PULP’s CLIC extensions for fast interrupt handling
Status: In Progress
- Type: Semester Thesis
- Professor: Prof. Dr. L. Benini
Today’s real-time systems require fast interrupt handling to manage asynchronous requests coming from the surrounding environment, for example external master devices interacting with the system through I/O peripheral interfaces. This means that the underlying HW has to guarantee fast propagation of the interrupt lines towards the processor, with a per-interrupt, fine-grained control over each line, and support interrupt preemption and nesting according to each event priority.
ControlPULP is a RISC-V, real-time platform envisioned as an integrated Power Management Unit. Developed at IIS, it is based on PULP    and relies on FreeRTOS   to implement a Power Control Firmware (PCF)  routine. RISC-V Core Local Interrupt Controller (CLIC)  is an interrupt controller for RISC-V cores subsuming the original RISC-V local interrupt scheme (CLINT). It promises pre-emptive, low-latency, vectored, priority/level based interrupts.
An implementation of the CLIC has been developed at IIS  and integrated in ControlPULP. It integrates most of the features described in the specifications. Nonetheless, the basic architecture can be extended - leveraging either HW and SW - to support additional features, such as dynamic switching between legacy CLINT and new CLIC interrupt controllers and the mnxti CSR for horizontal interrupts handling in machine mode . Inspiration can be taken from existing CLIC implementations (Nuclei’s ECLIC , T-Head’s CLIC ).
The goal of this project is to extend the CLIC with the aforementioned additional features, and compare against the current baseline. In particular:
- Study CLIC current specification draft  and PULP’s CLIC implementation 
- Implement CLINT/CLIC dynamic switching to handle CLIC selection at runtime
- Implement xnxti horizontal interrupt transition with non-vectored interrupts
- Implement interrupt tail-chaining 
- Synthesize the design, giving particular attention to the CLIC overhead with respect to the original baseline
- ASIC Backend flow. The CLIC IP and extensions integrated in ControlPULP will be taped-out in TSMC65 nm technology.
- 15% Literature / architecture review
- 20% RTL implementation
- 20% Software layer
- 25% Evaluation
- 20% Backend
- Experience with digital design in SystemVerilog as taught in VLSI I
- Must have visited VLSI II in a previous semester or take it alongside the thesis
- Knowledge of C programming language as from previous bachelor/master courses