Magic Blue Smoke

 

Physical Structure of Special Cells

Today let me give a quick insight into various special cells and their physical structure

(a) Level Shifters: In general there are 3 types of Level Shifter.

(i) High2Low LS: Shifts the Voltage from High 2 Low. Physically these are single row height cells and have 2 rails VDD and VSS. Conceptually they are nothing but high drive buffers and nodes gets charged upto the VDD(low), even though input is driven from higher voltage source. One of the key requirement for this kind of cell is they got to be placed in Lower Voltage Domains.

(ii) Low2High LS: Shifts the Voltage from low to high. Physically these are 2 row height cells and has 3 rails, Primary VDD(Low), Secondary VDD(High) and VSS(Ground). Ideally these are placed in Lower Voltage Domains.

(iii) Generic LS: These cells can shift the voltage in either direction. They are generally used in designs that deploy DVFS technique. Physically these are 2 row height cells.

(b) Isolation Cells: In general there are 3 types of Isolation cells.

(i) Simple Isolation Cells: Physically these are single height cells
(ii) Enable Level Shifters: These are combination of a Isolation cell and Level Shifter. Physically they are similar to level shifter explained above
(iii) AON Isolation cells: These are dual rail cells, physically these are 2 row height cells and has 3 rails, Primary VDD(ON/OFF VDD), Secondary VDD(AON VDD) and VSS(Ground)

I will discuss about the other special cell’s physical structure in my next post tommorrow.