i.MXRT1064: Can CCM_CLK1 pins be used for main 24 MHz clock?

[cpm72]

Hello,

We're using an i.MX RT1064 processor in a new board design and are considering a number of options for clocking the device.

If we have a 24 MHz crystal on the XTAL pins for boot, do you know if we can also feed an external, non-differential 24 MHz clock into one of the CCM_CLK1 pins and use that for the ARM core, PLLs and peripherals when our application starts up?

Section 3.1 of the datasheet and the clock configuration tool in the MCUXpresso IDE seem to allow this configuration, but the NXP document 'Hardware Development Guide for the MIMXRT1050/MIMXRT1060' says in section 4 that the CCM_CLK1_P and CCM_CLK1_N pins are for internal test only and 'please make CLK1_N/P pairs remain unconnected'.

If we can use CCM_CLK1 in this way, which pin should we feed the external clock into (datasheet says CCM_CLK1_P), what do we do with the other pin and what logic levels are valid? The XTAL pins require a 1.1V clock, but we were wondering if a 3V clock signal could be used.

Best regards

[kerryzhou]

Hi @cpm72 ,

  From the datasheet, you can use it:

And the CFG tool also can connect it:

But, if you are using for the product, I think you can follow the 'Hardware Development Guide for the MIMXRT1050/MIMXRT1060', still use the external 24Mhz as the PLL input. Just testing, you can this that pin.

As, the external 24Mhz as the PLL input should be more stable in the product design.

Wish it helps you!

Best Regards,

Kerry

[cpm72]

Hello,

Thanks for your reply. I just wanted to clarify things a bit more.

Are you saying that for production, we should following the Hardware Development Guide and avoid using the CCM_CLK1_P pin as the main 24 MHz clock input, but for testing purposes it's okay?

If we did still want to use the CCM_CLK1_P pin as an input, what voltage swing can it tolerate? Table 4.3.1.3 in the datasheet only gives the LVDS output characteristics.

Also, for boot ROM start up, do we always have to provide a 24 MHz crystal or external clock on the XTAL pins even if we want to use the CCM_CLK1_P pin?

Regards

[kerryzhou]

Hi @cpm72 ,

   Just confirm with you at first, do you want to use the CCM_CLK1_P input the clock to the PLL also for the production?

   Then, I will help you to double check with our internal expert,  and also the voltage swing and ROM boot situation, thanks.

  Please help to confirm it!

Best Regards,

Kerry

[cpm72]

Hello,

Yes, we would like to know if the CCM_CLK1_P pin can be used as the main 24 MHz clock input in the production units.

We wondered if we can boot first from a 24 MHz crystal and then switch to a more accurate 24 MHz  reference clock on the CCM_CLK1_P pin that comes from another board.

We still want the MCU to start up and run if the other board is not present though, hence the need for the crystal and why we're not just connecting the external clock into the XTALI or XTALO pins.

Regards

[kerryzhou · Accepted solution]

Hi @cpm72 ,

  I talk with our internal expert about your issues.

  We highly don't recommend you use the CCM_CLK1_P clock source for the product, as that is just the internal test usage, not the product usage.

  If you want to input the 24Mhz accurate clock, you totaly can use the XTALI, it is the reference clock, not the crystal.

So, don't consider the CCM_CLK1_P as the main clock source, even our AE and our support engineer, didn't used that before, just the internal testing usage.

So, you can input the 24Mhz to the XTALI, then select the reference clock, instead of the crystal.

Wish it helps you!

Best Regards,

Kerry

[cpm72]

Hi Kerry,

Thank you very much for the clear guidance. We will do as your tech experts suggest.

Regards,

Chris