QUESTIONS ON LOW POWER

What is isolation cell ?

The cells which are used to isolate the output of the power down domain is called isolation cells.

Why output of the power down domain is isolated ?

The signals of the power down domain reaches to some intermediate values due to power gating effect and if those signals are fed to the power up domain, than it will effect the functionality of the power up domain. So to avoid that we isolate the outputs of the power down domain.

What is crowbar current?

When a particular power domain is shut down than outputs of it reaches to some intermediate vales due to the power gating effect and when it is feed to the power up domain inputs than it results in the crowbar current.

What is isolation enable signal ?

Isolation enable signal decides when the output of the power down domain should be isolated.

What kind of cells are used for isolation?

For clamp "0" we used AND gate and for clamp "1" we use OR gate.

What is level shifter ?

When we want to shift a signal from one voltage level to another than we use level shifter. Level shifters are generally used in the multiple voltage designs.

What is AON cells ?

When there is a power domain which is at shutoff condition and some logic of that domain on need to be "ON" at that particular time than we use always on cells in that case. AON cells have two power supplies one is primary and another is secondary.

What are power switches ?

The switches that are used to shutoff the pull up or pull down network when the design is not in functional, than we call it as power switches.

What is power gating ?

The technique in which either pull up or pull down network is shutoff with the help of switches, than we call it as power gating mechanism.

What is coarse grain and fine grain power gating techniques ?

In fine grain power gating technique, for each cell we use one power switch and in coarse grain for bunch of cells one power switch is used.

In fine grain technique cell density is more so generally it is not preferred and coarse grain technique is used widely.

What is rush current ?

When a power domain is in shutoff condition and it is powered on suddenly, than all the internal capacitors start to charge simultaneously and huge charge is drawn which leads to the rush current.

What is ramp up time ?

The time by which a shut down domain is powered up is called ramp up time.

What is header and footer switches ?

The power switch which is used to gate the VDD we call it as header switch and the switch which is used to gate the VSS than we call it as footer switch.

What happens if we will not use level shifter in multi voltage designs ?

If the voltage given to the input of the transistor exceed the breakdown voltage, than the dielectric layer damages. Breakdown voltage is very large than the supply voltage. So in a multi voltage design when there is a signal propagating from high to low voltage domains, then it may lead to breakage of the dielectric layer if that voltage exceeds the breakdown voltage. When a signal is propagating from lower to higher voltage domains, than it may happen that this low voltage cannot exceed the threshold voltage, in that case transistors will not become on and it will affect the functionality. To avoid that that we use high to low level shifter and low to high level shifters.

What is Intra polation ?

When the tran and cap values lies whithin max limit but not exactly the values present in the table, than tool tries to calulate it to the nearest possible value. This process is called intrapolation.

What is Extrapolation ?

When the tran and cap values lies above the max limit than tool tries to map it the highest value in the look up table, this process we call it as extrapolation. With this process as the delay values calculated are less as compared to the actual delays, tool falsely meet the setup timing.