Timestamp Peripherals for Precise Real-Time Programming

On microcontrollers, timer devices provide high-precision timing, but that precision is lost when using high-level languages without suitable abstractions for temporal behavior. So, for timing-sensitive applications, programmers resort to low-level languages like C which lack expressiveness and safety guarantees. Other programmers use specialized precision-timing hardware which is expensive and difficult to obtain. In this work, we achieve sub-microsecond precision from a high-level real-time programming language on the RP2040, a cheap, widely available microcontroller. Our work takes advantage of the RP2040's Programmable I/O (PIO) devices, which are cycle-accurate coprocessors designed for implementing hardware protocols over the RP2040's GPIO pins. We use the PIO devices to implement timestamp peripherals, which are input capture and output compare devices. We use timestamp peripherals to mediate I/O from programs written in Sslang, a real-time programming language with deterministic concurrency. We show that timestamp peripherals help Sslang programs achieve the precise timing behavior prescribed by Sslang's Sparse Synchronous Programming model.