Electronic Design

Simulation Mismatches Can Foul Up Test-Pattern Verification

Design for testability (DFT) works to make a circuit more testable to ensure that it was manufactured correctly. Alfred Crouch explains the purpose of DFT in his book, Design-For-Test for Digital ICs and Embedded Core Systems: "To exhaustively test a combinational circuit with N inputs, a sequence of 2N test vectors must be applied and observed to fully exercise the circuit." With that goal in mind, the primary purpose of DFT is to increase testability of a given netlist by increasing controllability and observability.

A design can be considered testable if a satisfactory set of test patterns is generated, evaluated, and applied to improve quality and minimize time-to-market. Therefore, a testable system implies better fault coverage, a shorter testing time, a higher-quality product, and a shorter time-to-market. Two tasks must be accomplished in DFT: generating efficient test patterns with maximum test coverage, and then verifying test patterns. Afterward, these generated test patterns are applied to the real design, and timing information is considered.

During test-pattern verification, problems occur when the results generated by the automatic test-pattern generation (ATPG) tool don't match the simulated results-when timing information is considered. This is known as simulation mismatch.

Scan Design Methodology
The ultimate goal of applying this methodology during a design's creation and implementation phase is to ensure that hardware hooks in the final design will enable the tester to apply test vectors that can achieve a high quality of test. This methodology is applied to the general design structures, and it behaves as combinational logic during test mode.

In functional mode, the design comes in the form of sequential logic. Circuit operation involves many clocks to get data through memory elements. In a sequential design, back tracing for vector generation isn't possible. Therefore, these sequential elements are transformed to pseudo-combinatorial elements using various scan architectures. Designers generally use three scan architectures:

1. Mux-DFF: A mux-DFF cell contains a single D-type flip-flop with multiplexed input lines that allow the selection of either normal system data or scan data (Fig. 1). In normal operation (SC_EN = 0), system data passes through the multiplexer to the D input of the flip-flop, and then to the output (Q). In scan mode (SC_EN = 1), scan input data passes to the flip-flop, and then to the scan output (SC_OUT).

2. Clocked-Scan: The clocked-scan architecture is similar to the mux-DFF architecture, but it uses a dedicated test clock instead of a multiplexer to shift in scan data (Fig. 2). It ensures data hold for non-scan cells during scan loading. In normal operation, the system clock (SYS_CLK) clocks system data into the circuit and to the output (Q). In scan mode, the scan clock (SC_CLK) clocks scan input data (SC_IN) into the circuit and through to the output (SC_OUT).

3. Level-Sensitive Scan Design (LSSD): In normal mode, the master latch captures system data (DATA) using the system clock (SYS_CLK) and sends it to the normal system output (Q) (Fig. 3). In test mode, the two clocks (ACLK and BCLK) trigger the shifting of test data through both master and slave latches to the scan output (SC_OUT).

Scan Operation
The scan chain is a set of cells in which one scan cell's output port is connected to the dedicated scan input port of another scan cell (Fig. 4). In this way, a serial scan chain is constructed. If all sequential elements in a design are converted to scannable elements, the architecture is known as full scan. If some non-scanned sequential elements are left in the design, the architecture is known as partial scan. The test procedure is based on the following sequences:

1. Enable the scan operation to allow shifting.
2. After loading the scan cells, hold the scan clocks off and then apply a stimulus to the primary inputs.
3. Measure the outputs through primary outputs.
4. Pulse the clock to capture new values into the scan cells.
5. Enable the scan operation to unload and measure the captured values.

In Figure 5, the scan cells are shifting the scan data (scan data1, scan data2, and scan data3) with respect to the scan clock. After completion of the shift operation, the scan enable signal is disabled and applied to the stimulus from the primary inputs. Finally, the outputs are measured through the primary outputs.

Once the test circuitry is inserted and the test procedure is established, the ATPG tool can be used to generate the test patterns. However, the ATPG tool will perform design rule checking (DRC) to ensure the test circuitry and test procedure can allow the tool to generate the test patterns with high fault coverage.

Simulation Mismatches During Test-Pattern Verification
Test-pattern verification is based on the device timing information to ensure that the patterns can be applied to the real circuits. If any mismatches occur between the timing-based simulation results and the expected results, the test patterns must be corrected. If test patterns aren't corrected, good devices would be rejected during the manufacturing test process. The most common simulation mismatches are as follows:

Timing Issues: Timing-related issues generally occur because of clock skew between the successive scan cells in the design. If the propagation delay at the data path between two successive scan cells is less than the propagation delay at the clock path, clock skew occurs. Conceptually, it would be ideal to have only one scan clock. But this actually creates more clock-skew problems, due to the multiplexing together of asynchronous clocks.

