nRF52840 Schematic questions

[eiadj]

Hello,

We are currently migrating a product to the 52840 and had several questions regarding design:

1.  Looking at reference schematics provided by Nordic (file attached), there are capacitors designated as "N.C." Are there any downsides to leaving these capacitors completely out of our design? specifically;
   
   1. C10 connected to DEC4 and DEC6
   2. C13 connected to DEC2
2.  based on reference schematics, designs where USB is not used has "VBUS" shorted to ground. Is this necessary or can it be left floating? If we choose to have a 0-ohm jumper as an option to short, is it ok if the resistor is far from the pin?
3.  We are having trouble with space on our layout for the 52840 based on the assumption that most critical components should be near the 52840, are there any of the critical discrete components (i.e. anything shown in the reference schematic attached) that can be moved further away?

Thank you!

Eiad Jandalinrf52840_qiaa_var5_schematic.pdf

[eiadj]

Building off question #3:

• are there any components mentioned in the reference schematic that are not necessary and be removed? 
• based on the reference schematic BOM (attached), can any of the components be shrunk to smaller packages? (e.g. 0402->0201)nrf52840_ckaa_var5_bom.xls

[Kaja]

Hi,

 Looking at reference schematics provided by Nordic (file attached), there are capacitors designated as "N.C." Are there any downsides to leaving these capacitors completely out of our design? specifically;

1. C10 connected to DEC4 and DEC6
2. C13 connected to DEC2

 - No, you are fine leaving these capacitors out of your design, they are there mainly for historical reasons, as it may be needed in some corner cases.

 based on reference schematics, designs where USB is not used has "VBUS" shorted to ground. Is this necessary or can it be left floating? If we choose to have a 0-ohm jumper as an option to short, is it ok if the resistor is far from the pin?

 - Our recommendation is to ground VBUS, and leave DECUSB, D+ and D- floating when USB is not in use, and I would recommend that you ground it close to the pin. 
 

We are having trouble with space on our layout for the 52840 based on the assumption that most critical components should be near the 52840, are there any of the critical discrete components (i.e. anything shown in the reference schematic attached) that can be moved further away?

 - All of the components in the reference is recommended to be placed close to their pin, decupling capacitors, DEC capacitors and DCDC indcutors. The placement of the RF components, and RF path should be directly copied from the reference layout. 
 

eiadj said:

are there any components mentioned in the reference schematic that are not necessary and be removed? 

 - reference no 6 is the schematic option with the smales amount of needed components:

eiadj said:

based on the reference schematic BOM (attached), can any of the components be shrunk to smaller packages?

 - Yes, in theory you can do this for all the components, but have in mind the the specs. for the components, and pin's. 

Best regards,
Kaja

[eiadj]

Hi,

Thank you for the detailed response. My only follow-up question for now is:

1. To ensure that we remove components responsibly, could you let us know the corner cases that would require C10 and C13?

Thanks!

[Kaja]

They are added there under our engineering face, and it was conducted that they are not needed, and therefor put to N.C. So you are safe to remove them. We have not had any situasjons where you need to add them back.. so you should be okay not having the footprint. 

Best regards,
Kaja

[Harsh Bhatt]

Hello Here In the schematic DEC4 And DEC6 Is connected.

So my question is that its mandatory to do this.

If yes than what is the problem if we dont connect this.

[Kaja]

You can end up having radio problems if you don't connect them.. as the decupling on the internal power supply will be wrong. Always follow the reference design for the IC you are using. 

FYI: This is an old case, so if you want help with your design, make a ticket and upload HW files and ask for a HW review. 

Best regards,
Kaja