STM32WL55JCI7 VR_PA stuck at 0V.

[Hbburnha]

I am currently designing a board that uses the STM32WL55JCI7 LoRa functionality. I am currently using the AT_SLAVE firmware provided by ST for testing. I am currently trying to Transmit a signal through the Antenna on my board but no signal has been been coming through. 

After some investigation I have noticed that the VR_PA voltage is stuck at 0V. 

I am trying to transmit in Low Power mode, and the voltage on the other pins is as follows:

VDDRF = 3.3V

VDDPA = 1.55 V

This software has worked on a different variation of my board so I do not believe it is a firmware issue. I am trying to figure out why this particular board is not working. 

[FilipKremen]

Hello,

can you please share the schematic?

Thank you.

Best regards,

ST support

[Hbburnha]

Yes, Below is the schematic for the RF and Antenna Portion of the board, I have also included the Schematic for the 3.3V Power Supply. Vsys ~ 25.6V.

[FilipKremen]

Hello,

thank you for the schematic, is the VBAT pin connected to VDD on your board?

Best regards,

ST support

[Hbburnha]

On this Board there is a VBAT and a VSYS, both should be around 25.6 V, the reason for the difference is that there is two power supplies on this board, a PV module and a battery. VSYS will be connected to one or both through a transistor. Interestingly a different variation of the board that was powered by the PV only worked perfect, while this variation that has both battery and PV does not work. 

The VDD for the STM32, is powered from the 3.3V output from the DC-DC converter as shown above, VSYS is the supply for that converter. 

[FilipKremen]

Hello,

what I meant is VBAT pin on the MCU. Is it connected to VDD?

Best regards,

ST support

[Hbburnha]

I see, below is our setup for our VBAT on our board. 

D5 is actually supposed to be Depopulated but was accidently populated on the board, however removing D5 does not change how the antenna behaves. 

[FilipKremen]

Hello,

can you please measure voltage on the VBAT pin? Is the MCU same as on the previous board variation? Same revision?

Best regards,

ST support

[Hbburnha · Accepted solution]

Hello, 

I actually determined the error and have been able to make the STM32 MCU work. 

The error was that the ferrite bead leading to the RF crystal and a capacitor were accidently swapped when the board was being assembled. 

L12 and C74 were swapped. This meant that the crystal was not receiving any voltage and there was no oscillation for the RF signal. 

Thank you for help, 

Hayden Burnham