Reading time on ZC706 through registers

[piyushj]

Hi everyone,

I am using Xilinx Zynq ZC706 with linux OS running in it made with yocto project. Though while reading the time using POSIX APIs "clock_gettime()" I see some inaccuracy in updating the time at about 3.3ms timer cycle. So I was thinking to read directly from the time registers. Can any one shed some light if anyone has experience this problem and how can I read the time more accurately?

[gudishak]

HI @piyushj ,

Did you try with devmem command to observe the delay?

[piyushj]

Thanks for the help. FInally I read the time through the Globaltimer registers. I simply opened the memory with open() and gave input of proper registers.

if ((mem = open ("/dev/mem", O_RDWR | O_SYNC)) == -1)
	fprintf(stderr, "Cannot open /dev/mem\n"), exit(1);

//base register values of Globaltimer
base_ptr = mmap (0, 8192, PROT_READ|PROT_WRITE, MAP_SHARED, mem, BASE_ADDRESS);
if(base_ptr == (void *) -1)
{
	printf("Memory map failed. error %i\n", base_ptr);
	perror("mmap");
}

valuetimer_low = (unsigned int*)(base_ptr + TIMER1_CONT_LOW);
printf("Timer1_low = %u \n", *valuetimer_low);

valuetimer_high = (unsigned int*)(base_ptr + TIMER1_CONT_HIGH);
	printf("Timer1_high = %u \n", *valuetimer_high);

Thanks,

[rfs613]

---

@piyushj wrote:

Though while reading the time using POSIX APIs "clock_gettime()" I see some inaccuracy in updating the time at about 3.3ms timer cycle.

Which clock source did you use for your experiments? You'd want CLOCK_MONOTONIC_RAW, otherwise you will see (intentional) adjustments to the clock, to keep it in sync with NTP.

[piyushj]

Hi, Thanks for the quick reply. I used CLOCK_MONOTONIC. What is the range of this adjustments?

[rfs613]

---

@piyushj wrote:

Hi, Thanks for the quick reply. I used CLOCK_MONOTONIC. What is the range of this adjustments?

---

It will depend on the actual frequency of the underlying clock (eg. the h/w oscillator). These tend to drift with temperature, too. But some crystals are considerably off, and will thus need quite frequent correction, while others will be much more steady.

Quoting from the manpage:

CLOCK_MONOTONIC
      Clock that cannot be set and  represents  monotonic  time  since
      some  unspecified starting point.  This clock is not affected by
      discontinuous jumps in the system  time  (e.g.,  if  the  system
      administrator  manually  changes  the clock), but is affected by
      the incremental adjustments performed by adjtime(3) and NTP.

CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific)
      Similar to CLOCK_MONOTONIC, but provides access to a raw hard‐
      ware-based time that is not subject to NTP adjustments or the
      incremental adjustments performed by adjtime(3).

[piyushj]

Thank yo very much for heads up!