Timers in VxWorks: Periodic and One-Shot Timers

🚀 Introduction

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Timers are critical in real-time operating systems (RTOS) like VxWorks.
They are used for:

• Periodic tasks (e.g., sensor polling every 100 ms).
• Timeouts (e.g., network retries).
• Watchdog mechanisms to detect software hangs.

VxWorks provides Watchdog Timers, which can be used for both one-shot and periodic timer functionality.

🧩 Types of Timers in VxWorks

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1. One-Shot Timer

   • Executes once after a specified delay.

2. Periodic Timer

   • Re-arms itself after expiration for repeated execution.

💻 Example: Using a Watchdog Timer

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We’ll create:

• A One-Shot Timer → executes once after 5 seconds.
• A Periodic Timer → executes every 2 seconds.

Code Example

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#include <vxWorks.h>
#include <wdLib.h>
#include <taskLib.h>
#include <stdio.h>

WDOG_ID oneShotWd;
WDOG_ID periodicWd;

// One-shot callback
void oneShotHandler(int arg)
{
    printf("One-shot Timer expired! arg=%d\n", arg);
}

// Periodic callback
void periodicHandler(int arg)
{
    printf("Periodic Timer triggered! arg=%d\n", arg);
    wdStart(periodicWd, sysClkRateGet() * 2, (FUNCPTR)periodicHandler, arg);  
}

// Init function
void usrAppInit(void)
{
    oneShotWd = wdCreate();
    periodicWd = wdCreate();

    // One-shot: 5 seconds
    wdStart(oneShotWd, sysClkRateGet() * 5, (FUNCPTR)oneShotHandler, 42);

    // Periodic: first trigger in 2 seconds
    wdStart(periodicWd, sysClkRateGet() * 2, (FUNCPTR)periodicHandler, 99);
}

📝 Explanation of the Code

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1. wdCreate()

   • Creates a watchdog timer instance.

2. wdStart()

   • Starts the timer with a given delay (in system ticks).
   • sysClkRateGet() → gets system clock ticks per second.

3. One-Shot Timer (oneShotWd)

   • Fires once after 5 seconds.

4. Periodic Timer (periodicWd)

   • Re-arms itself inside the callback, firing every 2 seconds.

⚡ What You’ll See

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When running, you’ll see something like:

Periodic Timer triggered! arg=99
Periodic Timer triggered! arg=99
One-shot Timer expired! arg=42
Periodic Timer triggered! arg=99
...

🔍 Key Takeaways

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• VxWorks timers are implemented using watchdog timers.
• One-shot timers fire once, while periodic timers must be restarted inside their callback.
• Timers are crucial for timeouts, scheduling, and periodic tasks.

✅ Wrap-Up

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In this tutorial, you learned:

• How to create and use one-shot timers.
• How to implement periodic timers in VxWorks.
• How watchdog timers can handle both use cases.

In the next blog, we’ll explore Semaphores in VxWorks: Binary, Counting, and Mutexes to manage synchronization between tasks.

👉 Stay tuned for Blog 13: “Semaphores in VxWorks: Binary, Counting, and Mutexes.”