Using an External Clock instead of XTAL

[kbell_hardware]

Hello,

When reading pages 13-14 of the IMX6Q hardware development guide it seems to suggest that you can use an external clock source instead of a crystal for the 24MHz input clock. However it is unclear whether this clock source must be hooked to the XTALI pin or whether you can leave the XTALI unconnected and use the CLK1_N, CLK1_P or CLK2_N, CLK2_P pins instead. The XTAL pins use the NVCC_PLL_OUT power group (1.1V) so it seems to me as though you would have to get an external clock that does not switch at a voltage level above 1.1V. I have had much difficulty finding an external clock source like this and i don't know if one exists.

1. Can the the XTAL pins be driven by a 1.8V clock source?

2. Can the CLK pins be used instead of the XTAL pins? (XTAL pins grounded)

Thanks,

KBell

[TomE]

Yuri wrote:

> > Can the the XTAL pins be driven by a 1.8V clock source?

> No.

There's always the option of using a level-converter chip or a clock buffer to change the voltage levels.

Or something as simple as two resistors. Am I missing something obvious here?

Tom

[grp62]

Hi,

I plan to use an +3.3V external 24Mhz oscillator with a level shifter to provide the main clock to the IMX6Q on the XTALI pin.

My question is:

Can i use the NVCC_PLL_OUT output to supply my level shifter or do i need to provide the +1.1V from an external LDO ?

Thanks

Greg

[TomE]

If you use two resistors as the level shifter then you don't need to power it:

http://en.wikipedia.org/wiki/Potentiometer#Theory_of_operation

> Can i use the NVCC_PLL_OUT output

Does the Data Sheet document how much current you can draw from it? Or is it only meant for connecting a filter capacitor to. If they don't specify using it as a power source then I'd advise against doing so.

Tom

[Yuri · Accepted solution]

From the Design Checklist of the Hardware Development Guide for i.MX 6 :

If feeding an external clock into the device, XTALI can be driven DC-coupled with XTALO floated.

The XTALI signal level must swing from ~80% of NVCC_PLL_OUT to ~0.2V.

So,

1.
> Can the the XTAL pins be driven by a 1.8V clock source?

No.

2.
> Can the CLK pins be used instead of the XTAL pins?

No. 24 MHz is default ref. clk for the PLLs after reset.
Therefore 24 MHz is needed at least for startup.

[gavinjones]

Hi Yuri,

Further to this discussion, please may I draw your attention to a new thread I recently started:

Input spec for XTALI pin when using external clock source

I do not yet have any replies to the new thread, but as it is essentially a continuation of this thread I am hoping that you (or others who have commented on this thread) may be able to help.

Best regards,

Gavin.

[kbell_hardware]

Thanks Yuri,

So then it would seem that i have no choice but to use a crystal for the 24MHz input since it doesn't appear to me as a 1.1V clock sources exist? Do you have any knowledge of this?

I find it sort of hard to believe that Freescale would suggest an external oscillator if there was no possibility of using one..........

Thanks,

KBell