Microwatt is an open source soft processor core originally written in VHDL by Anton Blanchard at IBM, announced at the OpenPOWER Summit NA 2019^[2] and published on GitHub in August 2019. It adheres to the Power ISA 3.0 instruction set and can be run on FPGA boards, booting Linux, MicroPython and Zephyr.^{[3][4][5][6][7][8]}

Design

Microwatt is a tiny 64-bit bi-endian scalar integer processor core, implementing a subset of the Power ISA 3.0 instruction set. It has 32× 64-bit general purpose registers and 32x 64-bit floating-point registers. It uses Wishbone for the memory interface.^[4]

The initial development was done in a couple of months, included the entire integer processing functionality of the instruction set; the bare minimum to make it compliant, with no memory management unit (MMU) and no floating-point unit.^[4]

Later additions to the implementation includes JTAG debugger interface, divider instructions, 16 KB instruction and 32 KB data caches, a non-hypervisor-capable MMU, pipelining^[4] and floating-point support.^[9]

It's designed using VHDL 2008 and the GHDL simulation environment.^[3]

Chiselwatt

A sibling project called Chiselwatt is another open processor core implementing the Power ISA 3.0 instruction set, written in the Scala-based Chisel instead of VHDL.^[10][11]

Implementations

• Microwatt itself
• Libre-SOC – A libre system on a chip under development that uses Microwatt as a reference design.^[12]
• Kestrel – A soft BMC in Raptor Engineering POWER9 based workstations.^[13][14]

History

It is the first processor written from scratch using the open Power ISA 3.0, and is released by the OpenPOWER Foundation as a reference design.

The project started as a demo, proof of concept and a reference implementation for the release of the opensource initiative regarding Power ISA 3.0.^[15] The goal for Blanchard was to see if he could make it, and as a software developer, taking on a very low level hardware project was a challenge.^[2][3]

Microwatt is set to be fabricated in 130 nm by Efabless "Open MPW Shuttle Program" in 2021.^[16] As of February 2024, there has been no update on the progress of fabrication on Efabless's Microwatt project page.^[17]