PicoRV32 - A Size-Optimized RISC-V CPU

PicoRV32 is a CPU core that implements the RISC-V RV32I Instruction Set.

Tools (gcc, binutils, etc..) can be obtained via the RISC-V Website.

PicoRV32 is free and open hardware licensed under the ISC license (a license that is similar in terms to the MIT license or the 2-clause BSD license).

Features and Typical Applications:

• Small (~1000 LUTs in a 7-Series Xilinx FGPA)
• High fMAX (>250 MHz on 7-Series Xilinx FGPAs)
• Selectable native memory interface or AXI4-Lite master

This CPU is meant to be used as auxiliary processor in FPGA designs and ASICs. Due to its high fMAX it can be integrated in most existing designs without crossing clock domains. When operated on a lower frequency, it will have a lot of timing slack and thus can be added to a design without compromising timing closure.

For even smaller size it is possible disable support for registers x16..x31 as well as RDCYCLE[H], RDTIME[H], and RDINSTRET[H] instructions, turning the processor into an RV32E core.

Note: In architectures that implement the register file in dedicated memory resources, such as many FPGAs, disabling the 16 upper registers may not further reduce the core size.

The core exists in two variations: picorv32 and picorv32_axi. The former provides a simple native memory interface, that is easy to use in simple environments, and the latter provides an AXI-4 Lite Master interface that can easily be integrated with existing systems that are already using the AXI standard.

A separate core picorv32_axi_adapter is provided to bridge between the native memory interface and AXI4. This core can be used to create custom cores that include one or more PicoRV32 cores together with local RAM, ROM, and memory-mapped peripherals, communicating with each other using the native interface, and communicating with the outside world via AXI4.

Performance:

The average Cycles per Instruction (CPI) is 4 to 6, depending on the mix of instructions in the code. The CPI numbers for the individual instructions are:

Instruction	CPI
direct jump (jal)	3
ALU reg + immediate	3
ALU reg + reg	4
branch (not taken)	4
memory load	5
memory store	6
branch (taken)	6
indirect jump (jalr)	6
shift operations	4-15

Dhrystone benchmark results: 0.280 DMIPS/MHz (493 Dhrystones/Second/MHz)

For the Dryhstone benchmark the average CPI is 4.606.

This numbers apply to systems with memory that can accomodate requests within one clock cycle. Slower memory will degrade the performance of the processor.

Todos:

• Optional IRQ support
• Optional write-through cache
• Optional support for compressed ISA