In this section we will elaborate on these fault models. For detailed discussion on exhaustive list of faults and modeling techniques the student is referred to [4,5,6].

2.1  Stuck-at fault

Stuck-at-fault in memory is the one in which the logic value of a cell (or line in the sense amplifier or driver) is always 0 or always 1.

Figure 2 shows the state transition diagram for a good memory cell. S0 is the state where the cell contains 0, while is S1 is the state where it contains 1. w1 (w0) indicates value of (0) 1 being written. The self loop from state S0 indicates 0 being written to the cell. Similarly, all transitions can be explained.

Figure 3 illustrates state diagrams for a cell under s-a-0 and s-a-1 condition.

Figure 3. State diagram for a s-a1 memory cell and s-a-0 memory cell

2.2  Transition fault

In transition fault a cell fails to make a (0 to 1) transition or a (1 to 0) transition when it is written; up transition fault is denoted as and a down transition fault is denoted as . Figure 4 shows state diagram for up transition fault. It may be noted from Figure 4 that a cell having up transition fault is same a s-a-0 fault, however, the cell can take and retain value 1 if a 0 has not yet been written to the cell. The dual happens for down transition fault.

Figure 4. State diagram for up transition fault in a memory cell