified by writing test benches. But it is very difficult to verify the complex designs. Also, if the design involves several memory elements it is very difficult to create such huge amount of memory locations. Designers face difficulties in loading test vectors in memory elements several times. In this post, a simple way of design verification is discussed.

Functional Verification Approaches

Functional verification of design can be carried out using two methods in general and they are

  • MATLAB/Any similar tool based.
  • XILINX/Any similar tool based.

MATLAB based Verification: MATLAB provides a very easy system verification platform as both the analog and digital part can be designed in same platform. XILINX provides an SYSTEM GENERATOR tool which can realize the digital blocks used in MATLAB based design. XILINX HDL files can also be imported to MATLAB for functional verification. Functional verification can be done using the newly introduced feature called FPGA In Line (FIL). Here FPGA is placed between the analog and digital design. The following diagram shows the functional verification scheme using MATLAB.


Figure 1 :Functional verification using MATLAB

First step is to design the analog part in SIMULINK environment. The input test vectors are converted to fixed point data using FIXED POINT CONVERTER tool of MATLAB. Here we can specify the data precision. The digital part is designed by using the digital blocks which can be converted to HDL codes. The FPGA output (Binary) is converted to decimal data type which MATLAB supports. Then the output can be observed in scopes or can be compared with the inputs. MATLAB provides predefined Computer – FPGA interface details of which are not known to the users.
MATLAB based verification provides an easy solution to the design verification. Most of the VLSI designers avoid this kind of verification as there are few scopes where a designer can improve the design. For an expert designer, who seeks to improve the performance of the design must do design and verification at XILINX level.

XILINX based Verification

XILINX also provides an easy way of functional verification. If a designer is designing a system in XILINX, then the designer need not import all the HDL codes to MATLAB. The general scheme for verification is shown below


Figure 2: XILINX based functional verification.

The XILINX based system verification can be explained by an example. Consider, a designer has to solve the linear equation y = ax, where y is an output vector, a is a square matrix and x is the input. The problem is to find the x by the equation x = inv(a)y.   To do that designer is supposed to find the inverse of a and multiply with y in FPGA.

To begin with this simple problem, the output y can be generated in MATLAB or y may be the output of an external analog system. The digital part of the system takes y and matrix a as inputs and produces x as output. Consider that the matrix a is constant and need to be stored in memory inside FPGA. So the only varying input is y. If y is generated through MATLAB then the interface between a COMPUTER and FPGA can done by serial UART protocol or using LAN or using SPI flash technique. If y is the output of an external system then the analog system can interfaced with FPGA using interfacing ADC with the FPGA. After interfacing, FPGA processes and produces the output x. The output x can be seen back in the MATLAB using same interface mechanisms mentioned above or it can be seen in a CRO screen using interfacing a DAC with the FPGA.

Following discussion focuses on designing the analog part in MATLAB and importing the FPGA output back into MATLAB for comparison. The input vector y is generated in MATLAB and stored in a XLSX file. The input vector y is converted to fixed point data by MATLAB code d2b.m. By this code we can vary the data precision. The fixed point data is now written in text file input_vector.txt using MATLAB code wrintext.m. The matrix a is fixed and need to be stored in ROMs in FPGA. The same steps are followed for matrix a also. The text files can read by the XILINX tool by the Verilog code memread.v. These steps eliminate the bottleneck of loading data in memory elements several times. The behavioral simulation is performed by XILINX and it produces the output x. The output x is again written in a text file output_vector.txt by the Verilog file memwrite.v. This output text file is then converted to decimal data format by b2d.m MATLAB file and written in a XLSX file. This file now can be used in MATLAB to compare with MATLAB results. These above mentioned steps are explained in the following flowchart.

Figure 3: Flowchart for XILINX based verification

We have considered handling of XLSX files using MATLAB. The codes are described and attached below. 

Reading from a text file.

The command fopen() opens a file input_vector.txt in read mode. In the loop of j, jth element of input_vector.txt (of0) stored in register A1. Then the content of A1 is stored in jth location of b which is an array of registers.

module mem_read(C1,ada,clk);
    input [9:0] ada;
	 input clk;
    integer of0;
    output reg [17:0] C1;
    reg [17:0] A1;
	 reg [17:0] b1 [1023:0];
	 integer j;
	 initial begin
	 of0=$fopen("input_vector.txt","r"); 
	 for (j=0;j<=1023;j=j+1)
	 begin 
        $fscanf(of0,"%d\n",A1); 
         #1;
        b1[j] = A1;
    end 
	 $fclose(of0);
	 end 
    always @ (posedge clk)
    begin 
	 C1 = b1[ada];
	 end
endmodule

Writing into a text file.

The value of the register A is written in the file output_vector.txt only if en is high and with positive edge of clock. The clock is considered to be of period 10 ns.

module mem_write(A,en,clk);
    integer of0;
    input [17:0] A ;
	 input en,clk;
	 integer j;
	 initial begin j = 0; end
	 always @(posedge clk)
	 begin 
	 if (en)
	      begin
	 of0=$fopen("output_vector.txt","w");
	   for (j=0;j<=4;j=j+1)
  	      begin
	 $fdisplay(of0,"%d\n",A); 
	 #10;
	      end
		   end
	 else
	 $fclose(of0);
	 end
endmodule

The MATLAB codes are not discussed here. They are attached with the post.

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