Accurate calculation of time delay detailed explanation of VxWorks timestamp

Project scenario ：

vxWorks Accurate to ms Time keeping method of , Use a timestamp sysTimeStamp() function

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Method of accurately calculating time delay Demo

void timeAccurate()
{
    
    int t2,t3,tt2,tt3;
    int time1,time2,freq; 
    int a=0;
  while(a<10) 
  {
     
	
	
	freq = (sysTimestampFreq()/1000); 
	t2=sysTimestamp();   /* Get the timestamp */
    tt2=tickGet();
    
    //msDelay(1);
    
    taskDelay(2);
    
    t3=sysTimestamp();   /* Get the timestamp */
    
    tt3=tickGet();
    
    //logMsg("is : t2=%d t3=%d tt2=%d tt3=%d t3-t2=%d tt3-tt2=%d\n",t2,t3,tt2,tt3,(t3-t2),(tt3-tt2));
    //logMsg("miao :%d ms %d\n",((t3-t2)/399783),a,0,0,0,0);
    //logMsg("freq is :%d\n",freq,0,0,0,0,0);
    //logMsg("time is : %d\n",((t3-t2)/freq),0,0,0,0,0);
    abb[a]=(tt3-tt2);
    acc[a]=((t3-t2)/freq);
    	a++;
  }
}

void printacc() /* Print uses the timestamp difference to calculate the delay time */
{
    
	for(int i=0;i<10;i++)
	{
    
		logMsg("time is : %d ms\n",acc[i],0,0,0,0,0);
	}
}

void printabb() /* To print using tick Difference calculation delay time */
{
    
	for(int i=0;i<10;i++)
	{
    
		logMsg("tick time is : %d ms\n",abb[i],0,0,0,0,0);
	}
}

void msDelay(int ms)   // Delay function encapsulation   Use timestamp to delay 
{
    
	int t,t1,t2;
	t1 = sysTimestamp(); /* Record the timestamp of the last round */
	while(ms--)
	{
    
		t = 0;  /* Counter reset */
		while(t < sysTimestampFreq()/1000)
		{
    
			t2 = sysTimestamp();  /* Read the current timestamp */
			if(t2 > t1)
				t += (t2-t1);
			else
				t += t2;
			t1 = t2;
		}
	}
}

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CPU Time stamp ：

sysTimeStamp() Also known as “ Time stamp ”. It is realized through the system clock . At first, I also felt puzzled , The timing cycle of the system clock is tick, How to realize high-precision clock ? By reading BSP Underlying code discovery ,sysTimeStamp In fact, high-precision timing is obtained by reading the current count value of the timer . adopt sysTimestampFreq() Function can get the frequency of the system timestamp , It often reflects CPU The reference frequency of the timer . Of course , Such a high resolution can only be an ideal value , Different systems may not be able to achieve . After all, the implementation of this timestamp has a fatal weakness ： Through query . The system clock timing interrupt is based on ticb： Unit , Further improve the resolution and read the count value of the timer (CPU A special function register of ), It can only be realized by query .

To get the precise time interval of milliseconds , And frequent calls do not cause the system CPU High occupancy .
I suggest taking adopt CPU Machine cycle solution .
Basically, most mainstream CPU All one. 64 Bit registers are used to record CPU After power on Number of machine cycles .
You can write an assembly function to get this 64 Bit count .

Then you start the program , Make a 10 Second timing , Get it once before and after the timing CPU Machine cycle .
And then subtract , You can get every second CPU Machine cycle ,
Divide this number by 1000 It's every millisecond CPU Machine cycle .
This value can be used as a constant in operation , Use us to name it Millisecond factor

Then you take the previous execution and the next execution when you perform the last execution ,
In the next execution, take the previous execution and the next execution .
Subtract twice to get Spaced CPU Machine cycle . Then use the above millisecond factor to convert it into milliseconds .、

Using this method, you just want to be accurate to nanosecond It's OK, too .

Tick And timestamp difference

vxWorks Under the tick and timestamp There's no correlation . Nor is it the relationship between seconds and minutes that you said .

tick In fact, it can be regarded as the frequency of kernel scheduling , It's also a time slice 、watchdog Overtime 、 Unit of task delay , So it is system clock.timestamp Can get high-precision timing , Follow RTOS Scheduling doesn't matter . For example, the count of reading registers at a certain time is A, Read again after a period of time as B, The main frequency is f. that B and A The time distance between them is ：T = (B-A)*f.( Suppose it is used as timestamp Of timer No, rollover) Corresponding to hardware ,tick as long as timer Capable of producing periodic The interruption is That's ok , Used as a timestamp Of timer Ask for a little more .

Timestamp common functions

If the support Timestamp,BSP The following functions should be provided
sysTimestampConnect() - Connection timestamp interrupted
sysTimestampEnable() - Enable timestamp
sysTimestampDisable() - No timestamps
sysTimestampFreq() - Frequency of obtaining timestamp
sysTimestampPeriod() - Get timestamp period
sysTimestamp() - Get a timestamp
sysTimestampLock() - No interruptions , Get a timestamp
sysTimestampInt() - Optional timestamp ISR