Crystal design on KEA platform

[rmaier]

Hello,

I am using the S9KEAZN64ACLH. I have an issue where the external clock occasionally fails to start. It is only an intermittent issue. Maybe 1-2% of the time, and is usually more common on certain PCBAs.

Attached is the schematic and layout of the crystal used. The crystal has a load capacitance of 20 pF and 16MHz frequency. The load caps are actually 43pF, so the resulting load capacitance here is 21.5pF. R76 has been tried with anything from 0.0 to 6.75k. Lower resistance usually helps, but is not a silver bullet. Replacing C42 and C43 helps, but the only thing that really works is replacing the crystal. So far, that seems to be the only 100% fix.

Any ideas on what could be causing this issue? Is it stray capacitance?

[jingpan]

Hi Robert,

Crystal oscillator circuit provides stable oscillations when gmXOSC > 5 * gm_crit. The gm_crit is defined as:
gm_crit = 4 * ESR * (2πF)2 * (C0 + CL)2
where:
• gmXOSC is the transconductance of the internal oscillator circuit
• ESR is the equivalent series resistance of the external crystal
• F is the external crystal oscillation frequency
• C0 is the shunt capacitance of the external crystal
• CL is the external crystal total load capacitance. CL = Cs+ [C1*C2/(C1+C2)]
• Cs is stray or parasitic capacitance on the pin due to any PCB traces
• C1, C2 external load capacitances on EXTAL and XTAL pins

In addition, please use a 0ohm resistor on R76.

AN3208 is a document for crystal oscillator troubleshooting. You can search it in NXP website.

Regards

Jing

[rmaier]

Thank you for your advice, Jing. Is the transconductance of the internal oscillator circuit provided anywhere? I have not been able to find it in the data sheet or the reference manual. This is for the S9KEAZN64ACLH.

[jingpan]

Hi Robert,

Yes, it seems this value is not list in datasheet. You have to calculate it. The formula above is from AN1783. It is very useful, it can guide you select crystal and measure amplifier characteristics.

Regards,

Jing

[rmaier]

Thanks, Jing.