When connecting flip-flops of different clocks onto the same scan chain, we try to buffer the path from scan out of one clock domain to the scan in of the other to provide hold time during scan shifting. This relaxes the clock-skew requirement between the flip-flops. Clock skew caused by the delay through the multiplexer can introduce hold-time violations in the scan mode. These violations occur when a non-inverted flip-flop Q output is connected to an inverted clock DI (scan) input.

In Figure 6, the second scan cell captures the updated data of the first cell instead of data at the input of the second cell before asserting the clock. This problem can be resolved by increasing the delay at the data path by adding a latch, called a lock-up latch. Another method involves setting an ATPG function to both input cells, which ensures both cells capture the data at the same time. Fix all hold-time violations in scan mode by adding buffers to delay the path (if two clocks are in the same scan chain).

Design-Rule Violations: Non-scanned latches in the pad logic are flagged as violations. But since these are bypassed in the scan mode, they won't affect the coverage. Even if all blocks in the design pass DRC, violations still appear at the top level due to connectivity problems, bus contention, loops, or clock interactions.

Clock-Rule Violations (C3 rule): Clock-rule violations (C3 rule) are handled by simulating multiple events per test cycle. Figure 7 illustrates the single-cycle multiple events. Event 1 corresponds to default simulation performed by the ATPG tools. State elements haven't yet changed, although all combinational logic-including that connected to clocks-has been changed. Event 2 corresponds to a time when level-sensitive and leading-edge state elements have updated as a result of the applied clocks. This simulation correctly calculates values for trailing-edge and level-sensitive state elements.

D7 Design-Rule Violations: A negative flip-flop present in the design causes D7 design-rule violations (Fig. 8a). The clock for this flip-flop is at logic level "1" during the off state. This happens because the clock is inverted and connected to the clock pin of the flip-flops (Fig. 8b). The negative-edge-triggered scan cell in a scan chain can capture data from the positive-edge-triggered scan cell in another scan chain. Unfortunately, the positive-edge-triggered scan cell captures the data before the clock cycle is completed. Therefore, the expected data generated from the tool for positive-edge-triggered cell is old data.

In timing-based simulation, the new data is the updated data of the negative-edge-triggered cell. This is because the negative-edge-triggered cell is capturing the data on the negative edge of the defined clock cycle. Sometimes, we can ignore the D7 violations. Otherwise, we need to inform the ATPG tool to update the scan-cell data.

Bus Contention During Scan Operation: The biggest problem in system-on-a-chip (SoC) designs with respect to ATPG is resolving how to control the internal three-state bus structures. Two careful considerations are necessary to avoid bus contention.

First, make sure there's no contention on the three-state buses during the scan-shifting operation. A scan-insertion tool automatically performs this task. Second, ensure that there's no contention on the internal three-state buses during the capture cycles while scan testing. The ATPG tool can't generate the test pattern for the bus contention, and it uses those test patterns to test the device that may be stressed to the production test. Therefore, avoiding the internal three-state bus contention is most important during test-pattern verification.

The bus contention problem will occur at two levels. The first is within a block design that contains multiple drivers for the three-state port. The second problem is at the chip level, where multiple design blocks interface with the same bus.

Multiple Clock Domains: A clock domain is a grouping of sequential elements sharing a single clock in the design block. If two design blocks share the same clock, then there must be clock skew between the two blocks. Clock skew is important because although it's easy to meet the setup timing requirement, it will create a problem with hold-time violations during scan testing. To avoid the hold-time problems during scan testing, add buffers in the clock path.

Bidirectional I/Os: Handle bidirectional I/Os with care during test-pattern verification. The ATPG tool can generate patterns when the bidirectional I/Os change direction as a result of the capture clock. Testers generally don't support this function. So, you should force the ATPG tool to generate scan patterns that don't change the direction of the bidirectional IOs during the capture cycle.

Finally, test-pattern verification is important prior to testing to ensure that the ATPG-tool-generated test patterns can be applied to the real design and to prevent the rejecting good chips off the manufacturing line. Simulation mismatches will reduce the fault coverage and yield. They should be resolved by analyzing the circuits as described here.

Recommended Reading:
Abramovici, Miron, Breuer, Melvin A., and Friedman, Arthur D., Digital Systems Testing and Testable Design, Wiley-IEEE Press, Sept. 1994.

Basto, Luis, Khan, Asif, and Hodakievic, Pete, Embedded X86 Testing Methodology, Advanced Micro Devices, 1999.

Crouch, Alfred, Design-For-Test For Digital ICs and Embedded Core Systems, Prentice Hall, 1999.

"Design-For-Test" documentation, Mentor Graphics Corp. (www.mentor.com/supportnet/documentation)

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