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| -rw-r--r-- | docs/bel_and_site_design.md | 25 |
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diff --git a/docs/bel_and_site_design.md b/docs/bel_and_site_design.md index fd85064..8f4519b 100644 --- a/docs/bel_and_site_design.md +++ b/docs/bel_and_site_design.md @@ -9,6 +9,15 @@ enables a place and route tool to succeed. Good design is hard to capture, but this document will talk about some of the considerations. +### Assumptions about cell placement and driver BEL pins + +One important note is that BELs represent a placable location for a cell, and +only one cell should be placable at a given BEL. This means that the cell +library design and BEL design strongly affects what is expressable by the +place and route tool. There will be some examples highlighted below that +expand on how this is important and relevant when discussing concrete +examples. + ## Granularity of the cell library It is important to divide the place and route problem and the synthesis @@ -188,3 +197,19 @@ So given the Versal site layout, the following BELs will be required (per SLICE - 1 LUT4 BELs that connect to the carry - 2 LUT5 BELs that connect to the output FF or output MUX - 1 LUT6 BELs that connect to the output FF or output MUX + +#### Implication of a wider BEL definition + +Consider the Versal structure, but instead of drawing four BELs per row, only have two +BELs per row. One BEL has the `O5_1` and `prop` output BEL pins and the +other BEL has the `O6` and `O5_2` BEL pin. In this configuration, if the cell +library does not expose a cell that maps to both the `O5_1` and `prop` output +BEL pins, then it will not be possible to map LUTs that leverage both output +BEL pins. + +In theory, the cell port to BEL pin map could map the output pin of a LUT4 +element to both the `prop` and `O5_1` output BEL pins, but then there will be +two output BEL pins driving the net connected to the cell port. Having +multiple BEL pins driving one net is not legal, except for the global logic 0 +and 1. + |