ISOLATION CELLS

 

         Isolation cells are placed  between two power domains . These cells are introduced at Synthesis stage to isolate the logic from Power ON/OFF domain ( Gated Domain) to AON  Domain .   

       Lets  assume PD_1 and PD_2 is two power domains  and Signal passing between from PD_1 to PD_2 . 

     At Normal conditions there is no issues , its simply passes the signal from PD_1 to PD_2  either 0 or 1  value based on PD_1 conditions But when PD_1 is Off & PD_2 condition which value will passes to PD_2 domain ?  The value of signal is either 0 or 1  not sure which one will passes , Hence its corrupt the functionality.

     

     To avoid this situvation we will use ISOLATION cells between the power OFF domain to other domains as seen in below . 

      

     In normal conditions ISOALTION cell act as Buffer , when PD_1 is OFF mode  the isolation cells clamps the output value either 0 or 1. Enable signal comes from the PMU( Power Management Unit )  module .

       ISOLATION cell as isolation enable signal that determine whether the isolation cell should provide isolation from PD_1 domain or act as buffer .  

           Generally there are two types of ISOLATION cells based on its clamp value . If its clamp value "0" , synthesis will insert by AND gate with input connected enable signal  from the PMU module .If its clamp value is "1" ,  Synthesis will insert by OR gate  with the input connected  inverted enable signal comes from PMU . Lets  see the below picture for better understanding . 

   

  Kind of Issues will see with ISO cells / Strategies  : 

• An isolation cell required for crossing signal but its not  present in the design  or Not in the UPF .
• An isolation cells present in the design but ISO strategy missing in the respect UPF file . 
• An Isolation cell missing in the design but Strategy is defined in UPF file .   
• Two different clamp values are present for the same Isolation cell in UPF file . 
• Clamp value of the isolation cells is not matching with design vs  defined value  in UPF .