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

[Bk37]

Long story short, I trusted the datasheet (big mistake) where it says an external crystal is optional. In previous builds, I had an external crystal onboard anyway, so this issue never arose, but now I don't want an external crystal, but it turns out the "optional" marking in the datasheet really means "there's no easy way to do this, oh ya, and no clear guide either".

1. I have no external crystal oscillator, how can I use USB as an HID device still?

2. Can I still use BLE reliably without the external crystal? I'm depending on 7.5ms intervals, 16 Kb/s throughput.

Please don't paste minute code snippets without telling me where to paste them. Thank you!

[Turbo J]

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

[Bk37]

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

[Bk37]

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.

[Håkon Alseth]

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

 

[Bk37]

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.

[Håkon Alseth]

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

[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.

[Håkon Alseth]

Hi,

 

Bk37 said:

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.

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