[NRF51822] debugging radio - low average voltage on VDD_PA pin.

Question

Hi, 
 I am trying to debug a custom PCB for the NRF51822 - I can run a modified version of the HRS application (S110) that has been verified on the PCA10004. The LF clock source has been changed as per a previous post in these forums, so as to use the internal RC. 
 Sanity checking with just flashing some LEDs also works. However, I am stuck in debugging the RF part of it - there is currently no transmission happening. The VDD_PA pin is showing an average voltage of 70mV as compared to the ~2V I am seeing on the PCA10004 (Tx power +4dbm). This is using a multimeter as I don't have access to an oscilloscope at the moment. 
 I removed all passives connected to the VDD_PA pin to discard the possibility of a low resistance path to ground - but I still get the same reading on the multimeter. 
 Any ideas on what could be going wrong? Or tips on how to further debug the VDD_PA pin behavior? 
 Thanks,
NK

Asbjørn · Accepted Answer

The VDD_PA pin will go to 1.8 V during transmission. So when the radio is actively sending, it will be 1.8 V and for the rest of the time, it will be 0 V. 
 I'm not sure what's wrong with your board as there could be many reasons. It's most likely something in your code that's not enabling the radio correctly or the code is stuck somewhere waiting. I would recommend you to use the debugger and make sure that the chip is actually going through all the steps you think it is and the especially the stack init routine is running correctly. Are you sure the 16 MHz crystal is present and running? 
 The fact that you are measuring 70 mV on average could indicate that the radio is on for very short time frames. How are you configuring the radio? Is it sending often? Is it advertising? Do you see it advertising with for example a sniffer or Master control panel?