Icicle

Introduction

Icicle is a 32-bit RISC-V system on chip for iCE40 HX8K and iCE40 UP5K FPGAs. It can be built with the open-source Project IceStorm toolchain and currently targets several development boards.

Current features

• RV32I core with a classic 5-stage RISC pipeline, static branch prediction, bypassing and interlocking. It currently implements the entire user ISA (except ECALL and EBREAK) and parts of the privileged ISA.
• Shared instruction and data memory (8 KiB, implemented with FPGA block RAM).
• Memory-mapped UART and LEDs.

Dependencies

• arachne-pnr
• GNU RISC-V toolchain
• GNU Make
• Icarus Verilog (master branch)
• Project IceStorm
• vim (for xxd)
• Yosys

Building and testing

Supported boards

Icicle supports several development boards:

• ice40hx8k-b-evn: iCE40-HX8K breakout board
• blackice-ii: BlackIce II
• upduino: UPduino

<board> should be replaced with the internal name of your development board in the rest of the instructions (e.g. ice40hx8k-b-evn for the iCE40-HX8K breakout board).

Building

• Run make BOARD=<board> syntax to check the syntax with Icarus, which has a stricter parser than Yosys. At the time of writing the master branch of Icarus is required as there isn't a stable release with always_comb/always_ff support yet.
• Run make BOARD=<board> to synthesize the design, place and route, compile the demo program in progmem.c and create the bitstream.

Programming

iCE40-HX8K breakout board

• Configure the jumpers for direct SRAM programming.
• Run make BOARD=ice40hx8k-b-evn flash to flash the bitstream.

BlackIce II

• Configure jumper on board for DFU Mode and connect both USB1 and USB2 on the board to host USB ports.
• Run make BOARD=blackice-ii dfu-flash to flash the bitstream.

Testing

• If your chosen board has built-in LEDs, some of the LEDs should turn on.
• Run picocom -b 9600 /dev/ttyUSBn (replacing ttyUSBn with the name of the serial port) to connect to the serial port. Hello, world! should be printed once per second.

Other targets

The make BOARD=<board> stat target runs icebox_stat and the make BOARD=<board> time target prints the icetime report.

The Makefile runs the IceStorm toolchain in quiet mode. Unset the QUIET variable to run the toolchain in verbose mode - e.g. make BOARD=<board> QUIET= ....

Planned features

• Use remaining block RAM tiles to eke out as much memory as possible.
• Use the SPRAM tiles on UP5K devices.
• Implement remaining bits of the user ISA.
• Implement machine mode from the privileged ISA.
• Interrupts/exceptions.
• Unaligned memory access support.
• Memory-mapped GPIOs.
• Memory-mapped XIP/SPI flash.
• Improved reset support (a reset signal + boot ROM to zero all the registers).
• Automated tests.
• Multiply/divide support.
• Compressed instruction support.
• Add flags to disable certain features (e.g. privileged mode) to save LUTs on smaller devices (e.g. the UP5K).
• Investigate using DSP tiles on the UP5K.

Size and performance

The entire system on chip currently occupies around 2,500 LUTs on an iCE40 when synthesized with Yosys.

It's currently clocked at 24 MHz but icetime estimates it could be clocked at ~30-35 MHz (depending on how lucky arachne-pnr is).

The core is capable of issuing and retiring one instruction per clock cycle, although the actual number of instructions per cycle will be slightly less than this in practice due to interlocking, branch mispredictions and the shared memory bus.

License

This project is available under the terms of the ISC license, which is similar to the 2-clause BSD license. See the LICENSE file for the copyright information and licensing terms.