Load caps on EVK-LPC55S28

[steve5528]

Good day!

EVK-LPC55S28 uses crystal CX3225GA16000D0PTVCC, and in the datasheet the load cap is specified to be 8pf. In the capbank appnote, it gives an example where the xtal loadcap with 8pF value does not need external caps. However the schematic and the BOM shows C77, C80 to be 8PF.

Could you explain why this is the case? what have you seen in your customer's production boards? is AN13057 still applicable?

Thank you.

[Habib_MS]

Hello @steve5528,

As mentioned in the chapter 13.3 called "Crystal oscillator" "In the crystal oscillator circuit, only the crystal (XTAL) needs to be connected with the option to connect capacitances CX1 and CX2 on XTAL32M_P and XTAL32M_N pins. Depending upon the computation of the required Capacitance Load, there is no need to add some capacitance on PCB if computation is less than 20pF (10 pF equivalent IEC), and if computation is greater than 20 pF (10 pF equivalent IEC), then additional capacitance is on PCB required. See Figure 28 and refer to the "Cap Bank API" chapter in the user manual."

I highly recommend see the chapter 13.3.1 called " Crystal Printed Circuit Board (PCB) design guidelines" in order to obtain more information.

Also, the AN13057 is still applicable.

BR
Habib.

[steve5528]

Hello @Habib_MS 

Thanks for the reply. Before I can comment, could you identify and link which document you are referring to for "chapter 13" - both LPC55S2x user manual or AN13057 do not have chapter 13.

Regards

[Habib_MS]

Hello @steve5528, 
I apologice, I forgot mention this information, it is in the datasheet.

BR
Habib.

[steve5528]

Hello @Habib_MS 

Thanks for the reference.  I went thru that doc and few others. My original question was about why EVK-LPC55S28 includes 8pF external load caps.

UM11126 section 12.4.1..1 gives an example of 8pF XTAL load capacitance, and the conclusion is that there is no need for the external load caps.

AN13057 section 4.1 presents the similar example with same result as above.

So, for me it was confusing as to why the EVK-LPC55S28 schematics includes two external 8pF caps. Then i found the following:

In UM11158, section 7.6 it states that "External capacitors are not required on the crystal pins, but are installed on Revision A1 and A2 boards. These may be removed for better performance if issues are observed."

So the question is: if external caps are included on the EVK-LPC55S28 already, then there is no need to use the capbank api, correct?

Thanks!

[Habib_MS]

Hello @steve5528,
In this case is correct, but please take in mind this comment that you mentioned:

"External capacitors are not required on the crystal pins but are installed on Revision A1 and A2 boards. These may be removed for better performance if issues are observed."

Also, if you experience any issue, do not hesitate to let me know.
BR
Habib