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Forums > Digital IC > Floorplanning, Place and route

Subject: Problem with addWellTap command
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EngHan
Posts: 65
Online: User is Offline
5/19/2007 6:55 PM  
Hi,

The standard cell I am using does not have substrate and n-well tap, so I have to placed well tape cell at regular interval in the core.

My perference is to use place them in checkerboard fashion. But I am hit with what I believe is a limitation in the command addWellTap. It is not easy to explain, so please be patience.

Let assume the max distance between the well tape is 40um (to simplify, we ignore the width of the well tap and the height of the cell row). So the command to add well tap should be:

addWellTap -cell wellTap -cellInterval 160 -checkBoard

The cell interal is 160um because a well tap can cover 40um on both side. Also, in between the two cell taps on the same row, there are two well tapes on the upper and lower row which are exactly in the middle. Hence the spacing is 160um. The following drawing explain this:

(Note: X is the tie cell)

           X             <---80um--->X 
X<--------------------->X<----------160um--------->X
           X             <-40-><-40->X  


The problem occur when the upper or lower row is not available to place the well tap cell (e.g. due to macro or placement blockage). In this case, the tool shold place the well tie cell 160/4 or a max of 40um away from the edge of the row. The tool now place it 160/2 or 80um away. This is incorrect from my application of the command:


.............             X
... Macro ...     X
.............             X
.............<-!->X
.............             X
      X           X
X                  X                      X


The distance marked "!" above is less than 160/2, but larger than 160/4. So an DRC will occur here.

Is there a workaround or hidden switch to get this command to work? Thanks.

Regards,
Eng Han
mohanch007
Posts: 52
Online: User is Offline
5/20/2007 9:50 PM  
Hello Friend ,
How about using the below options -fixedGap with addWellTap :

If not can you move your Macrocells accodingly to align .


Examples
The following command preplaces a well-tap cell xyz on every row in the floorplan, with the distance between two consecutive cells not exceeding 40 µm.

addWellTap -cell xyz -prefix welltap -maxGap 160 -fixedGap 40

And you can add option : -fixedGap


FYI.

-fixedGap
Ensures that the specified gap is always maintained. That is, this will not search for a legal location if it cannot place well-tap cells at the given -maxGap. Instead, it goes on to place well-tap cells at the next position.

-maxGap microns
Specifies the distance, in micrometers, from the right edge of one well-tap to the left edge of the following well-tap in the same row. If well-tap cells cannot be placed at this gap due to placement blockage, placement will search for a legal location: first towards the previous well-tap, and then away from the specified location. Under no circumstances can the gap between well-taps be less than 45% of the specified maxGap value plus the well-tap width. The distance is measured from the center of the cells.
EngHan
Posts: 65
Online: User is Offline
5/21/2007 12:55 AM  
Hi Mohan,

It is not easy to shift the macro as the design has more than 100 macro of different sizes.

The option "-fixedGap" does not take an argument. So we can only either say "-maxGap 160" or "-cellInterval 160". The issue is that the tool will always take 160/2 as the maximum allowable distance. This is not true for checkerboard style.

More info on where the issue can happened:
1. Between the left side of the core to the left side of a macro (a small area for buffering and JTAG cell)
2. Between the right side of a macro to the right side of the core (a small area for buffering and JTAG cell)
3. on the left and right side of the macro
4. A single row betweent two macros (at the right side of these rows)

Regards,
Eng Han
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Forums > Digital IC > Floorplanning, Place and route > Problem with addWellTap command


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