Knowing When To Panic

By Bhanu Kapoor
Sometimes we hear that the number of power domains in SoCs have increased significantly and that makes power management verification difficult. True, the numbers have gone up from say 2 or 3 to between 7 and 10, but these are not large numbers by any means and you can write tests to ensure that each of these power domains are covered for power related tests. Power-related tests refers to ensuring that sequencing related to power down and power up (timings of clock gating, reset, switching supply, isolation, and retention), as well as increasing and decreasing voltages in connection with voltage scaling are being done correctly. One point to note here is that power up sequence of a domain is completely distinct from power down, and similarly, voltage going up is distinct from voltage going down.

Then we sometimes hear that the difficulty really arises because of the number of possible power states and not just because of the increasing number of domains. This is backed up with a statement such as: If you have 7 power domains in a design then you could be dealing with 128 power states if you only consider on/off states and many more if you include different on voltages. True, but that’s not what you really see in practice. Let’s take an example of a cell phone processor (such as the one presented by Infineon at ISSCC 2006) and we find that the chip has 7 power domains and has only 11 possible power states. These power states account for the device being used in states such voice call, voice call with hands-free, sleep idle, paging idle, 3 different types of data download with different download bit-rates, video encoding, video telephony, music replay, and music replay with paging. It has 11 power states but not 128 or even more that can possibly exist. One can again write tests to ensure that each of these power states are covered for power related tests. So where’s the problem in getting power management verification coverage?

Each of these power states assign on and off power states to each of the power domains. The on state of a domain is further differentiated with different voltage/frequency possibilities. The real problem in verification coverage is in ensuring that all possible power state transitions are covered. On a cell-phone chip, a switch from the music mode to a video mode is completely different than a switch from the video mode to the music mode. It’s because of the points that we noted at the end of the first paragraph. And with only 11 power states, where you allow all possible transitions of power states, you are looking at 110 distinct power switching possibilities. This creates a verification coverage challenge. Since the number of state transition grows quadratically with respect to the number of power states in the design, future complex and feature rich power-managed devices are likely to find power management verification coverage even more challenging.

Bhanu Kapoor is the president and founder of Mimasic, a low-power consultancy.