Software Timer using xTimerCreate runs 33% slow

[vannoo67]

Hi Folks,

I am having trouble using FreeRTOS's Software timer. It works, but the duration of the timer is consistently 33% too long. To get the correct time, I need to scale my duration by 0.75.

I'm wondering if I have something wrong with my clock frequency setting?
Note: I am using Arduino IDE 1.8.9 with esp32 package 1.0.2
CPU Freq: 240 MHz (WiFi/BT)

My esp32 is an esp32 DevKit.

I setup my timer (initially with 1 tick duration) like so;

Code: [Select all] [Expand/Collapse]

1. channel[channelId]->xCountdownTimer = xTimerCreate(
2.   "CountdownTimer",
3.   1,
4.   pdFALSE,
5.   (void*) channelId,
6.   vCountdownTimerCallback
7. );

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(Note: I have a number of separate 'Channels', each with a xCountdownTimer and associated xStopSemaphore)
I use 4th arg TimerID to store the index into the channel[]


With callback function;

Code: [Select all] [Expand/Collapse]

1. void vCountdownTimerCallback(TimerHandle_t xTimer) {
2.   int channelId =  (int) pvTimerGetTimerID( xTimer );
3.   xSemaphoreGive( channel[channelId]->xStopSemaphore );
4. }

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I recalculate the required duration immediately before starting the timer;

Code: [Select all] [Expand/Collapse]

1. bool startCountdown() {
2.   return ( (pdPASS == xTimerChangePeriod(
3.                                   xCountdownTimer,
4.                                   pdMS_TO_TICKS( calcDuration() ),  
5.                                   pdMS_TO_TICKS(TIMER_SET_BLOCK_TIME) ))
6.  
7.                && (pdPASS == xTimerStart(
8.                                   xCountdownTimer,
9.                                   pdMS_TO_TICKS(TIMER_SET_BLOCK_TIME) )));
10. }

GeSHi © Codebox Plus Extension

Everything works perfectly, except that the duration is 33% longer than expected. As a work-around I am scaling the result of calcDuration() by 0.75

I have tried various sizes of delays, from seconds to minutes, but if I ask for 30 seconds, I get 40 seconds, ask for 60 seconds, I get 80 seconds.

I'm confident that my calcDuration function is working correctly (returns number of milliseconds as an unsigned long), but something is going pear-shaped in pdMS_TO_TICKS or deeper in the system.

Note: I've simplified my code a little, so if the above doesn't make complete sense, something was probably lost in translation.
eg; startCountdown() is actually a method of the Channel object stored in the channel[] so references to xCountdownTimer are local

Any suggestions will be gratefully received.

[ESP_Dazz]

Some points to consider

• How are you actually measuring the duration of your timer? The timer doesn't start until xTimerStart() returns, and xTimerStart() has a possibility of being blocked if the timer's internal command queue is full.

• The timer callback could be delayed in running if there is something a higher priority task preventing the timer daemon task form running. Check your timer task's priority in your project configuration (should be priority 1 by default).