Five years ago we started the Parallel Ultra Low Power (PULP) project at ETH Zürich together with the University of Bologna under the leadership of Luca Benini. Our goal was to explore novel computing architectures that are able to get the most out of the energy the system uses over a wide operational range starting from tiny systems used in the internet of things (IoT) all the way up to applications in high-performance computing (HPC). It was clear for us that to build such a system we would need a small but efficient processor core. But where to find such a processor core that we could use?
Exploring Commercial vs. Open-Source Processors
We first investigated available commercial processors. Since our research activities do not result directly in an income that would allow us to pay for regular commercial licenses, we would need to make use of the academic licensing agreements. In all fairness, most processor IP providers had university programmes with reasonable conditions. The problem was that most of these programmes would not allow you to make changes to the architecture, some would need additional licenses for manufacturing actual chips, and most importantly any collaboration we wanted to establish with new partners both from the industry and academia would require additional agreements to be signed that took quite a long time to set up.
So we started to look at open source processors. While there was plenty of Verilog and VHDL code available for various processor cores under different licenses, like many readers at the moment, we were not very sure how well these cores were designed and verified, whether or not they were actually used in practice.
RISC-V in the PULP Project
As an academic institution, we were able to take more risks with our choices and the freedom that an open source processor gives you, both in terms of making enhancements, and in allowing collaboration with anyone without lengthy negotiations about IP ownership was too tempting to pass up. For the first few years, the PULP project used a 32-bit OpenRISC based processor with great success. We were able to improve the performance of our own implementation of OpenRISC cores (called OR10N) to be competitive with state of the art 32-bit processors and these formed the backbone of our successful multi-core systems geared towards ultra-low-power operation.
Figure 1. The PULP system consists of efficient RISC-V cores, peripherals, interconnect solutions, and accelerators that are combined to create different platforms.
RISC-V as a Solution
At the same time, we saw how RISC-V ISA was gaining in popularity. By far the most important issue in deciding what processor to use in your projects is the availability of a development environment, preferably with widely known and used tools. No matter how clever you can get with developing your architectures, if you can not get developers to port their applications to your system with ease, you will not get the results you want. In my opinion, it is precisely this point that the RISC-V ISA made the most contribution to the open source hardware community.
By maintaining a clear and well thought ISA definition, maintained by the RISC-V foundation (of which ETH Zürich is a founding member), it was possible for a large community to provide ports of compilers and tools to support RISC-V. In the meantime, most of these ports were upstreamed and are available out of the box (for example for GCC since version 7.1 released in April 2017).
This is why, early in 2015, we made the switch to RISC-V as well. We were able to use most of the development for our OR10N core and derive our first core implementing the RISC-V ISA called RI5CY and we have continued to use and improve this core over tens of ASICs we have developed throughout the project. We have also adapted RISC-V cores for different use cases, and added specialized cores for applications that have tight resource constraints (Zero/Micro Riscy), as well as adding a 64-bit core called Ariane that is able to boot Linux operating system.
Figure 2. Our 9-core IoT processor, Mr. Wolf, on a small demonstration board.
After slightly more than 5 years on the project, the PULP project has resulted in 26 ASICs (seen in Figure 3 below) that were taped out and tested in various technologies from early trials in mature 180nm technologies all the way down to our latest design called Poseidon realized using Globalfoundries 22nm technology. And we are not done yet, we are currently involved in many projects and collaborations that make use of our PULP technology and will allow us to develop it further.
Figure 3. All 26 PULP chips that we have taped out in the last 5 years (not to scale).
The Reach of Open Source Projects
From the beginning, it was clear for us that we would keep PULP project open source, which gives us the flexibility to collaborate with other groups and companies as we see fit and ultimately allows contributions from all over the world. We have adopted the Solderpad Licensing scheme, a derivation of the Apache license introduced by the LowRISC project initiated by the University of Cambridge, which has helped it being adopted for various projects. Our GitHub repository contains the source code for various hardware we have developed as part of the PULP project, not only RISC-V processors, but peripherals (SPI, I2C, GPIO, JTAG) and complete systems form simple microcontrollers, to state-of-the-art multi-core IoT platforms all written in System Verilog.
But by far, the most rewarding part of this project has been to experience how well our work was received not only by academia but also in the industry. We learned at the last RISC-V conference in Barcelona, that Google, IBM, and NXP have all evaluated and used our cores and taped out designs based on our work successfully. EDA companies like Cadence and Mentor have used our designs as part of their training and evaluations, and Greenwaves Technologies, a startup from Grenoble in France has based their first product, the GAP8 processor, largely on our multi-core OpenPULP architecture that is available on our GitHub page for everyone.
When people speak of open source projects, they often mention that it is free. Most people only take this to mean that you do not pay for it, but the real freedom you get from open source projects is much more and I think more important. People using our RISC-V systems, are free to switch to RISC-V implementations from any other source when they want—after all, the same binaries will work on any RISC-V implementation that follows the standard.
Researchers can take what we provide and freely change it for their experiments, knowing full well that the system they are using is not a toy example, but something that also finds applications in real-world scenarios. Startup companies can build on what we provide as a starting point and concentrate their time and energy on the actual innovations they want to provide. People who are disturbed by various attacks on their systems have the chance to look inside and know what exactly is in their system. This may not automatically solve all security issues, but it is an important step towards increased security, by giving a larger community access to scrutinize the system you work with. And yes, all this freedom comes at no extra cost, while there are also commercial RISC-V implementations, our silicon-proven RISC-V systems remain freely accessible over GitHub.
RISC-V Benefits in Academia
When we started the PULP project 5 years ago, open source hardware was in its infancy. Today, there are still many skeptics, but by judging by the interest of companies both small and large in our project, it has already established itself as a viable alternative and the open collaboration model will surely drive it further. Together with our partners at the University of Bologna, we are very proud of being an active contributor to the RISC-V community and will continue to be a leading supporter of open source hardware in the academia.
All images in this article were provided by the author and used with his permission.
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