What exactly is required to run USB on an nRF52840 without an external crystal?

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 15:10:58 GMT

Hi,

[quote user="Bk37"]One last question as an aside, I assume that BLE will still require a 32.768 KHz external crystal, correct? Or is there a way to make BLE work with this configuration as well? I can open a new question thread if necessary, just curious.[/quote]

BLE does indeed require a LFCLK source, but it does not need to be a physical crystal.

You can use the 32k RC oscillator for this.

This is already handled with your previous setting of the LF clk source:

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y

So if you want to evaluate some bluetooth-samples, remember to include this configuration in addition to your DCDC-config.

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 14:58:17 GMT

One last question as an aside, I assume that BLE will still require a 32.768 KHz external crystal, correct? Or is there a way to make BLE work with this configuration as well? I can open a new question thread if necessary, just curious.

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 14:44:35 GMT

Hi,

Thank you for the kind words. I am always happy to help out, and I am so glad to hear that you got it working!

Hope you have a wonderful day!

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 14:41:34 GMT

This fixed it! Thank you very much! Since you mentioned the DCDC stuff, I also realized that I had a shared board file for this and another project that used some of the DC-DC components, so I removed that file, added your code, and it worked.

Thanks mate! Nordic support is #1 by far, the main reason I will stay loyal to the brand.

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 14:28:42 GMT

Hi,

Sorry, I oversaw that your device is in LDO operation.

If you are on the latest NCS v2.9.0, you should add this to the overlay (example: nrf52840dk_nrf52840.overlay) file:

&reg1 {
	regulator-initial-mode = <NRF5X_REG_MODE_LDO>;
};

Or if you are using older version, you can add this to your prj.conf:

CONFIG_BOARD_ENABLE_DCDC_HV=n
CONFIG_BOARD_ENABLE_DCDC=n

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 14:13:58 GMT

I added that line to the prj.conf, but still no change. When the device is plugged into USB, it's receiving power, and VDD is even outputting the proper voltage, but no device is showing up on my PC. Again, the only difference between this and my other functional board is the external 32 KHz oscillator.

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 13:55:13 GMT

Hi,

That is great! The module does indeed have a 32 MHz xtal in-place.

[quote user="Bk37"]Should USB still will under this layout? As far as my research tells me, if the 32 MHz crystal would suffice for USB, it would be automatic, like it's automatic when a 32 KHz crystal is connection.[/quote]

R30 and R31 should be 0-ohm, as the resistors are internal to the nRF.

Other than that, I do not see any issues.

XTAL will be requested by the driver itself, so no worries there.

To select internal 32k RC oscillator for your LFCLK, please append this option to your project:

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 13:31:19 GMT

I am using a 3rd party board with this chip, after doing some research, it turns out that it does in fact include an external 32 MHz crystal, it does NOT include a 32 KHz crystal.

Should USB still will under this layout? As far as my research tells me, if the 32 MHz crystal would suffice for USB, it would be automatic, like it's automatic when a 32 KHz crystal is connection.

This is the schematic we are using:

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 12:34:08 GMT

Hi,

[quote user="Bk37"]Why does it specifically mark a 32 KHz crystal as "optional" if a crystal is not optional at all?[/quote]

32 MHz is required, while the 32.768 kHz is optional. Please see the former:

[quote user="hkn"]

As mentioned in the comments here, the 32.768 kHz external crystal is optional, but the 32 MHz one is required:

https://docs.nordicsemi.com/bundle/ps_nrf52840/page/ref_circuitry.html

[/quote]

Could you please confirm your circuitry?

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 11:54:24 GMT

But doesn't the MCU have a built in crystal/clock? I assume I cannot reliably use the built-in ones for USB or BLE?

Why does it specifically mark a 32 KHz crystal as "optional" if a crystal is not optional at all? That doesn't make sense to me.

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 11:49:07 GMT

Hi,

As mentioned in the comments here, the 32.768 kHz external crystal is optional, but the 32 MHz one is required:

https://docs.nordicsemi.com/bundle/ps_nrf52840/page/ref_circuitry.html

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 11:26:02 GMT

I do not have an external crystal present, no. The datasheet only marks an external crystal as "optional" under USB power. I only have whatever is present within the nRF52840.

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Håkon Alseth — Wed, 12 Mar 2025 11:10:24 GMT

Hi,

Do you have a 32 MHz xtal present on your design?

Kind regards,

Håkon

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Bk37 — Wed, 12 Mar 2025 10:35:10 GMT

On my old builds (which ran perfectly), I just used a single external 32 KHz crystal.

RE: What exactly is required to run USB on an nRF52840 without an external crystal?

Turbo J — Tue, 11 Mar 2025 21:52:55 GMT

Which crystal did you omit? The 32kHz one is optional, the 32MHz one is mandatory for BTLE.