Full-Custom Layout of an SRAM-Based FPGA - University of Toronto

4. Routing
Routing consists of the 4 wiring tracks running horizontally and 4 running vertically.
They can be interconnected inside the switch boxes using programmable switches. For
this purpose, we plan to use pass gates using only nMOS transistors, as shown in the
following figure.
The figure shows that any track can connect to any of the three possible directions
(continue the same way, or turn left or right), or more than one direction, thus providing
the fanout. Since there are 4 tracks coming in from each side, and 6 transistors (and
corresponding SRAM cells) are required to be able to route each of them in any direction,
we will use segmented routing. Segmented routing means that not all of the wiring tracks
are of the same length, but there are segments of various lengths. In our case the chosen
lengths are 1, 2, 4, 4, as shown in the following figure.
As can be seen from the figure, if the segments are distributed in a proper way, there
are only 8 wires (2 from each side) coming into the switch box, which reduces the
number of transistors (and SRAM cells) needed to implement routing to 12.
Since our routing uses pass transistors, we will use feedback transistors on all the
inputs coming from the wiring tracks and other pass transistors, to pull the signal back to
VDD, as shown in the following figure.