SPI on RTU

Processors in microcontrollers are optimized for speed executing tasks in order to minimize completion time. Often burdened with interrupts, nondeterministic execution, auxiliary tasks, and non real-time operating systems, processors are unable to make any timing guarantees on most operations. In a traditional setup, interfacing with external components is done via protocol-specific hardware peripherals embedded into silicon. This hardware-based implementation puts restrictions on both the types of communication protocols that can be used and the number of external ports implementing a certain protocol. Furthermore, fixed pin functionality mapping may also cause sub-optimal PCB routing. To resolve some of these issues, we explore using a timing-predictable co-processor attached to generalpurpose input/output (GPIO) pins. Digital communication protocols can be implemented using bit banging, a technique in which software is used to control signals, giving flexibility. The co-processor can also offload cycle-consuming hardware tasks from the main processor, allowing the main processor to utilize its resources elsewhere. Such a co-processor is called a real time unit (RTU).