About DECAP filler cells used in floorplaning

[pandit_vlsi]

filler cell decaps

hi all.
1.I want to know why these are used.If they are used in reducing PSN ,why cant we use Bypass for the same purpose.
2.What the structure of these cells.
3.How they help in reducing power supply noise.
4.How they are diffrent from Bypass capacitors
I
am in desparate need of these thigs.iam preparing a report.so if possible some links &books.
...pandit

 

[coolrak]

Hi Pandit,

Decaps are used to provide the power, to meet the power requiremnets when the dynamic switching happens. Decaps are placed similarly how u place filler cells. Normally put where there is more switching activity .

 

[linuxluo]

hi,
decap is used for IRDrop. in fact it is a capacitor cell which save power normal and provide power when circuit switching. and normally it is under power line .

 

[eternal_nan]

In response to OP: They are bypass capacitors, no more - no less. One thing to keep in mind when using them is that although it is easy to just fill all empty space with them, there is a down side - leakage power consumption, so they should be judiciously used.

 

[nav_vlsi]

decap is one solution to many power ground integrity issues. mainly it reduces ground and supply bounce. dynamic IR drop occurs whenver there is a switching in cell , when ever logic 0 to 1 transition occurs , the load capacitance has to discharge via VSS power line.if this is happening too fast , there wont be sufficent time for nmos to fully discharge load cap which results in an elevated ground. similarly for 1 to 0 transition in VDD line. as a result of this get supply bouce.when decap are inserted they get charged , and during switching they act as a local power source, there by reducing bounce.

the main disadvantage of using decaps is that its bulky and consume large amount of useful chip area

so carefully handle them