An update on Ibex, our microcontroller-class CPU core

At the beginning of many chips projects, there’s a dream. Could we create a more future-proof chip by embedding an FPGA fabric into it? Could we measure glucose levels more accurately by integrating a small bio lab onto a chip? Could we more reliably recognize kittens in a set of pictures by implementing neural network inference in hardware?

In implementation, this dream becomes a piece of hardware, with digital or analog logic, sensors, actuators, and much more. Let’s get it produced and try out the real thing! But wait. How do you control the hardware block? How do you feed data to it? How do you make sure the startup sequence is done exactly in the right way? The answer often is: add an embedded micro-controller core to handle the control logic. A small, efficient, and rock-solid core. Where could you get such a core?

Say hello to Ibex: a small, 32-bit microcontroller-class RISC-V CPU core written in SystemVerilog. Reliable, unpretentious, getting the job done.

  • Ibex supports the standard RV32I/EMC instruction set, allowing you to tap into a large ecosystem of compilers and software libraries.
  • Its two stage pipeline design balances a small area overhead with good performance. It gives you an IPC of 0.67 in CoreMark (2.44 CoreMarks/MHz) with an area of just 18.9 kGE. That’s 0.027 mm² in a 65 nm technology! (Reference) But Ibex works equally well on FPGAs. The same configuration utilizes 2.5 k 6-input Slice LUTs and 1 DSP slice on low-end Xilinx 7-series FPGAs when targeting a clock frequency of 50 MHz.
  • And best of all: there are no usage restrictions. Ibex is open source under the Apache 2.0 License, download it today and get started!

So if you need a microcontroller-class CPU core, look no further and give Ibex a try! Our friendly community awaits you over at GitHub to discuss enhancements, issues or help you get your favorite feature added to Ibex!

Ibex block diagram

If you’re interested in using Ibex in your design, you’d be in good company. Researchers at the University of Manchester (with support from Andrew Attwood from the STFC Hartree Centre) are already working on taping out a design including the Ibex core, with reconfigurable FPGA instruction set extensions. Do get in touch at info@cl.cam.ac.uk/~jrrk2 if you’re looking to integrate Ibex into one of your designs…

University of Manchester design with Ibex core

Ibex stands on an impressive mountain of engineering, erected by many great people mostly at ETH Zürich and the University of Bologna. Before it was contributed to lowRISC in December 2018, Ibex was called Zero-riscy. The history of the core can be traced back to a CPU core called “OR10N”, which was first taped out in 2013. Since that time, many, many hours of engineering, testing, and benchmarking went into what is now Ibex. That’s why it is such a solid design, and that’s the legacy lowRISC is proud to build upon.

lowRISC started working on Ibex around the time I joined the engineering team, since then we’ve been able to make a range of improvements in collaboration with our partners at Google and ETH Zürich, with Pirmin also joining the effort recently.

One of the first things we did with Ibex was the replacement of the debug system with one that is compliant to the RISC-V Debug Specification. (Thanks to Robert Balas and Davide Schiavone who started this work on a similar CPU core!) We also cleaned up the code in many places to make it easier to read and extend. And just recently Tao Liu from Google added a UVM testbench to make it easier to verify that the core works as expected.

Importantly, this is an active and ongoing effort. With the engineering and project maintenance resources lowRISC and our partners are putting into Ibex and related IP blocks, we aim to make it the make Ibex the go-to microcontroller-class CPU core. Our issue tracker on GitHub should give you a rough idea of what’s coming.

Do you want to join us making Ibex even better? For example, do you want to set up world-class continuous integration for this core? lowRISC is hiring, and we’re looking for a broad range of engineers: hardware, software and tooling. Join us!

You want to know more about Ibex and how lowRISC develops it to be a piece of high-quality free and open source IP? Join Philipp for a talk at the Week of Open Source Hardware (WOSH) in Zürich, Switzerland on Friday, June 14.

Philipp Wagner