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S32K388 FXOSC GM_SEL recommended values for 8/16/20/40 MHz crystals

Hi NXP community,

I am working on an S32K388 project and configuring the FXOSC (Fast Crystal Oscillator). I can see in the reference manual that GM_SEL selects the transconductance of the FXOSC amplifier, but I could not find a clear recommendation table mapping crystal frequency to GM_SEL value.

Current configuration:

Crystal: 16 MHz passive crystal
GM_SEL = 12 (0xC, ~0.7016x)
EOCV = 157
OSC_BYP = 0 (crystal mode)
COMP_EN = 1
This works fine for 16 MHz.

My question: I would like to support multiple crystal options: 8 MHz, 16 MHz, 20 MHz, and 40 MHz.

Could anyone share the recommended GM_SEL values for each frequency? Or point me to an NXP application note or table that provides this guidance?

Specifically:

Is there a recommended GM_SEL value for each standard FXOSC frequency (8/16/20/40 MHz)?
For a 20 MHz crystal, should GM_SEL be increased from 12 to 15 (maximum) for reliable oscillation?
Does GM_SEL selection depend mainly on crystal frequency, ESR, load capacitance, or all of them?
Any guidance or reference would be greatly appreciated.

Best regards

Re: S32K388 FXOSC GM_SEL recommended values for 8/16/20/40 MHz crystals

Hi @xlele 

In section 3.2 of the Hardware Design Guidelines for S32K3xx Microcontrollers, Rev. E2, included in the S32K3 MCUs for General Purpose Hardware Design Package, it is mentioned that “the crystal oscillator circuit provides a very safe and stable oscillation when GM > 5 × gmcrit”, and it also defines how gmcrit is calculated.

Additionally, in section 11.4 of the S32K3xx Data Sheet, Rev. 14, for ALC disabled mode, a generic recommended setting of GM = 4'b0010 is provided.

By last, if for example, according to the crystal specification a transconductance of G = 12 mA/V is required to determine the CTRL[GM_SEL] value:
12 mA = 18.5 mA × N
N = 12 mA / 18.5 mA = 0.6486
The closest value supported by the register settings corresponds to CTRL[GM_SEL] = 1010b → 0.6681×

BR, VaneB