VLSI Physical Design
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The EDA companies complain about not being able to attract new talent, but maybe if they paid better? The other problem with the EDA companies (as someone who has worked in EDA in the past) is that they're just plain stodgy - they feel like some kind of old boys club. We had C++ code from the early 90s and to a large extent they still coded C++ in a 90s style. C++ Templates? Nope, not allowed in the group I was in.
I mean just go look at that post from yesterday where I complain that yosys uses C++ to script flows and the response to the complaint - I'm starting to think these people just enjoy being miserable.
Software development is unique in that tools for building software are also software. This hammer is great and all, but I make hammers, and I know this one could be better
The real reason is the hardware vendors are entrenched and don't make money off the software at all. The situation is exactly the same as it was in the proprietary compiler space before gcc (and then clang) - no incentive to improve or open source the compilers -> no access to good tools -> proprietary tools remain only option.
Yosys though, being an open source attempt, is just making unforced errors, hence my ranting and raving in the other thread.
(*) Excluding emulators or simulation accelerators.
did you miss the part where the comment i'm responding to is about VTR?
The book is considerably out of date, but a lot of the concepts and definitions are still relevant.
https://www.eng.biu.ac.il/temanad/files/2018/11/Lecture-4-Sy...
A good starting point for more modern techniques would be something like this:
https://people.eecs.berkeley.edu/~alanmi/publications/2006/t...
I don't know how up-to-date the information is. Two of the authors (Korte, Vygen) wrote a textbook on combinatorial optimization. I haven't read it and I don't know enough about the subject to be able to say how useful the contents are to VLSI design. I think they focus more on the theory than the application: https://link.springer.com/book/10.1007/978-3-662-56039-6
Another aspect, at least what I have experienced, is that these tools are built on top of themselves and layered over older versions of themselves, making the code base fragile and hard to work on. For example, Xilinx's Vitis HLS is based on AutoESL, an academic work that underwent commercialization and transitioned into a Xilinx product. You can tell in Vitis HLS that there are many layers on top of simple tool actions that go through custom TCL functions and then down to the most likely Java/C++ software, which also calls out to outdated versions of LLVM tools. I would imagine this is not a pleasant code base to work on and add new features to.
Also, making changes to an EDA tool has large implications for downstream customers who depend on your FPGA chips and are maximizing the utilization on the chips, making them super sensitive to any changes in the EDA algorithms used. This also makes the software development process for these commercials a little sticky. This makes it hard to even plug into and build on top of these tools as an academic.
However, this is no excuse for bad software engineering. I agree that a big investment in software engineering for EDA could be a slam dunk. I think Synopsys and Cadence realize this payoff for their use cases (mostly ASIC), while maybe the payoff is not as big for FPGA companies? Who knows what they are thinking.
On the technical side, more open-source EDA tools help even if they are not fully practical in a commercial setting. Even the C++ codebase for VPR is readable and mostly easy to follow. As a thought experiment, a Rust-based port of that would be interesting from an API and program architecture perspective. There are also some DARPA programs that are pushing for better open-source tools for EDA flows as well as better EDA algorithms for large-scale, 3D, and heterogeneous semiconductor design, so I think they see the technical problems we are behind on.
https://link.springer.com/book/10.1007/978-0-387-93820-2
It doesn't really focus on theory but more the principles of timing analysis and how to wrangle chip design tools into complying with your desired compromises in order to achieve your goals.
https://books.google.com/books/about/Timing.html?id=80gOBwAA...
I’ve worked with Andrew, one of the authors on occasion in the past, and he and his team of students are among the best academic teams in the world on this topic.
I do think a lot of the secret sauce lives as trade secret with Cadence, Synopsys, Mentor… They see all the real problems in designs from all their customers in bleeding edge nodes like 3nm and beyond.
https://books.google.com/books?id=EkPMBQAAQBAJ&printsec=fron...
Handbook of Algorithms for Physical Design Automation edited by Charles J. Alpert, Dinesh P. Mehta, Sachin
The information is available if one looks for it. It's a tough subject though.
That why for commercial PCB packages with support designs with 1000s of nets, such as Cadence's Allegro, one does see support for automatic routing of PCBs. And it's quite good.
Just trying to understand.
What I mean is not all nets are created equal. Is this an edge sensitive gpio or a level sensistive? Is this a net with a cap a current that's being integrated or edge rate control on a clock driver? A person knows because they can read the datasheets. The way for the algorithm to know is routing constraints, and their fairly tedious to write correctly which means for there to be good ROI on the time spent writing them, the algorithm needs to do a lot of work for you.
Yeah, you've listed off ~100 nets vs 1000s+ nets. But...
Many MPUs are BGA200 to BGA400 in size, and seemingly designed for 4-layer to 6-layer boards (available from OSHPark). Which means I expect around... maybe 200 nets or so in practice (a lot of the pins are either power-or-ground. But many other pins would be in fact connected to "something" and be their own net). More than your first number, less than your second number.
In your opinion, is that still within the feasibility of a hobbyist laying things out by hand? A lot of SiP MPUs have "internal DDR2" in sufficient quantities to boot Linux, although I'm also thinking about routing my own RAM for maximum flexibility.
There is a huge difference between autorouting in VLSI and autorouting in PCBs. There are particular differences:
1) The number of available layers
VLSI autorouting pretty much sucked until you had 5+ layers. You pretty much need local, horizontal, vertical, global, and pad as a minimum and the more layers the better the autorouter gets. Note that this is already more layers than generally exist on most PCBs which are typically 4 layer (upper, gnd, power, lower).
2) Changing layers in VLSI is relatively cheap without obstacle while changing layers in PCB is expensive and creates obstacles.
Changing a layer in VLSI is generally a couple of vias and no big deal. The vias are typically smaller than the metal track (but not always) and generally only connect the two layers in question.
Changing a layer on a PCB is generally expensive. PCB vias are generally larger than the metal trace and generally penetrate the entire PCB creating an obstacle on all layers. Take a look at PCB boards for FPGAs: they are generally 8+ layers and use expensive blind vias for routing breakout. Those kinds of PCBs are relatively expensive, and even those don't lend themselves to autorouting that well.
3) A lesser difference is that for high-speed signals PCB physical dimensions can be on the order of magnitude of the size of the wavelength of the signal being transmitted while VLSI dimensions are typically much smaller.
For example, a quarter wavelength for 6GHz is roughly 1cm which is generally bigger than most chips and significantly smaller than most PCBs. The design considerations for VLSI autorouting generally can ignore differences in routing length or layer changes (but not always).
On the other hand PCBs often need to be length matched fairly carefully for things like RAM buses. They also sometimes need to account for the fact that PCB materials are inhomogeneous--your routing track has different electromagnetic properties depending upon whether it is over glass fiber or epoxy. Thus, your PC motherboard tends to be routed at odd angles like 7 or 10 degrees to average out the effect.
Because of all of this, PCB autorouters are HARD and the KiCad developers have (rightfully, IMO) decided that their time is much better spent on the myriad other features and bugs rather than developing a PCB autorouter.
However, I'm sure that if you were willing to take a crack at it, the KiCad developers would be ecstatic to give a pointer to your amazing autorouter. Just be prepared that it's going to be quite difficult and you're going to get LOTS of complaints about how much it sucks.
I look forward to seeing your efforts.