VxWorks is one of the most widely adopted real-time operating systems in the embedded domain, with extensive applications across industrial control, medical devices, telecommunications, aerospace, and defense. As a 32-bit real-time OS, it has been in use since the 1980s, developed by Wind River. Its strong real-time performance, support for multitasking, compact size, and high customizability have made it a popular choice among developers and enterprises worldwide.
In systems that demand strict real-time behavior, timers play a crucial role. However, VxWorks does not provide a built-in, universal, and efficient timer component. To address this limitation, this paper introduces a shared watchdog timer mechanism, which serves as a general-purpose timer solution tailored for VxWorks environments.
**1. VxWorks Timing Methods**
**1.1 Using the taskDelay Function**
The `taskDelay(int ticks)` function allows a task to sleep for a specified number of system clock ticks. This method is often used when precise timing is not required. The default tick rate is 60 ticks per second, though this can be adjusted using `sysClkRateSet()`. While simple to implement, the timing accuracy is limited due to task scheduling delays, making it unsuitable for high-precision applications.
**1.2 Watchdog Timer (watchDog)**
VxWorks offers a built-in watchdog timer, managed through four key functions: `wdCreate()` to initialize, `wdDelete()` to remove, `wdStart()` to activate, and `wdCancel()` to suspend the timer. The watchdog timer operates within the system’s clock interrupt service routine (ISR), ensuring high-priority execution. It is commonly used to trigger a specific function after a set period. However, it must follow ISR rules, such as avoiding blocking calls, and is typically used for time-critical operations.
**1.3 POSIX Standard Timer**
VxWorks also supports IEEE POSIX 1003.1b standard timers, offering a portable interface for application development. These timers allow tasks to send signals to themselves after a certain interval, based on system clock and signal mechanisms. They are ideal for cross-platform applications that require portability across different operating systems.
**2. Efficient Timing Mechanism Based on Watchdog**
**2.1 Shared Watchdog Timer**
In many embedded real-time systems, multiple tasks may need to schedule timed events. However, using individual watchdog timers for each task can lead to resource inefficiency and reduced system responsiveness. To overcome this, a shared watchdog timer mechanism is proposed. This approach allows multiple tasks to share a single watchdog timer, reducing overhead while maintaining real-time performance.
**2.2 Timing Algorithm**
The core principle of the shared watchdog timer is dynamic management of scheduled tasks. For example, consider three tasks A, B, and C with different timing intervals. Task A schedules a 500ms delay, followed by Task B at 200ms, and Task C at 150ms. The shared watchdog timer maintains a list of pending tasks, sorting them based on their next scheduled execution time. When the system clock reaches the scheduled time, the corresponding task is executed, and the timer updates its internal list accordingly. This ensures efficient resource usage and avoids unnecessary task creation or context switching.
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