Periodic timers revisited: The real-time embedded system perspective

Common sense dictates that single-shot timer mechanisms are more suitable for real-time applications than periodic ones, specially in what concerns precision and jitter. Nevertheless, real-time embedded systems are inherently periodic, with tasks whose periods are almost always known at design-time. Therefore a carefully designed periodic timer should be able to incorporate much of the advantages of single-shot timers and yet avoid hardware timers reprogramming, an expensive operation for the limited-resource platforms of typical embedded systems.In this paper, we describe and evaluate two timing mechanisms for embedded systems, one periodic and another single-shot, aiming at comparing them and identifying their strengths and weaknesses. Our experiments have shown that a properly designed periodic timer can usually match, and in some cases even outperform, the single-shot counterpart in terms of precision and interference, thus reestablishing periodic timers as a dependable alternative for real-time embedded systems.

A properly configured periodic timer can match the single-shot approach in terms of performance and interference and outperform an equivalent single-shot mechanism when the requested period exceeds the maximum hardware period. The overhead of reprogramming the hardware timer in the single-shot event handler is 5 times higher using an 8-bit microcontroller.Figure optionsDownload as PowerPoint slide