16 July 2018 by Phillip Johnston • Last updated 16 August 2021

The crect project (pronounced correct) is a C++14 library for generating a scheduler for Cortex-M microcontrollers at compile-time. crect uses the Cortex-M’s Nested Vector Interrupt Controller (NVIC) to implement a Stack Resource Policy (SRP) scheduler which guarantees deadlock-free and data-race-free execution.

crect is built upon the Kvasir Meta-programming Library, which is also the foundation of the Kvasir framework. Use of C++ meta-programming and C++14 features allows priority ceilings and interrupt masks to be calculated at compile time. Resource locks are handled through RAII, and resource access is handled using a monitor pattern).

The most impressive thing about this framework is the minimal resource requirements:

• ~400 bytes of memory for the static elements (linked list, SysTick, time implementation)
• 4-5 instructions per job for initializing the NVIC
• 2-3 instructions per queue element for initializing the asynchronous queue
• 3-4 instructions + 4 bytes of stack space for a lock
• 1-3 instructions for an unlock
• 2-4 instructions for a pend/clear
• 20-30 instructions per item in the queue for async

If you are working on a bare-metal ARM program with real-time concerns, crect is an RTOS alternative that can provides protection against common multithreading concerns like priority inversion, deadlocks, and race conditions.

Further Reading

For more on crect:

• crect on GitHub
• Real-time for the Masses
• Stack-Based Resource Allocation Policy for Realtime Processes
• Abstract Timers and their Implementation onto the ARM Corte-M Family of MCUs
• Real-time for the masses, step 1: Programming API and static priority SRP kernel primitives