Timestamp timer problem

[Altera_Forum]

Hi , 

 I am having some problems in using the timestamp timer. Basically , when i am measuring the time to perform a single instruction, i occasionally(but not always) get a negative answer.I have taken care to call alt_timestamp_start() whenever I use the timer, to ensure that it does not overflow.  Also when i don&#39;t change the program and run it again, each time i get a slightly different number for the number of cycles taken. What could be the reason?  My code is as follows :    //CODE   if(alt_timestamp_start() < 0)  {  printf("Timer init failed \n");  exit(0);  }   time1 = alt_timestamp();  time2 = alt_timestamp();   timer_overhead = time2 - time1;  printf("\n timer overhead is %d",timer_overhead);   ......... .......... .......   time1 = alt_timestamp();  *(peripheral_base ) = 0x210f1007   time2 = alt_timestamp();  printf("\n time to perform 1 write to hw is %d" ,(time2 - time1 - timer_overhead));   printf("\n time1 is %lu",time1);  printf("\n time2 is %lu",time2);   //END OF CODE    I get the following output : //OUTPUT     timer overhead is 246 time to perform 1 write to hw is -12  time1 is 143  time2 is 377  //END OF OUTPUT    Any suggestions ? Am i missing out something ? Thanks, Karthik

[Altera_Forum]

you have to turn on bit31 of the address to uncache the io port access. 

or use IORW macro.

[Altera_Forum]

feynyan : 

you have this problem but otherwise you can read value returned by  al_timestamp() funciont. In my case this function returns always 0  I cannot understand why  Do you have some idea? ciao  walter

[Altera_Forum]

You need to start the timestamp first.  

Also the timestamp doesn't roll over, so if it has reached its maximum value it needs to be started again.