CV32RT: Enabling Fast Interrupt and Context Switching for RISC-V Microcontrollers

Abstract

This work introduces CV32RT, an enhanced, open-source 32-bit RISC-V core designed to overcome the latency shortcomings of standard RISC-V interrupt handling in embedded systems. CV32RT integrates the RISC-V CLIC specification and features a custom extension called "fastirq" to enable pre-emptible, low-latency vectored interrupts. The resulting core achieves an interrupt latency as low as 6 cycles, positioning it as the first fully open-source RV32 core competitive with proprietary architectures like the Arm Cortex-M series.

Report

Key Highlights

• Target Core: CV32RT, an enhanced version of the open-source CV32E40P MCU-class RISC-V core.
• Innovation: Implementation of the Core Local Interrupt Controller (CLIC) augmented with a custom extension called fastirq.
• Performance Metric: Achieves extremely low interrupt latency, reported as low as 6 cycles.
• Competitive Position: CV32RT is claimed to be the first fully open-source RV32 core with interrupt-handling features competitive against mature proprietary architectures like Arm Cortex-M and TriCore.
• Secondary Benefit: The proposed extensions also demonstrably improve task context switching performance in Real-Time Operating Systems (RTOS).

Technical Details

• Baseline Problem: The default RISC-V Core Local Interruptor (CLINT) lacks the configurability necessary for real-time systems, specifically in interrupt prioritization and preemption.
• Solution Foundation: Uses the RISC-V Core Local Interrupt Controller (CLIC) specification, which allows for pre-emptible and low-latency vectored interrupts.
• Core Implementation: CLIC and fastirq are implemented on the CV32E40P, an industrially supported open-source core.
• Custom Extension: The fastirq extension is responsible for driving the interrupt latency down to 6 cycles.

Implications

• Real-Time Viability: By achieving competitive interrupt latency (6 cycles), CV32RT significantly closes the performance gap between open-source RISC-V hardware and established proprietary real-time microcontrollers.
• Ecosystem Growth: This development enhances the suitability of RISC-V for demanding embedded use cases, particularly those requiring hard real-time constraints and flexible, predictable event handling.
• Open Source Leadership: CV32RT provides the RISC-V ecosystem with a verified, high-performance, fully open-source core suitable for RTOS deployment and deeply embedded applications, encouraging wider adoption and future development in the MCU space.