Adaption of Internal Load Capacity of i.MX 8QM Crystal Interface

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Adaption of Internal Load Capacity of i.MX 8QM Crystal Interface

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Contributor III

Hello,

 

we are trying to change the Load-Capacity for the 32kHz and the 24MHz crystals, which has to be done inside the i.MX 8QM.

How can we do so? We found BRD_OSC_CAP_TRIM_VALUE_24M and BRD_OSC_CAP_TRIM_VALUE_32K in the reference manual, but changing them in U-Boot is forbidden. Inside the firmware we can not find any settings.

 

I hope you can help us here,

 

Best regards,

Adrian

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NXP TechSupport
NXP TechSupport

Hi Adrian

additional details were sent via mail.


Best regards
igor
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Contributor III

Hello Igor,

we followed the inctuctions inside the HW Developers Guide. Trying to disable the Load-Capacitors (by setting them to 0pf) we used this command:

fuse prog 0 768 0xA0080000

So this should mean:

BRD_OSC_CAP_TRM_VALUE_32K[19..16] = 1000 (0 pF)

BRD_OSC_CAP_TRM_VALUE_24M[3..0] = 0000 (0 pF)

BRD_OSC_24M_TRIM_VALID = 1 (Valid)

BRD_OSC_32K_TRIM_VALID = 1 (Valid)

Measuring the Load-Capacity with these Fuse settings, shows that something went wrong.

The Capacity is still about 15 - 16 pF at each of the four Load-Capacitors.

Have you verified the information given in the Dev Guide? Or is there something we missed?

Best regards,

Adrian

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NXP TechSupport
NXP TechSupport

Hi Adrian

yes Dev Guide was verified internally.

Could you create document (pdf, word) with detailed explanation

how you measured load-capacity. So document should show:

a) unblown fuses, test result.

b) blown fuses, test result, expected result.

If this is custom board please also provide schematic, you can create service request for that.

If crystal is used, please provide its datasheet with load capacitance, ESR data recommended

by crystal vendor. 

Best regards
igor

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Contributor III

Hi Igor,

thank you for the information. We are now able to change the settings. Do you have any further information about the Capacity Tolerance of the internal Load-Capacitors? The i.MX 8 datasheet only states typical values.

Best regards,

Adrian

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NXP TechSupport
NXP TechSupport

Hi Adrian

 

sorry I have not such info. In general, the parasitic capacitance is about 4-5 pF,

depending on the PCB. This capacitance needs to be added to the trimmed value.

So if you trim for example 16 pF, Cload should be 16/2 + 4-5 = 12 pF. Given the

dependency of the PCB, matching test is needed for each design.


Best regards
igor

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Contributor III

Hi Igor,

thank you for your efforts.

We need to be sure about the frequency error resuling from the internal load capacitors. Elsewise we have to use external load capacitors.

Aren`t there any requirements from NXP Side to there design? Or are those internal capacitors just made for usecases where the frequency is not that important?

Best regards,

Adrian

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NXP TechSupport
NXP TechSupport

Hi Adrian

24MHz frequency is important and described in

sect.7.9.1 Oscillator tolerance (24 MHz)

i.MX 8QuadMax/i.MX 8QuadXPlus Hardware Developer’s Guide

Best regards
igor

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