Question, i.MX28 RTC

[Aemj]

Dear team,

I would like to ask about RTC inside of i.MX28.

The issue is seen on my customer’s lab.

The issue is RTC gains 50 seconds per 7 hours.

The issue occurs on the customer’s board and EVK as well.

They use LinuxBSP, L2.6.35_1.1.0.

The source clock of RTC is 32.768KHz crystal.

The procedure to see the RTC is as below.

- Right after Boot-up,

     Set-up system clock by ‘date’ command.

     Copy the date to RTC by ‘hwclock –w’ command.

- Left for 7 hours.

- Get system clock by ‘date’ command.

- Get RTC by ‘hwclock’ command.

As the result,

The system clock seems to be right, but RTC gains 50 seconds.

Do you have any ideas on the cause of this phenomenon?

Thanks,

Miyamoto

[Yuri · Accepted solution]

Hello,

Basically, RTC accuracy is fully defined by accuracy of the external crystal.

Nevertheless, please try using 24 MHz as RTC source.

Have a great day,
Yuri

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[Aemj]

Hi Yuri,

The customer believes that using 24MHz for RTC source is not possible for their application, because they have to use battery-backup mode. In that mode, 24MHz clock cannot be used for RTC, right?

The customer thinks that the 32.768KHz crystal signal is weak and it can be affected by a noise from internal DCDC. Because they saw the issue did not occur when i.MX28 is in back-up mode.

Is it possible for you to reproduce the issue on your EVK?

Here is the procedure that they did.

The customer saw this phenomenon occur on your EVK as well.

For i.MX28 EVK, they use LinuxBSP L2.6.35. And they did the following command to switch RTC clock source to 32.768KHz.

memtool 80056064=00000021

Thanks,

Miyamoto

[Yuri]

Hello,

I found on my i.MX28 EVK drift of several seconds per day.

To decrease it one can use more stable crystal / oscillator

or perform calibration for each board.

Regards,

Yuri.

[Aemj]

Hi Yuri,

Thanks a lot for your support.

OK, I will tell it to my customer.

Thanks,

Miyamoto

[Yuri]

Hello,

Have You tried just to use sysfs interface ?

$cat /sys/class/rtc/rtc0/date

$ cat /sys/class/rtc/rtc0/time

Regards,

Yuri.