nRF5340 with internal load caps for 32M XTAL

[mm784]

Hello,

The target MCU on nRF5340 DK seems to use internal load caps for the high frequency 32MHz XTAL because the board does not have external load caps. 

The nRF5340 specification sheet mentions that the internal caps must be configured before enabling them. 

Does it mean that every sample application configures this on the nRF5340?

When I load the "blinky" example, I see the following in the Output .config

1. In what config file is this configured and what are the default values?

2. What is the default startup clock of the SoC? Since the internal caps for the crystal must be configured first, I imagine the SoC must use another clock source before the external XTAL is enabled. 

[Einar Thorsrud]

Hi Mikael,

There is no need to do any adjustments in firmare to get the HF clock running with BT, as taht is always handled by the stack.

The error you get here indicate that the app core is not able to communiacte with the net core. Have you programmed the bluetooth controller (hci_ipc or hci_rpmsg depending on SDK version) to the network core?

[mm784]

Hi Einar,

The only thing I changed was enable this config, so LFCLK is synthesized from HFXO

CONFIG_CLOCK_CONTROL_NRF_K32SRC_SYNTH=y

instead of using internal RC oscillators

 

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y

I am assuming the RC oscillators are not stable enough for BT stack to initialize. 

Thank you,

Mikael

[Einar Thorsrud]

Hi,

Using synthesized LFCLK is usefull in some cases (particulrily during testing), but it is not much used in production, because it leads to a high current consumption. It is also not used during our BLE testing, so unless you have a good reason for using it(?) I suggest you do not.

mm784 said:

I am assuming the RC oscillators are not stable enough for BT stack to initialize. 

Thsi sounds strange. Can you elaborate? The BT stack works well with th ecalibrated LFRC, but that is used for keeping track of time (waking up on connection events etc). When the radio is used, the HFXO is started, and when the radio is no longer needed, it iss topped (this could be for instance between BLE connection events or advertising events or similar).

I am a bit unsure about the background for these quiestions. Are you facing another issue that you are tryign to resolve? Perhaps you can backtrack a bit and explain what you are trying t achieve/fix?

[mm784]

Hi Einar,

Thank you for your detailed response and your explanations. I am not trying to do anything unconventional. To test out the radio, I am running the example from Nordic Bluetooth Fundamental course, specifically "Lesson 2, Exercise 2". In this exercise the device is supposed to start advertising as a Nordic Beacon. I am using the code provided in the solution for this exercise as is - I did not change anything in the source code. But since I don't have an external 32.768kHz XTAL, I added following to prj.conf to enable RC Oscillators and I chose the lowest jitter at 20ppm. 

#Enable Internal 32kHz RC Oscillator

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y

CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC_CALIBRATION=y

CONFIG_CLOCK_CONTROL_NRF_K32SRC_20PPM=y

#Disable use of 32kHZ XTAL

CONFIG_SOC_ENABLE_LFXO=n

#Configure 32MHz Clock

CONFIG_SOC_HFXO_CAP_INTERNAL=y

CONFIG_SOC_HFXO_CAP_INT_VALUE_X2=24

With this build, the beacon does not work and in RTT logs I get the following:

-----------------------------------------------------------------------

Then I tried another option of generating the LFCLK, this time by using the HFXO and generating LFCLK from it. I changed my prj.conf to the following

#Synthesize LF Clock

CONFIG_CLOCK_CONTROL_NRF_K32SRC_SYNTH=y

#Disable use of 32kHZ XTAL

CONFIG_SOC_ENABLE_LFXO=n

#Configure 32MHz Clock

CONFIG_SOC_HFXO_CAP_INTERNAL=y

CONFIG_SOC_HFXO_CAP_INT_VALUE_X2=24

With this build, the beacon works. I see it in the nRF Connect App and RTT logs show the following:

----------------------------------------------------------------------

I do not want to jump to conclusions. My assumption that RC clocks are not stable enough to initialize the BLE stack is not based on empirical data, but rather on the RTT logs "HCI driver open failed" and "Bluetooth init failed".

Q. Could there be board to board variations that cause this?

Mikael

[Einar Thorsrud]

Hi,

mm784 said:

CONFIG_CLOCK_CONTROL_NRF_K32SRC_20PPM=y

PS: When using LFRC, you should set CONFIG_CLOCK_CONTROL_NRF_K32SRC_500PPM=y (not 20 ppm). This does not actually make a difference if you are ony doing advertising, but for connections etc you need to set the correct accuracy for the LFRC, which is 500 PPM (the sleep clock accuracy is used for window widening in BLE).

mm784 said:

My assumption that RC clocks are not stable enough to initialize the BLE stack is not based on empirical data, but rather on the RTT logs "HCI driver open failed" and "Bluetooth init failed".

I see. That is not the case though. If you enable the LFRC  with calibration, it will be within 500 ppm which is good enough for BLE. And regardless, an inaccurate clock would not result in this error.

mm784 said:

Q. Could there be board to board variations that cause this?

There could be board to board variations on the LFRC as well as other parameters, but that would not explain what you are seeing. BLE products without the 32.768 kHz clock is common.

The error you are seeing (HCI driver open failed (-11) does not indicate a clock issue specifically. Instead, it indicate that the BT host is not able to communicate with the BT controller (which should be running on the net core). So it seems you have not programmed the BT controller. This should normally happen automatically though. Another potential issue is that something went wrong in the controller iside (it might be clock related as you see a difference when you use synthesized LF clock).

Some questions in order fo rme to understand more:

1. These failure, are you seeing them on the nRF5340 DK or a custom board?
2. If it is a custom board, can you share your board files?
3. Also, can you share the project with all your changes? Please include the build folder so that I can see how you configure and build your project.
4. Which SDK version are you using?
5. Does the standard Zephyr beacon sample work on your device or do you see the same or other issues there?