Some questions I encountered when drawing the nRF52840 core board PCB

[fuyinyuezhang]

Hello Nordic Official! I am currently doing the PCB design of NRF52840 based on your datasheet, but I have some questions.

The manual I refer to is this: [ https://infocenter.nordicsemi.com/pdf/nRF52840_PS_v1.8.pdf ], page 602, "7.3.7 Circuit configuration no. 7 for QIAA aQFN73".

Here are some of my questions:

1. Is the following understanding correct?

Pin VDDH (Y2) supplies power to the chip. When Pin VBUS (AD2) is powered on, it only tells the chip that a USB Host is now connected. Pin VBUS does not supply power to the chip. If you are making the nRF52840 core board and there is only 5V power supply on the PCB, Pin VDDH must always be connected to 5V, and Pin VBUS can be left unconnected. 

2. For newer chips, C9 (820pF) is not needed. Should Pin N24 be changed to N.C., or C9 should be changed to N.C.?

3. Can the pull-up power supply of Pin nRESET use the 3V voltage output by nRF_VDD? When the system is just started, is it possible that the register controlling the nRF_VDD output has not been initialized, resulting in unexpected reset behavior?

[Kazi Afroza Sultana]

Hello,

Pin VDDH (Y2) supplies power to the chip. When Pin VBUS (AD2) is powered on, it only tells the chip that a USB Host is now connected. Pin VBUS does not supply power to the chip. If you are making the nRF52840 core board and there is only 5V power supply on the PCB, Pin VDDH must always be connected to 5V, and Pin VBUS can be left unconnected.

 The recommended operating conditions of VDDH is in between 2.5 V to 5.5 V. It depends on the requirement of your application. 

High voltage mode with DC/DC enabled can regulate the voltage in desired voltage. 

When using the USB peripheral, a 5 V USB supply needs to be provided to the VBUS pin. The operating voltage is 3.3V. That can be achieved by voltage regulator.

VBUS and either VDDH or VDD supplies are required for USB peripheral operation. If nRF usb pin is not in used that Pin VBUS can be left unconnected.

2. For newer chips, C9 (820pF) is not needed. Should Pin N24 be changed to N.C., or C9 should be changed to N.C.? 

You can do that.

3. Can the pull-up power supply of Pin nRESET use the 3V voltage output by nRF_VDD? When the system is just started, is it possible that the register controlling the nRF_VDD output has not been initialized, resulting in unexpected reset behavior?

The internal pull up resistor of RESET pin can be powered by VDD. If the operating voltage of VDD is in the range of recommended ratings, most likely there will not have unexpected behavior. 

[fuyinyuezhang]

Thank you very much for your answer.

I laid out the PCB according to [ 7.3.7 Circuit configuration no. 7 for QIAA aQFN73 (page 602) ] in [ https://infocenter.nordicsemi.com/pdf/nRF52840_PS_v1.8.pdf ].

And provide 5V power supply from USB to the VDDH pin,

Will the chip automatically recognize the DCDC circuit and automatically start DCDCEN0 and DCDCEN1?

[Kazi Afroza Sultana]

Hello,

If you are using USB, you can connect VBUS and VDDH, as in config. 1. 

To ensure good RF performance when designing PCBs, it is highly recommended to use the PCB layouts and component values provided by Nordic Semiconductor.

Regarding your most recent question, to use the DC/DC you have to enable DCDCEN0 and DCDC in the respective registers as well. This is described here.