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Impact of Power Straps on Die Size
If we have a metal power supply using fraction p of the metal resources, and a core side of x, then the increase in the length of the core is not (x×p)%.
referring to the figure on the right.
For example, for a power strap percentage of 15% both horizontally and vertically, this means that the increase in the core side is not 15% but 17.6%.
We can see this from the drawing on the right. Before power strap insertion, the die area is
and after power strap insertion it is
|A =||x2 × (1-k(2)p) × (1-k(3)p)|
So that the ratio between the areas after and before power strap insertion is
|(1-k(2)p) × (1-k(3)p)|
and the ratio in the length of the side is
In the special case where we make the horizontal and vertical power straps equal, then k(2)=k(3)=1 and a core side of length x will increase to
Both metal-1 and metal-3 horizontal power straps will block the placement of standard cells underneath. This means that one should aim to keep the fraction of metal-1 used for power straps the same as the fraction of metal-3. That is, p1 = p3, and they are used interchangeability in this paper.