Time-Triggered Communication with UARTs

The simple UART (Universal Asynchronous Receiver and Transmitter) encoding is used in two novel fieldbus protocols for real-time systems, the Local Interconnect Network (LIN) and the Time-Triggered Protocol for SAE class A applications (TTP/A). These protocols use a time-triggered communication schedule to achieve a predictable timing behavior. The employment of standard components like a standard UART reduces costs, but the issues of clock drift, send jitter, and the adjustability of the send frequency have to be considered. This paper examines common timing problems with standard UARTs and imprecise oscillators and presents a calculation of upper bounds for the timeliness of UART driven communications. Furthermore we discuss methods to solve the timing problems when imprecise on-chip oscillators are used. The synchronization support of time-triggered fieldbus protocols often relies on a configurable UART without send jitter. Standard hardware UARTs usually do not hold this requirement. It is possible to use a software UART implementation but at the cost of node performance. We have developed an enhanced UART architecture that behaves better for time-triggered systems than standard UARTs. Together with proper protocol synchronization support this approach allows the integration of nodes with imprecise clocks in time-triggered real-time systems.