Kronos is a 3-stage in-order RISC-V RV32I_Zicsr_Zifencei core geared towards FPGA implementations.

Finding prime numbers (sf_prime.c) with Kronos implemented on iCEBreaker (Demo Snowflake SoC)

Arduboy on KRZ SoC - https://www.youtube.com/watch?v=nveWIcuFHzo

Features

• RISC-V RV32I_Zicsr_Zifencei compliance tested with riscv-compliance suite.
  • Complete implementation of the Unprivileged Architecture:
    • RV32I Base Integer ISA, v2.1
    • Zifenci Instruction Fetch Fence extension, v2.0
    • Zicsr Control and Status Register extension, v2.0
  • Partial platform-specific implementation of the Privileged Architecture:
    • Machine-Level ISA, v1.11
• Optimized for single cycle instruction execution.
• DMIPS/MHz of 0.7105 on the KRZ SoC.
• Direct mode trap handler jumps.
• Dual Wishbone pipelined master interface for instruction and data.

News

• Officially listed - https://riscv.org/exchange/cores-socs/
• Kronos makes the news! Article on Hackster.io - LINK
• A group at Taiwan Tech (NTUST) compared their work on IEEE Access against Kronos RISC-V. I am truly honored! I didn't think this day would evere come. As you can see, in single-issue comparisions, Kronos smashes the crowd favorite PicoRV32. A corrective note to add is that the utlization of 5280 is the entire size of the iCE40UP5K, out of which the Kronos takes up 1600 to 2100 LUTs (LUT-4) depending on bells and whistles added.
• arduboy-pmod v0.1 pcb has been layed out and fab'd.
• Native Arduboy port is coming along well. Checkit out:

Documentation

https://sonalpinto.github.io/kronos/

Integration

All of the HDL for the Kronos core is located in the rtl/core directory, with the top design file being kronos_core.sv.

Instantiation template and IO description at docs/integration

KRZ SoC

Kronos Zero Degree (KRZ) is the System-on-Chip packaged in this project to show-off the Kronos core. It is designed for the iCE40UP5K with the following features.

• 24MHz system clock.
• 128KB of RAM as 2 contiguous banks of 64KB.
• 1KB Bootrom for loading program from flash to RAM.
• UART TX with 128B buffer.
  • Configurable baud rate
• SPI Master with 256B RX/TX buffers.
  • Configurable SPI Mode and rate.
  • Max 12MHz.
• 12 Bidirectional configurable GPIO.
  • Debounced inputs.
• General Purpose registers

Read More - KRZ SoC

Performance

Kronos running on the KRZ SoC hits a DMIPS/MHz of 0.7105. making it one of the fastest RISC-V cores to run on the the iCE40UP5K, an FPGA with 5280 LUTs. More details here.

The following single-thread algorithm tests have been ported as well, and the CPI (Clocks Per Instructions) is recorded. Yes, they all pass.

Test	Cycles	Instret	CPI
median	7972	4152	1.92
multiply	33442	20899	1.60
qsort	220435	123505	1.78
rsort	291505	171131	1.70
spmv	3143547	1947328	1.61
towers	10847	6168	1.75
vvadd	14037	8026	1.74

Status

• The kronos_core is feature complete and RISC-V compliant.
• Kronos: Zero Degree, KRZ, a Kronos-powered SoC is ready.
• Todo:
  • Make the project yosys/nextpnr-ice40 friendly.
  • Rigorous verification of the core (riscv-torture, riscv-formal, etc).
• Goal: Run Arduboy on the iCEBreaker. Custom loader to boot any game from a library on the onboard flash. Interface: 1 oled, 6 buttons and a piezo speaker.

License

Licensed under Apache License, Version 2.0 (see LICENSE for full text).

Miscellaneous

I initially started this project to build some street cred as a digital designer. The RISC-V ISA and the open-source community that has grown around it is absolutely beautiful, and I want to be a part of it. If it wasn't for the maturity of the riscv-toolchain and the effort the community has put into it, I wouldn't have attempted to build this core. A core can only shine when it runs awesome software. As a bonus, I also get something neat to present during job interviews, instead of just my prosaic grad school work on formal theory.