FXOSC clock module

[vishnuvariyath_2]

I am using S32K311 MCU and there are 4 clock options are available 

1.FIRC

2.SIRC

3.FXOSC

4.PLL

If I am using external crystal, FXOSC is active, which has two modes Crystal mode and bypass mode, actually I didn't get the idea about these modes. What will be the FXOSC_clk_out when I am using these two modes?

[danielmartynek]

Hello @vishnuvariyath_2,

The Crystal mode uses a crystal at the XTAL, EXTAL pins.

The Single-Input Bypass mode does not use any crystal but an external oscillator connected to the single EXTAL pin.

Regards,

Daniel

[vishnuvariyath_2]

@danielmartynek 

Thank you for your support. In my understanding if we are only using an external crystal, then the FXOSC is always in Crystal mode. Right?

In the datasheet of S32K311, it is mentioned that the crystal value must be 8-40MHz and input capacitance of Pins is 7pF.There is no other information regarding the selection of crystal, Now I am using 16MHz crystal with Load capacitance 8pF, Is that okey? 

[danielmartynek]

Yes, that is correct.

Refer to the S32K3xx Hardware design guidelines:

https://www.nxp.com/webapp/Download?colCode=S32K3_HW-DesignPackage

BR, Daniel

[vishnuvariyath_2]

@danielmartynek 

Regarding the FXOSC, understanded the modes. But how it will operate under in crystal mode.

Regarding the Crystal selection, In the design guidelines (Rev. D3 – September 2023). There is no exact information about the crystal selection. Only the range is given. Are there any things I missed?

[danielmartynek]

Hello @vishnuvariyath_2,

In the datasheet, there is min. EXTAL_SWING_PP specified (Table 42).

Crystal oscillator circuit provides stable oscillations when gmXOSC > 5 * gm_crit (Figure 40).

The gm_crit is defined as: gm_crit = 4 * (ESR + RS) * (2πF)2 * (C0 + CL)2

HWDG: Table 21 footnote 1:

"The Rs and load capacitors values are dependent on the specifications of the crystal and on the board capacitance. It is recommended the customer develops evaluation and characterization of the crystal on their PCB with the part manufacturer."

Regards,

Daniel