Need technical assistance on crystals used in MIMXRT1024CAG4B

[Muthulakshmi]

Hi,
We are using the MCU part number MIMXRT1024CAG4B in our design. We have checked the reference manual and the evaluation board schematics for that part number. Kindly confirm the below points.

1. In EVK schematics, for 24MHz crystal XRCGB24M000F2P29R0, 2 capacitors valued 4.7PF is being used (Refer page number 4 in EVK schematics). Can you please give us suggestions on how the capacitor is being selected and need to be selected for 24MHz crystal. Also please confirm there is resistor valued 2.2M ohms is being connected with XTALI pin, is needed or not.

2. In EVK schematics, for 32.768KHz crystal FC-135 32.7680KA-AG3 with load capacitance 7PF, 2 capacitors valued 10PF is being used (Refer page number 4 in EVK schematics). But in datasheet, it is mentioned that the capacitance implemented on either side need to be twice the load capacitance of crystal (Refer page number 16 on datasheet). From this I can understand that we need to use 14PF capacitor on either side. Please suggest us on this regard.

[EdwinHz]

Hi @Muthulakshmi,

1. You can find information about the proper selection of a crystal for XTALI/XTALO on the Hardware Development Guide for the
MIMXRT1020 Processor. I believe this information will prove useful for this inquiry.

2. The datasheet is not trying to say that the requirement is that the capacitors on either side must be twice the load capacitance of the crystal. It mentions that they are "about twice the crystal load capacitor" but that "the board capacitors need to be reduced to account for board and chip parasitics." This is the reason why the EVK does not place 14pF capacitors for the 32.768KHz crystal, but rather 10pF ones. My recommendation would be to follow the EVKs values instead.

BR,

Edwin.

[Muthulakshmi]

Hi @Edwin,
1. Regarding query 1, I have checked the hardware development guide you have provided. It has the information related to selection of 24MHz crystal only. It doesn't have any information about the capacitors and resistors selection. Kindly help with the selection of capacitors and resistor

2. Thanks for the information provided for query 2

[EdwinHz]

Hi again @Muthulakshmi,

The MCU does not determine the the value of the capacitors for the crystal, this is really determined by the crystal itself, as stated by my colleague on the following community post:

https://community.nxp.com/t5/i-MX-RT/24MHz-Crystal-Load-Capacitance-Value-in-RT105x/m-p/1317583

That said, the value to use would depend on the crystal of choice. I believe the following information will prove useful for your inquiry on capacitor selection for an oscillator crystal:

https://www.nxp.com/docs/en/application-note/AN3251.pdf

https://electronics.stackexchange.com/questions/121659/how-to-select-capacitor-for-a-crystal-oscilla...

https://blog.adafruit.com/2012/01/24/choosing-the-right-crystal-and-caps-for-your-design/

BR,

Edwin.