Radio not working when RTT is in use

[Nathan Morsa]

We are using a nRF52840 (QIAAAA) on a custom board which replicates exactly the schematic used by the development kit (PCA10056). When using Segger RTT for debugging purposes, the board does not emit any BLE packets (no errors are reported).

When trying to use BLE, for example by sending advertising packets, everything works fine until J-Link RTT Viewer is connected. The device then suddenly stops sending any BLE data (if it was advertising no more advertising packets are detected, if it was connected the connection is lost). However if J-Link RTT Viewer is closed, advertising packets are immediately being sent once again. The device does not reset nor produce any visible errors during this operation. The exact same firmware works as expected (sending BLE data when RTT is being used) when flashed onto a PCA10056 board.

We have thoroughly checked our hardware for any difference with the reference schematic and haven't found anything significant.

Has anyone encountered this problem before? If the firmware are schematic are identical, when could be the source of this problem? Are there differences in nRF52840-QIAAAA batches?

[Matt]

Do you have JLink OB in your custom board or you connect the JLink to the SWDCLK and SWDIO lines of the N52840 on your custom board? I'm no 52840 user (I use N52832) and I'm just wondering if it is the actual Jlink h/w connection that is the problem here.

[Nathan Morsa]

@matthewlai: I'm using an external J-Link Base. The connector on the board is as shown here (pin 6/P1.00 is SWO). However, I have connected the same J-Link Base on the development kit (PCA10056) through the Debug In (P18) connector, and everything worked as expected. I have also tried to fully replicate the wiring of the Debug In connector by removing the trace pins (pins 5 and 9 grounded, pins 7 and 8 floating), but that didn't change anything. I also tried to change the SWD clock speed in RTT Viewer (from 50kHz to 50MHz), but that didn't change anything either.

[Jørn]

I think we will need to see your schematics and layout to investigate this one. If your project is not confidential you can upload it to your original question here. If your project is confidential you can create a mypage case at nordicsemi.com and refer to this post.