Firmware configuration if VBUS is less than 4.3V

[Clayton Karmel]

We would like to power VBUS on nRF52840 from 3.8V.  We recognize that the chip will not generate an attachment event when VBUS is below 4.3V.   But our USB interface is to an on-board host (and not detachable), so we only have to wait for power supplies to settle.

Can we set ENABLE register and USBPULLUP register to both enable the USB peripheral and pull up D+ so that the host will enumerate?

[Håkon Alseth]

Hi,

 

It looks like the pictures wasn't attached correctly, could you try to upload them again?

We would like to power VBUS on nRF52840 from 3.8V.  We recognize that the chip will not generate an attachment event when VBUS is below 4.3V.

With such low voltage, you are not compliant with the USB specification, as described here:

https://www.beyondlogic.org/usbnutshell/usb2.shtml

From an nRF perspective, your events for power detect/remove will be influenced by having such a low voltage:

https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html?cp=5_0_0_4_2_7_6#unique_506524919

We cannot guarantee the operation when running the USBD outside of the specification. That being said, you can have a look at this thread, which has a similar setup (voltage wise):

 USB CDC ACM transmission is broken on voltage drops 

The root-cause in the above case was the usage of series-resistors on the D+/D-, which shall be 0 ohm (or directly connected without any resistors).

Can we set ENABLE register and USBPULLUP register to both enable the USB peripheral and pull up D+ so that the host will enumerate?

My official recommendation is to power the VBUS within the range shown in the product specification.

For your scenario, I would strongly recommend that you thoroughly test to ensure that it behaves per your requirements.

 

Kind regards,

Håkon