Memory testing in Java Generator Code 3/9 in Java Memory testing

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, Sm 1 d Code 39 Full ASCII for Java escribe the condition for sensitizing the fault; Si {0, 1, , } for 1 i m 1.. Single-cell faults The memory cell arr j2se Code-39 ay faults involving only a single cell are: the stuck-at fault, stuckopen fault, transition fault and data retention fault. Stuck-at fault: The stuck-at fault (SAF) can be de ned as follows: The logic value of a stuck-at cell or line is always 0 (an SA0 fault) or 1 (an SA1 fault), i.e.

, it is always in state 0 or in state 1 and cannot be changed to the opposite state. The notation for an SA0 fault is /0 (Note: denotes any operation; {0, 1, , }); and for an SA1 fault /1 . A test that has to detect all SAFs must satisfy the following requirement: from each cell, a 0 and a 1 must be read.

A state diagram for a good memory cell is shown in Figure 14.8(a). The cell contains the logic value 0 in state 0 (denoted by the node labeled S0 ), and a 1 value in S1 .

When a write 1 (denoted by the arc with label w1 ) operation takes place in S0 , a transition is made to S1 ; when a w0 operation takes place in S0 , the cell remains in S0 . Figure 14.8(b) shows the state diagram for a memory cell with an SA0 fault, while Figure 14.

8(c) shows the state diagram for an SA1 fault; regardless of the type of write operation (a w0 or a w1 operation) the cell remains in the same state. Stuck-open fault: A stuck-open fault (SOpF) means that a cell cannot be accessed (Dekker et al., 1990), e.

g., due to an open WL (see Figure 14.2).

When a read operation is performed on a cell, the differential sense ampli er has to sense a voltage difference between the bit lines (BL and BL) of that cell. In the case of an SOpF, both bit lines will have the same voltage level. Consequently, the output value produced by the sense ampli er (SA) depends on the way it is implemented: The operation of the SA is transparent to SOpFs.

When the SA has only a single input (Figure 14.6(a)), an SOpF will always produce a xed output value. The SOpF will appear as an SAF.

The operation of the SA is non-transparent to SOpFs. The implementation of most differential sense ampli ers (see Figure 14.6(b)) is based on a latch.

An SOpF may have the effect that the latch is not updated because of the negligible difference between the voltage levels of BL and BL. A read operation will then produce the value of the last, i.e.

, previous, read operation. The notation for this fault is /L , where L denotes the value produced by the last read operation. Transition fault: A special case of the SAF is the transition fault (TF).

It is de ned as follows: a cell or line which fails to undergo a 0 1 transition when it is. 14.3 Reduced functional faults w0 S0 w1 w0 S1 (a) State diagram of a good cell w0 S0 w1 (b) SA0 fault w1 S0 w0 ^ Figure 14.8 State diagrams for single-cell faults w1 S1 w0 (c) SA1 fault S1 w1 written is said to contain an up-transition fault; similarly, a down-transition fault is the impossibility of making a 1 0 transition. The notation for the up-TF is /0 , and for the down-TF /1 . If a cell has a /0 TF and is in state 0 upon power-on, it is effectively an SAx cell (where x = 0); when it is in state 1 upon power-on, it can undergo one 1 0 transition.

The same argument can be used for /1 TFs. A test that has to detect all TFs, should satisfy the following requirement: each cell must undergo a transition (state of the cell goes from 0 to 1), and a transition, and be read after each transition before undergoing any further transitions. The state diagram for a memory with a /0 TF is shown in Figure 14.

8(d). The cell may be in S1 , e.g.

, after power-on; once the cell has entered S0 it cannot leave that state any more. The fault is sensitized by a w1 operation while in S0 ; a successive read operation will detect the fault because a 0 will be read while the expected value is a 1 . Data retention fault: A data retention fault (DRF) occurs when a cell fails to retain its logic value after some period of time (Dekker et al.

, 1990). A DRF may be caused by a broken (open) pull-up device within an SRAM cell (see Figure 14.2).

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