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DFFRAM Compiler

Open in Google Colab

License: Apache 2.0 CI Status Invite to the Skywater PDK Slack Code Style: Black

Standard Cell Library based Memory Compiler using DFF/Latch cells.

✨ Installation & Usage

See this document.

You can try the Google Colaboratory, but it is a bit out-of-date at this point.

Platform Support Status

Configured Platform Working Silicon-proven*
sky130A Yes Yes
sky130B Yes No
gf180mcuD No* (Hold violations in the Netlist) No

* Silicon proven does not imply that you should use it without whole-system, timing-annotated simulation to make sure that it works for your circuit.

There may be design-specific complications that may render DFFRAM (and indeed the entire chip) unusable. Proceed with caution.

Overview

The objective of this project is to develop a DFF/Latch-based RAM, Register File and Cache custom compilation flow that utilizes standard cell libraries following a standard ASIC (Application Specific Integrated Circuit) implementation approach. Different views (HDL netlist, HDL functional models, LEF, GDS, Timing, …) are all generated for a given size configuration.

The layout targets highly compact designs (85%+) as the cells are placed on the floor plan using a custom placer. Moreover, the custom placer ensures that the routing will be relatively simple. Currently, the compiler uses OpenROAD routers to route the macros with great success.

The Compiler relies on basic building blocks to construct the layout of different RAM/RF/Cache configurations. Check the compiler documentation for more info. The following shows how a 32x32 memory (DFF based) is constructed.

The generated layouts by the DFFRAM compilers for RAM32 as well as its building blocks are as follows:

  • First, a byte, which is just 8 bits placed together... GDS layout of a byte

  • Put four of those side by side, and you get a 32-bit word... GDS layout of a word

  • Stack eight of those for an 8 word bank of RAM... GDS layout of 8 words stacked vertically

  • And stack 4 of these 8 words for a kilobit of RAM! GDS layout of 4x8 words stacked vertically

  • We can keep going, but these images aren't getting any smaller. As a bonus though, here is 64 kilobits: 8kbytes

That stuff you see on the right of each image? It's clock gates, decoders and the like. Don't worry about it.

Currently, the can compiler generate the layout of the following configurations:

1RW1R variants are temporarily disabled due to a bug.

  • RAM
    • 32 words with byte write enable (1RW and 1RW1R).
    • 128 words with byte write enable (1RW and 1RW1R).
    • 256 words with byte write enable (1RW and 1RW1R).
    • 512 words with byte write enable (1RW and 1RW1R).
    • 1024 words with byte write enable (1RW and 1RW1R).
    • 2048 words with byte write enable (1RW and 1RW1R).
  • Register File
    • 32 x 32-bit words (2R1W)

The OpenLane/ folder will contain good known OpenLane configurations to build DFFRAM different macros.

File Structure

  • .github contains files for GitHub actions.
  • docs/ contains documentation (😮)
  • rtl/ contains RTL RAM file generators for benchmarking and comparison purposes.
  • platforms/ contains PDK-specific files:
    • <pdk-name>/
      • <scl-name>/
        • _building_blocks/ contains a hierarchy of building blocks supported by the compiler.
  • placeram/ is the custom placer Python module.
  • scripts/ has assisting scripts used by the flow.
  • dffram.py is the compilation flow going from building blocks to LVS.

Comparisons

The following table compares the areas and bit densities of RAM macros generated using different means.

Size1 OpenRAM2 DFFRAM Compiler DFFRAM/OpenLane RTL/OpenLane
Dim WxH (μm) Bit Density (bits/mm2) Dim WxH (μm) Bit Density (bits/mm2) Dim WxH (μm) Bit Density (bits/mm2) Dim WxH (μm) Bit Density (bits/mm2)
512 bytes N/A N/A 396.52 x 388.96 26,557 527.46 x 527.46 14,722 680.25 x 690.97 8,714
1 kbytes 386 x 456 46,541 792.58 x 397.12 26,027 975.01 x 985.73 8,523 1,050 x 1,060 7,360
2 kbytes 659.98 x 398.18 62,372 792.58 x 786.08 26,297 1374.46 x 1385.18 8,605 1,439.615 x 1,450.335 7,847
4 kbytes 670.86 x 651.14 75,014 1,584.24 x 788.8 26,196 1940.45 x 1951.17 8,654 2,074 x 2,085 7,578
8 kbytes N/A N/A 1,589 x 1,572 26,229 TBD TBD 2,686.610 x 2,697.330 9,043

1 All support 32-bit word reads and 1, 2, and 4 bytes writes.
2 Values are based on the original layout produced by the compiler. OpenRAM macros are typically wrapped to be useful w/ automated PnR ASIC flows.

⚖️ Copyright and Licensing

Copyright ©2020-2023 The American University in Cairo

Licensed under the Apache License, Version 2.0 (the "Open Source License"); you may not use this file except in compliance with the Open Source License. You may obtain a copy of the Open Source License at the root of this repository (see the file 'License') or at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the Open Source License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the Open Source License for the specific language governing permissions and limitations under the Open Source License.