Understanding Power Supply + SWD target voltage

[jrjradler]

Hey!

I designed and fabricated a PCB using the NRF52840, but I'm encountering some power-related issues. The board has three separate power supplies - 4.2V, 3.3V, and 1.8V. I decided to power the NRF52840 with the 4.2V USB charging supply (or when disconnected from USB, the 3.7V LiPo supply). To do this, I configured the power supply into High Voltage mode (LDO only) and connected the VDDH pin to the 4.2V supply with a decoupling capacitor. I also connected the VDD pin to ground through a capacitor (as seen in the image below).

The NRF52840 appears to be operational, as I'm able to connect to it through Bluetooth. However, I'm having trouble reprogramming the chip through SWD. Considering I'm using 4.2v to power the chip, what interface voltage should I expect? My EDU J-Link only supports 3.3V and is unable to connect to the device.

 

[Simonr]

Hi Jake

Okay, so the VDD is in fact connected to VDD_nRF internally I assume? 

Regarding the SWD pins, they follow VDD_nRF, which can be configured in High Voltage mode using the REGOUT0 to output a set maximum voltage. By setting this to 3.3V that should let you connect to and program the external board with J-Link device. This case has a customer doing so for the nRF5 SDK. This does require a reprogramming of sorts though, so it might be hard reprogramming an already flashed device.

Best regards,

Simon