 # [Part 1]

### Solution to these challenges:

The way we can solve this is by inserting a Level Shifter between the two power domains. A level shifter takes an input (say, S1) from one domain (B) and 'translates' the input voltage level (0.9V) to a value (3V) such that the logical value of that signal (1'b0) remains the same in both domains. A level shifter may translate the input voltage up or down depending on the power domain where the output goes to. This is shown as P1 and P2 in Figure 3. ## What happens when you shutdown a power domain?

When a power domain is shut off, any signal generated by that domain becomes logical X. This creates two unique challenges to other domains that are not shut off at that time. As an example, let us consider the case when domain B is shut off when domain C is not. Of particular interest here is the signal S1 that runs from B to C.

First, let us assume the case when S1 is not required in domain C when domain B is shut off. Even though it is not required, S1 will propagate X through C. There has to be some way to block X propagation of S1 in C.

The second case is when the last legal value of S1 (when B was ON) is still required in C even when B is shut off. In this case, we need special arrangement to retain the last legal value of S1 and supply it to C.

### Solution to these challenges:

A solution to the first case above (when S1 propagates X) is to stop S1 to propagate X by passing it through an Isolation Cell whose output is known (can be either a stable 1'b1 or 1'b0). (Imagine P1 in Figure 3 is now an isolation cell).

The solution to the second problem is to use a Retention Cell. A retention cell remembers the last legal value of the signal S1. (This time assume P1 in Figure 3 is a retention cell).

## Next time

So far we have discussed the problems of designing low power systems. We have also seen how these challenges can be mitigated using special-purpose logic cells, such as, a level shifter, a retention cell or an isolation cell.

The next logical question is: can we specify the power domains in a structured way so that the design process itself takes care of instantiating these special-purpose cells? This is the question that motivated the use of a modeling language to define the power domains. Next time we will see how this is done using Unified Power Format (UPF).    Join Verification Management Group

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