Hi HN, I have always been interested and inspired by the idea of speed in computing, which raises a logical question: why do we use general-purpose processors for tasks that require minimal latency and predictability? The question is interesting, so a couple of months ago I thought: can I make a DLinear-type time series model? DLinear is a simple but effective tool for time series analysis, to implement it directly into silicon using a PDK with an open Sky130.
Well, it wasn't as easy as it seemed... My first attempt at direct synthesis led to a nightmare. OpenLane (the RTL-to-GDSII flow I used) reported a setup slack of -7.88ns. Essentially, the signal was too slow to travel through the 64-tap window within a 100MHz clock cycle.
I spent weeks refactoring the architecture in Chisel. I moved to a fully unrolled, 4-stage deep pipeline. The hardest part was balancing the binary adder tree; I had to ensure that the 6 levels of addition didn't bottleneck the entire chip.
I also realized that I could cheat a bit: instead of using a resource-heavy divider for the moving average, I used a static bit-shift (>> 6). In hardware, that’s just re-wiring, which costs zero nanoseconds and zero gates.
The final result is an 86,443-cell design that is LVS/DRC clean.
Of course, it now runs at 100 MHz on a 130 nanometer process, but physics shows that in theory it is possible to achieve 1.2GHz on a 7 nanometer process, which will reduce the delay to about 3.3 nanoseconds (yes, I did not try ASAP7 in OpenLane, the project is too controversial and I was not sure if it would give realistic results)
I think we are now approaching the point where a software-defined interface is becoming too slow for line rate networking or high speed control loops. All GDSII layouts and surfer waveforms could be found in repo
If someone is interested, I'd love to get feedback from the community about the architecture, as there are still some problems with synchronization in the chip, which can cause power consumption to suddenly jump when the chip is running, which leads to the chip going into reboot mode or maybe someone knows a more elegant method for handling the summation of large windows in Chisel.
