BLE runs only while debugging on custom board (probably RTT issue)

[Florian Zierer]

I have problems with my custom board.

I developed the firmware with the nrf52840-PDK and there the firmware runs standalone (with RTT connected and without) or when debugged via eclipse.

Both boards (nrf52840-PDK and my custom board) have the nrf52840 QIAAAA and I'm working with Softdevice 140 and SDK 14.2.

If I run the firmware from my own custom board, everything seems to work when debugging via eclipse or when RTT is not connected. The advertising starts and the device can be found from e.g. nrf-connect and connected.

Usually I use RTT for debugging (on nrf52840-PDK) and everything works fine. The custom board is programmed via jlink from an external nrf52 DK.

If I connect via RTT to my custom board, the advertising does not work correctly. Without RTT, it works.

The power profile (with PPK) looks the same if connected with RTT and without, the advertising seems to have started but nrf-Connect can't receive advertising packets.

Could it be some timing/crystal issue? What are the differences when connected via RTT and when not?

I changed NRF_SDH_CLOCK_LF_SRC from NRF_CLOCK_LF_SRC_XTAL to NRF_CLOCK_LF_SRC_SYNTH and the behaviour is the same. So the 32khz crystal should not be the problem.

It seems to have something to do with the hardware as I can't see this behaviour on the nrf52840-PDK.

21.03.18 Updated LF test, updated to clarify the problem. There was another connection problem but it seems they are different issues (see https://devzone.nordicsemi.com/f/nordic-q-a/32590/error-133-0x85-gatt-error-with-htc-u11-life)

[Florian Zierer]

Can you please also set

    NRF_CLOCK->TASKS_HFCLKSTART  = 1;
    while ( NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);

otherwise the firmware does not start at all without RTT connected.

With RTT I can see the output:

682420203: <info> app: Heart Rate Sensor example started.
682420203: <info> app: Fast advertising.

but can't connect with nfconnect. The device does not appear when scanning.

If I flash the same hexfile on my DK I can connect to it.

[Florian Zierer]

Why did you want me to test to start the HFCLK manually?

As I said, in another firmware, If I remove those lines, it wont start at all. Does this mean there is and issue with the crystal?

We are using a 12pF load crystal with 22pF Capacitors (calculation would give 21pF but there are only 22pF and 20pF capacitors available.

Would it help to switch to a 8pF load crystal (with 12pF capacitors)? Can this be the problem, that the crystal does not start fast enough?

[Vidar Berg]

"Why did you want me to test to start the HFCLK manually?"

To get an indication whether this issue was clock related. This replicates the clock behavior when the chip is in debug interface mode.  

I think wrong load capacitance can explain the behavior you have been seeing. Please refer to Sigurd's answer in this thread: https://devzone.nordicsemi.com/f/nordic-q-a/30204/hfxo-crystal/119935#119935. 

[Florian Zierer]

Ok, we will try with another crystal with 8pF load capacitance.

But one question. How is the load capacitance in the datasheet (12pF) meant? I thought it is the load capacitance for the crystal

The crystal we are using has 12pF load capacitance (should be in accordance with the spec). But with the formula we needed to add 21pF capacitors to get to the 12pF load capacitance. So with our 22pF we should not be too far off.

The reference circuitry does also not use exact capacitors, the formula would say you have to use 13pF (for 8pF load capaticance) and you use 12pF.

[Florian Zierer]

Hey Vidar,

we exchanged the crystal with a 8pF load crystal and use 12pF capacitors as it is done in the reference circuits.

Unfortunately this does fix the problem. Is there any other hint you could give us to figure out the root for the problem?