Custom PCB design for nRF52840

[Fernando Fontes]

Hello,

I've been implementing a ZigBee end device for a while now. At this stage, I feel already comfortable enough to move to the PCB design part. 

However, since this is a very agile development I haven't enough time to explore everything I did correctly. Just to contextualize these are the main components of my code:

•  3 Custom Zigbee Clusters;
•  Watch Dog Timer;
•  FreeRtos;
•  UART serial communication.

I did go into some trouble while developing the UART serial communication, which according to your suggestions lead me to use the external clock source for better precision (avoid bit errors). 

Moving to the question itself, I want to know if I really need to have both clocks like on the PCA1009 board (32.768KHz & 32MHz). I'm a bit confused about what source of the clock the FreeRtos is pending on, but I think that the watchdog timer needs the 32.768KHz.... 

Should I leave both clocks in my custom PCB? Should I start to use the internal clock for the serial communication and install only the 32.768KHz for the watchdog timer?

Are there any implementations of the PCA1009 forKICAD out there

Best regards,

Fernando

[Kenneth]

The 32MHz is required for radio operation.

I don't believe an external 32kHz crystal is needed, you should be able to use the internal 32kHz RC oscillator instead (for watchdog and timing), but I can double check.

Kenneth

[Fernando Fontes]

Hi Kenneth,

Thank you for the reply. I have another question regarding the power supply. What do you think is better, having an external 5V regulator and use the nrf52840 internal voltage regulator for the 3V3 or just having an external regulator for the 3V3 and skip the nrf52840 internal voltage regulator?

Thanks in advance,

Best regards,

Fernando

[Kenneth]

If the regulator specifications for the internal 3V3 meets your requirements in terms of tolerance and maximum external current draw that should be fine:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html#unique_392235502

Kenneth

[Fernando Fontes]

Hi Kenneth,

Thank you for the reply. Well, I think that 25mA is enough. However, it's best to give a margin and give the option for future expansion. For that reason, I'm trying to use an external dc-dc regulator to power everything. I went to this page https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fref_circuitry.html&cp=3_0_0_6_2 and I think that option 6 is the best fit for me. Can you help me to confirm that? Are the EXTSUPPLY, DCDCEN0, and DCDCEN1 registers related to the internal regulator of the NRF52840 right? So, I don't need them. 

My last question is related to the antenna port. I noticed different values for the passive components in the eagle design files and also some circuitry differences between options 6 and 7 for instance. Which one is the most updated version? I want to use an external antenna... 

Best regards,

Fernando Fontes

[Kenneth]

Fernando Fontes said:

Are the EXTSUPPLY, DCDCEN0, and DCDCEN1 registers related to the internal regulator of the NRF52840 right? So, I don't need them.

 Correct. 

Fernando Fontes said:

Which one is the most updated version?

Always check the latest reference design files from the product page:
https://www.nordicsemi.com/Products/nRF52840/Compatible-downloads#infotabs 

Best regards,
Kenneth