lowRISC / IMC internship week one - VGA output

Begnning on Monday, June 27th, we had a team of four University of Cambridge undergrads begin a 10 week internship working on the lowRISC project at the Computer Laboratory, kindly sponsored by IMC Financial Markets (who are also helping to advise this project). The team will be blogging regularly over the course of the summer - I’ll pass over to them to introduce themselves.

After some initial brainstorming, we decided to aim to extend the current lowRISC SoC design to enable video output, with the final goal of playing video smoothly at a resolution of 640x480 on FPGA. The photo below shows the four of us (left to right: Gary Guo, Profir-Petru Pârțachi, Alistair Fisher, Nathanael Davison).

The final goal has been decomposed into several milestones: adding VGA functionality to lowRISC, adding an in-memory framebuffer, implementing a video codec for RISC-V and designing and creating a 2D accelerator to speed up video decoding. Our plan for the augmented SoC architecture is shown the in the diagram below:

In our first week, we’ve succeeded in adding VGA output to lowRISC, a demonstration of this is shown in the video below. The demo shows lowRISC instantiated on a Nexys4 DDR board (Artix-7) displaying a static image that has been loaded into its BRAM. This image is read from SD card by a bare-metal program on the RISC-V application core, which then loads it in to the memory-mapped BRAM we hooked up to the AXI-Lite bus. The on-chip BRAM is obviously a very limited resource, so our next step is to use the board’s DRAM to hold the framebuffer and make use of the BRAM for a line-buffer.

We aim to publish something every week, either in the form of a blog post like this or as a more detailed guide showing how to repeat our work. Next week we’ll share a guide on how to enable VGA in lowRISC. By the end of the summer, as well as a working technical demo, we will also have produced detailed documentation on the whole process of adding a customised accelerator to lowRISC.