Magic Blue Smoke

 

Physical Structure of Special Cells (Cont’d)

last week we saw physical structure of LS/ISO cells. Today lets look at

(a) Retention Registers: These are typically either dual height cells or single height wide cells. They have Primary Rail and Secondary Rail. Primary Rail is switchable and Secondary Rail supplies power, when Primary Rail is off. They are typically placed in a shut-down region. One important factor is, Secondary Rail need not be of same potential as Primary Rail in ON state. Having a potential just enough to retain the state while primary is off would be the most efficient way to optimize leakage. Higher the Secondary Rail potential more will be the standby leakage current.

One of the biggest challenge in the design process is how can we automatically identify, number of registers in the switchable block to be retained? This could be done primarily based on the design architecture knowledge and also based on how fast one wants the switchable block to be up and running. Latter part can be automated in a way using some of the formal/property checking tools.

Other challenge is in verifying the functionality of Retention Register during functional simulation. Some of the verification challenges are listed below.

(i) Simulate Save/Restore Functionality.
(ii) Verify the functionality of ” When Save is asserted Secondary Rail is already active with the required potential”.
(iii) Verify the functionality of ” When Restore is asserted Primary Rail is completely ON”.
(iv) When Retention Register is in Save mode and if Secondary Rail drops(because of adjust block waking up) to a value below the required potential, how do we recover ?
(v) What if Save/Restore Lines are not treated as ALWAYS_ON ?

……etc

Lets look at the the other 2 important cells in the next post.