Testing of Combinational Circuit using Input Vector Monitoring BIST Architecture with S-RAM

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Testing of Combinational Circuit using Input Vector Monitoring BIST Architecture with S-RAM

Selvamani B

PG Scholar

    1. VLSI Design

      TRP Engineering College (SRM GROUP), Tiruchirappalli 621 105, India

      Padmapriyaa M

      Assistant Professor

      Department of Electronics and Communication Engineering TRP Engineering College (SRM GROUP),

      Tiruchirappalli 621 105, India

      Abstract In this paper using concurrent BIST architecture, simultaneously normal circuit operation as well as testing is performed. This technique overcomes the performance degradation which was observed while off-line testing. In an offline testing we encounter stall which interrupts the normal operation causing degradation of performance. Additionally in this approach we use S_RAM (Static Random Access Memory) to store the relative location of the vectors (set of inputs).Here the test pattern is generator using LFSR (Liner Feedback Shift Register) and that pattern is compared with the normal input. Here the circuit simulation is obtained by using Xilinx-Spartan 3E and the respective analysis of the waveform is done.

      Keywords Built-In-Self-Test, concurrent testing, on-line, off- line.

      corresponding response is observed in the Response Verifier (RV).


        Built-In-Self-Test (BIST) techniques provide an attractive solution to test modules deeply embedded in complex integrated circuits. BIST technique reduces the product development cycle and cost-effective system maintenance .BIST utilizes a Test Pattern Generator (TPG) to generate the test patterns which are applied to the Circuit under Test (CUT). BIST are classified into off-line BIST and on-line BIST. In offline BIST the normal operation of the CUT is stalled in order to perform test. To avoid this performance degradation input vector monitoring have been proposed with exploit input vectors arriving at the input of the CUT during the normal operations [2-10].

        The block diagram of an input vector monitoring concurrent BIST architecture is shown in fig1.The CUT has n inputs and m outputs and it is tested exhaustively; Hence the test size is N=2n. The technique can operate in either normal or test mode, depending on the value of the signal T/N. In fig 1 mux is used to select whether the mode ii test mode or normal mode depending on the value of T/N.

        If T/N=0 then the mode is normal mode, if T/N=1 then the mode in test mode. During the normal mode A[n:1] input is get by the mux and the same input is applied to CUT and CBU(Concurrent BIST Unit),the input is applied to the CBU is compared to the applied test vectors if any matches exists between these two inputs then hit occurs. If hit occurs the input is removed from the applied test set and the

        Figure1. Concurrent BIST Architecture

        During the test mode output of CBU is applied as input to the CUT. In order to calculate the time required for the test during the normal mode the CTL (Concurrent Test Latency) schemes in used. The brief is organised as follows. In section II, we introduce the literature review and in Section III, we introduce the proposed approach. In Section IV, summarizes the simulation results. Finally Section V summarizes the conclusion of this brief.


        In the conventional off-line method uses Test Pattern Generator (TPG) and fault simulation algorithms to find test set vectors. This test vectors is used to find the fault, test vectors can be applied externally or from the chip which already stored in it. The test vectors that is applied from the chip during the test phase is called off-line BIST technique. Four types of test pattern is used in BIST. They are exhaustive, pseudo-exhaustive, pseudo random pattern generator and deterministic.

        In exhaustive testing, test pattern generator and fault models are not required and also not required to store the test vectors in the chip, here the test pattern generated by LFSR (Linear Feedback Shift Register) with nonlinear circuitry added to include all zero pattern. This method cannot be used for large number of inputs since it takes more time for testing the circuit. To overcome these disadvantages pseudo exhaustive testing [4], verification testing [5] and testing using random inputs [6] can be used. The above mentioned method give high hardware overhead .To reduce the hardware overhead for systems where it continuous functioning is of utmost importance, online concurrent testing is the only solution.


        Let us consider the combinational circuit that has n inputs shown in fig 2 which can generate 2n possible test vectors. The proposed method based on the idea of monitoring input vectors every times it checks the occurrence of hit, when test vectors are matched with input if hit occurs then the RV in enabled. The applied input vectors are subdivided into two groups, higher order (H) and lower order (L) bits respectively.

        Figure 2. Proposed architecture

        Higher order bits is used to check whether the input has been into consideration. Low order bits is used to identify the location of the incoming input vectors. The proposed architecture is shown in fig 2. In this architecture T/N signal is used the mode of operation. When T/N=0 the architecture operates in normal mode that time the normal input is applied to CUT and the same input is applied to the CBU as higher order and lower order bits. The higher order bits are applied to the one input of the comparator and the other input is applied from the TG output. The lower order bits are applied to the decoder.

        Fig 3 shows the operation of the modified decoder of proposed architecture it operate as follows, when tge signal is enabled all the outputs of the decoder is enabled. When cmp is disabled all the outputs of the decoder is disabled, when tge is disabled and cmp is enabled the module operates as normal decoder.

        Figure 3. Modified decoder circuit used in proposed architecture

        Figure 4. Proposed Architecture for n=5, w=3, and k=2

        The logic module of the proposed architecture is shown in fig 4. It consists of group of cells, sense amplifier, flip-flop and w-stage counter .The logic module is used to store the test vectors. The counter drives the tge signal through unit flip- flop delay. By using the clock signal the clk and clk is enabled. We describe the operation of logic module, presenting the following cases: 1) reset of the module; 2) hit of the vectors 3) tge operation.

        1. Reset Of The Module

          The reset signal is applied to the module. After the reset signal is applied because of this the tge signal is activated and the output of all the decoder is enabled. (Fig 3)

        2. Hit Of Vector

          The same set of input is applied to both CUT and CBU, TG in the CBU generates the test vectors for the applied input. The higher order bits of the normal input is given to comparator, test vectors generated by TG is applied as another input of the comparator. The comparator compares both the input if there is a match hit occurs then cmp is enabled. If there is no hit then tge signal is enabled.

        3. Tge operation

          When all the cells are full, then the value of the w- stage counter is all one. Hence, the activation of the rve signal causes the counter to overflow; hence in the next clock cycle (Through the unit flop delay) the tge signal is enabled and all the cells (because all the outputs of the decoder of Fig. 3 are enabled) are set to zero. When switching from normal to test mode, the w-stage counter is reset. During test mode, the w-bit output of the counter is applie to the CUT inputs. The outputs of the counter are also used to address a cell. If the cell was empty (reset), it will be filled (set) and the RV will

          be enabled. Otherwise, the cell remains full and the RV is not enabled.


        1. Normal mode output

        2. Hit not occur

        3. Occurrence of hit

        4. IC view

            1. Device Utilization


During off-line BIST the normal operation of the circuit is get stalled to perform testing. The proposed techniques operates on on-line BIST it overcomes the performance degradation which was causes by off-line BIST where the TG in the Concurrent BIST Unit (CBU) generate the test vectors for the applied inputs. By using comparator we can compare the normal inputs with test vectors. If any match is found between these two inputs hit occurs and also the proposed schemes perform testing on the order of few Pico seconds. Simulation has been carried out using Xilinx and by using modelsim waveforms are analyzed.


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