Proper Timed I/O: High-Accuracy Real-Time Control for Conventional Operating Systems

Abstract

We propose a novel high-level abstraction for real-time control, called Proper Timed I/O (PTIO). The abstraction allows user-space programs running on a stock operating system (without real-time extensions) to perform high-resolution real-time digital I/O (setting pins high or low, responding to input transitions, etc.). PTIO programs express their real-time I/O behavior in terms of a timed automaton that can communicate with the user-space program. Simple behaviors are encoded in the timed automaton; complex behaviors are implemented by the user-space program. We present two implementations of the PTIO abstraction, both for Linux. One utilizes a deterministic co-processor that is available on some ARM-based system-on-a-chip processors. This implementation can achieve timing accuracy of 100ns or better and can perform millions of finite-state transitions per second. The other implementation uses hardware timers that are available on every system-on-a-chip; it achieves a timing accuracy of 6µs or better, but it is limited to about 2000 state transitions per second. Both implementations guarantee that the PTIO never fails silently: if the mechanism missed a deadline, the user space program is always notified. In many cases, PTIOs eliminate the need for bare-metal programming or for specialized real-time operating systems.