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Feedthrough insertion is a subtly crucial task that naturally arises in hierarchical digital design. There are several types of approaches we can use to allow signals to traverse across a chip, but the most common and effective I've seen is where buffers are inserted in neighboring partitions. This eliminates top-level routing and more importantly takes the top-level timing closure task and makes it part of block-level timing closure. In Encounter this capability is accessed through the TCL command insertPtnFeedthrough.
Here's a picture to describe what I'm talking about. Say we have 3 partitions: a, b, and c. A signal that originates in "a" and connects to "c" traverses over "b":
If we don't perform feedthrough insertion, what we'll get is a net that traverses over partition "b". This is unacceptable in scenarios where all layers are reserved for partitions, and in scenarios where some layers are reserved for over the block-routing we get into a difficult situation because we can't insert buffers on nets that exceed the maximum distance required for rebuffering. See below what the design would look like after buffering without feedthrough insertion:
If we use insertPtnFeedthrough, the tool will:
Once the buffers have been inserted we can place the partition pins (assignPtnPin) and commit the partitions (partition is the command). The result should be a design where connections between neighboring partitions consist of only 2-pin nets between abutted edges (or in the case of design with small roughly 10 microns channels, short jumper wires connecting the partitions):
So that's how feedthrough works in its simplest sense. In this context it's tempting to consider writing a script to do this at the logic synthesis stage. But feedthrough insertion is much more complex in practice. In upcoming blog posts I plan to focus on more advanced scenarios to give you a sense of what to prepare for in terms of making sure the tools you're using are up to the task.
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nice explanation for a fresher to get an idea..