IMX8MM With MEMS Oscillators

[user1092]

Hello,

I am trying to design a board using the IMX8M Mini, the design requires the use of MEMS oscillators.

The datasheets are not very clear for this arrangement.

IMX8MMIEC:
3.1.4 states that the RTC_XTALI is connected to an external oscillator and RTC_XTALO is connected to VDD_SNVS_0P8.

There is then a statement about the XTALI pin (assumed to be 24M_XTALI) with no description of what to do with the XTALO pin, is this left unconnected?

There are high/low thresholds given for the RTC clock but nothing given for the 24MHz clock, what are these thresholds?

IMX8MMHDG:
4.4 mentions CLK1_P/N I cannot find any other reference to this pad in any other document.

There is also a mention of CLKIN_1/2, the use of these pads is rather unclear with the only reference I can find is in the register definitions. I found this post (https://community.nxp.com/t5/i-MX-Processors/Processor-CLKIN-and-CLKOUT-pin-functionality/m-p/100707...) from 2020 with NXP stating it can bypass the XTALI pad and apologising that it is poorly documented, It still isn't very clear.

Should the 24MHz MEMS oscillator be connected to 24M_XTALI or CLKIN_1?

If no crystals are used should "VDD_24M_XTAL_1P8" still be powered as this is omitted from section 2.1 in IMX8MMIEC?

Thanks,
Chris

[Manuel_Salas]

Hello @user1092 

I hope you are doing very well.

It is Only recommended and validated option - use crystal as recommended
in official NXP document - Hardware Guide, sect.Table 12. Oscillator/Crystal
recommendations.

Best regards,

Salas.

[user1092]

Hello Salas,

After re-reading the datasheet there is a section (3.1.4) stating the XTALI can be fed from an external oscillator:

This chip was chosen based on this statement.

The documentation seems to be a bit of a mess, as it also states in this section that the RTC can only be connected to an external oscillator. However, section 3.4.2 discusses how to use the RTC with a crystal oscillator.

You never answered the question about the CLKIN pins and whether these pins can be used if the XTALI pin cannot be driven from an external oscillator.

I have spent almost a year developing this product, I do not want my time to have been wasted. The datasheet states XTALI can be driven from from an external oscillator.

Please could you help, as I need a solution as soon as possible.

Regards,

Chris

[Manuel_Salas]

Hello @user1092 

From internal team:

It sounds like a MEMS resonator on one instance (uses both XTAL pins) and an MEMS clock generator in the other (single ended input, only one).

As far as using a resonator to replace a crystal, no one has asked this to my knowledge (been covering i.MX since around 2008). Will need to do some research on this.

Have to dig on using an external, single ended clock generator since some parts are not able to work with an external oscillator. I don’t recall off hand which one it was so I need to dig on that too.

Do you have any part numbers/dara sheets?

Best regards,

Salas.

[user1092]

Hi Salas,

"It sounds like a MEMS resonator on one instance (uses both XTAL pins) and an MEMS clock generator in the other (single ended input, only one).

As far as using a resonator to replace a crystal, no one has asked this to my knowledge (been covering i.MX since around 2008). Will need to do some research on this."

I'm unsure as to what they are referring here, but a MEMS resonator will not be used by us. To be honest, I had not heard of them and they don't seem too readily available. We will be using a single ended clock source.

"Have to dig on using an external, single ended clock generator since some parts are not able to work with an external oscillator. I don’t recall off hand which one it was so I need to dig on that too."

Please that would be appreciated.

"Do you have any part numbers/dara sheets?"

The part will most likely be DSC6001HI2B-024.0000, See attached for datasheet.

Thanks,

Chris.

[Manuel_Salas]

Thank you @user1092 

I will update to internal team and wait for their answer.

Best regards,

Salas.

[user1092]

Attached file after failed post removed it.

[Manuel_Salas · Accepted solution]

Hello @user1092 

Following up on your inquiry, we have reviewed the RTL and datasheet of the PAD provided by the IP vendor. Based on this review, we can confirm that the PAD supports the use of an external 24MHz reference clock.

As per the IP datasheet:

"In the case of external clock without physical crystal, a differential signal must be applied between PADI and PADO."

This means that the external 24MHz clock must be a differential (complementary) signal applied to the XTALI and XTALO pins. Specifically:

• The signal should swing between 0V and 1.8V.
• When XTALI is at 0V, XTALO should be at 1.8V, and vice versa.

To achieve this, you have two options:

1. Use an oscillator that provides complementary differential outputs.
2. Use a single-ended oscillator along with an external inverter to generate the complementary signal.

Best regards,

Salas.

[user1092]

Hi Salas,

Thank you for the response, this is good news.

Could you also please confirm the data sheet is correct in regards to the RTC. With RTC_XTALO being tied directly to VDD_SNVS_0P8, this sounds like a strange arangement and I would like to just confirm it is the correct arrangement.

Thank you,

Chris

[Manuel_Salas]

Hello @user1092 

Yes, I can confirm that the user's guide is correct, in that case you must tie RTC_XTALO to VDD_SNVS_0P8.

This is from an internal discussion:

Best regards,

Salas.

[user1092]

Hi Salas,
Thank you for clarifying and for your assistance.
Chris.

[Manuel_Salas]

@user1092 

Happy to help, and thank you for your patience.

Best regards,

Salas.

[Manuel_Salas]

Hello @user1092 

Sure, I am waiting for an answer from internal team. I will let you know when any update.

Best regards,

Salas.

[user1092]

Hello Salas,

The design cannot use a crystal oscillator as it is used in a pressurised environment.

You say the only recommended/validated way is with a crystal, this suggests it is possible but not tested. Could you provide information on if it is possible and if it is possible, how?

Could you please provide some information on the purpose of the CLKIN_1/2 as there is little to no documentation about these pins. https://community.nxp.com/t5/i-MX-Processors/Processor-CLKIN-and-CLKOUT-pin-functionality/m-p/100707... suggests the pins can be used to feed the main clock instead of the crystal. Is this correct?

Thanks
Chris.