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iMX6Q Internal oscillator frequency (for WDT)

Question asked by Łukasz Majewski on Feb 17, 2018
Latest reply on Feb 19, 2018 by igorpadykov

Dear Support,

 

Problem:
========

When I do set WDT to fire after 60s, it actually fires after ~42s.


My current findings:
====================
1. Errata for iMX6Q [1]:

ERR007926:
"The internal ring oscillator is less accurate than a crystal oscillator and may be up to two times
faster than a 32 kHz external crystal oscillator"

So the internal frequency may be up to 64 kHz? Is this correct?

2. WDT uses "Low frequency clock" (ckil_sync_clk_root) as input clock for WDT counter.

3. My PCB:

RTC_XTALI connected to GND via 1K, RTC_XTALO N.C. (floating) -
Please correct me, but it seems to me that iMX6Q will be forced to use its internal oscillator.

Just to compare - Wandboard QUAD rev. C uses 32.768 kHz oscillator connected to
RTC_XTALI and RTC_XTALO [3].

4 Measurements on my system (the CLKO1 output)

The ckil_sync_clk_root = 42 kHz (not expected 32 kHz or 32.768 kHz).

5. From the Applications Processor Reference Manual [2]:

Chapter 71. Cristal Oscillator (XTALOSC)

"Supply another ~32 kHz clock source based off an independent internal oscillator if
there is no oscillation sensed on the RTC_XTAL bumps(contacts) (32 kHz specific
feature). The internal oscillator will provide clocks to the same on-chip modules as
the external 32 kHz oscillator."

"Automatically switch to the external oscillation source when sensed on the
RTC_XTAL bumps(contacts) (32 kHz specific feature)."


Conclusion:
===========
===========

Please correct if I'm wrong, but it seems to me like:

- My iMX6Q board is forced to use internal oscillator

- The internal oscillator can generate frequency up to 64 kHz.

- I shall connect the 32.768 kHz oscillator to RTC_XTAL contacts to
have proper WDT operation.

References:
[1] https://www.nxp.com/docs/en/errata/IMX6DQCE.pdf
[2] i.MX 6Dual/6Quad Applications Processor Reference Manual, Rev. 4, 09/2017
[3] http://download.wandboard.org/wandboard-imx6/documentation/wb-edm-imx6-rev-c1.pdf
page 5/8


Thanks in advance for help,
Łukasz Majewski

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