RE: Firmware configuration if VBUS is less than 4.3V

Håkon Alseth — Thu, 16 Feb 2023 14:53:38 GMT

Hi,

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

[quote user=""]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. [/quote]

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).

[quote user=""]Can we set ENABLE register and USBPULLUP register to both enable the USB peripheral and pull up D+ so that the host will enumerate?[/quote]

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