Page 282 - A Practical Guide from Design Planning to Manufacturing

252 Chapter Eight

A1 B1 C1
A1 B1 C1

A0 B0 C0
A0 B0 C0

Without bit pitch With bit pitch
Figure 8-9 Bit pitch.

completely device or wire limited. Variations in layout density can be
because of varying levels of skill and experience among different mask
designers, but often differences in density are because of the different
types of circuits being drawn.
To try and improve layout density and allow more reuse, most layout
is drawn with a specific bit pitch. It is common that many different bits
of data will have to go through similar logic. It is convenient if each row
of layout that processes a single bit has a uniform height. Figure 8-9
shows how assembly is more efficient for cells of a uniform bit pitch.
If layout cells A, B, and C are each drawn with a different height, then
a great deal of area is wasted when trying to assemble them into two iden-
tical rows. By creating each cell with the same height and only varying
the width as needed, assembly is easier and more efficient. The bit pitch
is typically chosen based on the number of wires expected to be routed hor-
izontally within each row. The layout density of some cells will inevitably
suffer as a result of being forced to maintain a uniform height. The high-
est density for each cell would be achieved by allowing each to be drawn
only as high as was optimal for that cell alone, but after the cells are
assembled the overall layout density will typically be far higher with a
uniform bit pitch. The area saved in assembling large numbers of cells
together can easily outweigh any area wasted inside the cells.
Acommon and painful mistake when estimating the space required for
routing signals within each bit pitch is forgetting to allow tracks for a
power grid. In addition to the signal lines routed to each logic gate,
every gate must have access to supply (V ) and ground (V ) lines. All
dd
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the power supplied to the die will come from the chip package to the top
level of metal. The V and V voltages must be routed down through each
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level of metal wiring to reach the transistors themselves. All the lines
running in parallel at each level are tied together by the perpendicular
lines of the levels above and below, forming a continuous power grid.
This grid acts like the electrical power grid of a city, supplying power
to each logic gate the way electricity is routed to each home. In cities