The oscillation of LFCLK 32768Hz external crystal looks weird and nRF52832 cannot run properly.

[WFKwok]

Hello, we are using a BLE module nRF52832 (MDBT42-P512KV2).

We also add the external 32768Hz crystal to our application with this module.

However, we recently found that part of the PCBA cannot run properly, just like hanging up without any operation.

We measure the external 32768Hz crystal pin and find a glitch to GND periodically. (once every 590ms)

For a good module, there is no such glitch in the crystal oscillation.

Please refer to the attached picture and the schematic.

The crystal is good if we swap the 32768Hz crystal with another good PCBA.

Is there any reason why there is a glitch to GND periodically?? We guess this glitch led to the BLE module not operating.

[Asbjørn]

Hi

1. Is it only the crystal pins that this happens to? Is it XL1 or XL2 that drops to 0V?
2. It seems the drop changes across time and does it occur a few times before the FW halts?
3. Does the chip reset when this happens?
4. Is there anything else on the other GPIOs happening at the same frequency as the faults are happening or related to power supply? My thinking is; could it be possible that there's a short somewhere else causing the chip to reset and it's visible on the XL pins, but actually it's coming from some short or breakdown somewhere else? I know it doesn't make sense as it's only occuring on some of the boards. 

Best regards

Asbjørn

[WFKwok]

Hi Asbjørn,

1. Only XL2 that drops to 0V (XL1 is no problem)
2. The drop is not charging across time and I don't know whether there is FW halts. But it seems no a few times occurrence.
3. I don't know whether the chip is reset or not. But the reset pin is stable without change.
4. The VCC voltage is measured, and it looks stable over time.

We are still looking into it and see if the reply above can help further analysis.

And one more picture of the waveform of XL1 and XL2 pins is attached for your information.

Thanks.

[Asbjørn]

Hello

Q1: XL2 is the only pin you observe this on,  Are you able to share the crystal specifications and datasheet? 

Q2: I'm not sure if  I understand your reply. The drops in voltage on XL2, do they make the FW halt or stop every time they occur or is it only at some occasions that the FW halts?

Q4: Is there anything else on the board/chip that is switching with the same frequency as the drop on the XL2 pin? The LEDs or SPI traffic?

Q5: Just to be sure, you're not reconfiguring the XL2 pin in FW?

Q6: Are you able to move the 32kHz crystal from a good board and place it on a board that is showing the error and see if the error follows the board or the crystal?

Best regards

Asbjørn