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AM3358: Frequency tolerance and frequency stability of 24Mhz Crystal

FRANK1
Expert 8545 points
Part Number: AM3358

Hi Team,

A customer is currently using AM3358BZCZ100 for which i am using a Crystal of 24Mhz

since the crystal is in EOL the alternative parts available are having higher tolerances than mentioned in the Spec sheet ,

I want to understand how will it affect the board.
Frequency tolerance and frequency stability

i would also like to understand ,TI suggests to 24Mhz for AM3358BZCZ100 master clock with tolerance of + - 50PPM , 

If I am using a crystal which has a tolerance of +-100 PPM how will it affect the Performance of the Processor AM3358BZCZ100,

also can you suggest the tests i need to carry out once i change this 24MHz crystal to validate it for the processor AM3358BZCZ100

Thanks.

Regards,

  • 0
    TI__Guru 62855 points

    The accuracy of the AM335x reference clock source should be based on system level requirements. For example, you may need a 30 PPM reference clock if an AM335x timer is being used to operate something that needs this level of accuracy. 

    The 50 PPM limit was defined for AM335x because the RMII Ethernet standard requires this accuracy. The other Ethernet standards only require 100 PPM, so you may be able to back-off on this 50 PPM requirement if not using RMII. However, this depends if you have other system function that require an accuracy of 50 PPM. I would not recommend going above 100 PPM as this may begin to effect other peripheral interfaces.

    Keep in mind, a crystal has three contributions to accuracy. There is a parameter that defines initial accuracy, another parameter that defines accuracy over operating temperature, and aging parameter that defines how much the resonate frequency changes over time due to aging effects. When selecting a crystal, you must combine all of these to determine frequency accuracy across all operating conditions for the life of the product.

    Reliable start-up is also a concern with crystal selection. The max ESR of the crystal is one of the primary concerns. The AM335x oscillator may not have enough gain to reliably start oscillation if the crystal ESR is too low. Many crystal datasheets define a worst case max ESR value for the entire family of crystals rather than a specific max ESR for the crystal being selected. In most cases the higher frequency crystals in the family will have a much lower max ESR value that what is found in the datasheet. Therefore, you may need to contact the crystal manufacture and request a device specific datasheet for the crystal part number you plan to use.

    One way to confirm you have start-up margin, is by inserting a resistor in series with the selected crystal and checking for reliable start-up across all operating conditions. I suggest you begin with a resistor value that is about 5x the max ESR of the crystal. If you have start-up issues with this value you can reduce it to 3x. There is not enough gain margin if oscillation will not reliably start with a 3x resistor. You would need to select a crystal with lower ESR if you find it will not reliably start with a 3X series resistor.

    Regards,
    Paul

        

  • 0 in reply to peaves
    TI__Expert 8545 points

    Hi Paul,

    Thanks for the response , thanks you mentioned about the ESR values , the AM3358BZCZ100 specifies to use a an crystal with Max ESR 48 Ohm ,

    So can i use use an Crystal with ESR of 30 Ohms, Please see the attached Datasheet of Exctal ,
    Also can you suggest the most suitable Crystal among
    AA-24.000MAHJ-T and ECS-240-18-30-JGN-TR

    AA-24.000MAHJ-T.pdf

    ECS-240-18-30-JGN-TR.pdf

  • +1 in reply to FRANK1
    TI__Guru 62855 points

    You neve said if you were using RMII and needed 50 PPM system performance. Therefore, I cannot comment how were these options align with your system requirements.

    I noticed the Digi-key description for your existing crystal, which they are selling under part number 7A-24.000MAAJ-T was cut for an 18pF load, so it must not be one of the standard options shown in the datasheet. What load did you apply to this crystal in your product? Are you willing to change load capacitor values in your product when you change crystals? How much capacitive load does you signals apply to the crystal circuit?

    I do not think a crystal cut for 18pF load was ever appropriate for AM335x. Each load capacitor would need to be approximately 33 - 36 pF to achieve a 18pF load for the crystal and the AM335x datasheet only allows load capacitors in the range of 12 - 24 pF. 

    The highest load capacitance crystal you would be able to use is 12pF to remain in the ranged defined by the AM335x datasheet.

    The AA-24.000MAHJ-T datasheet shows their standard offering is a crystal cut for a 8pF load. However, they provide an option to specify a load capacitance. I did not see an aging parameter in this datasheet, so not sure how PPM will change over time on this crystal.  It may be a viable option as long as you apply the correct load capacitance for the crystal your purchase while remaining compliant with AM335x datasheet. However, you should validate startup reliability using the method described previously to ensure ESR in not going to be a problem.

    The ECS-240-18-30-JGN-TR datasheet does not clearly show an option for buying a crystal cut for a lower capacitive load and 20 pF is too high. You may be able to use this as a viable option if they would sell you one that was cut for 10 pF.

    Regards,

    Paul