MKR NB 1500: no modem communication anymore


I got my MKR NB 1500 some times ago, testing with a Swisscom M2M Demo Kit.
Had mitigate success with it as I found the setup was quite unstable (losing connectivity without auto recovery, not sure at which stage exactly, but probably TCP session timing out within the SARA module, as AT+USOWR returns ERROR randomly after some time). Also during debuging it looks like SARA module doesn't reset properly.

Anyway today's topic is different: I was using a 500 mAh LiPo for short time testing, even though no battery was necessary for what I was doing.
Yesterday I connected a 2400 mAh I just got and connected the MKR to USB for charging the LiPo up.
Charging LED is on. I did let it charge for some hours.
In the evening I wanted to continue my troubleshooting of the board when I realized none of my sketches were working anymore.
I uploaded the passthrough sketch to send some AT commands, but absolutely noting comes back on the terminal. I can feel the SARA module heating up a bit as it does in normal operations, but I completely lost communication with it.
Testing with different serical terminals, different PC, different OS, same behaviour.

I'm unable to figure out if the problem is the module, micro, or anything else on the board.

Has anybody experienced the same? Any guidance on how to troubleshoot further?

I also sent a mail to Arduino support as there are 95% chances the board became defective.

I added this code and everything is working again. It will reset the modem.

// enable the POW_ON pin
digitalWrite(SARA_PWR_ON, HIGH);

// reset the ublox module
digitalWrite(SARA_RESETN, HIGH);
digitalWrite(SARA_RESETN, LOW);


Unfortunately modem was not answering even after several days out of power, so this code would not help.

Arduino support agreed the board has a hardware issue and are sending a new one.



I have experienced the same issue on two MKR NB 1500s. The modem appears to be dead, and I have no real way of probing the modem pins to check power. I never used a LiPo with them but I used the Vin pin to supply 5V. It worked for about a week before dying.

Does anyone have an idea of the root cause? It would be great to understand the problem a little better, so I can avoid it when I deploy my modules.

I would guess it is a power issue, but it would be great to have some confirmation on that. Are these boards particularly vulnerable to sudden power failures?

Interestingly, there is a Sodaq board with the same SARA-R410m modem, and they experience the same issue.