Setting VEXT at a different level to VTG

[Eliot Stock]

I have an nRFgo motherboard on which I can set the VTG voltage to anywhere from 1.9V to 3.3V using nRFgo Studio. I'm trying to get a UART connection working to a GSM modem which runs at 1.8V. The UART connection is using port 2 on the motherboard, pins P2.0 to P2.3, plus ground.

I need VTG to be 3.3V in order for the Segger to work so I can flash the device. If I flash the device at 3.3V then switch the voltage to 1.8V and restart, I can't then debug my code.

Are the pins P2.0 to P2.3 running at VEXT or VTG? If VEXT, how can I set this to be 1.8V (or 1.9V, say) but keep VTG at 3.3V so I can continue to debug?

[Hung Bui]

Hi Eliot,

For your issue, I think you should consider adding a level shifter for your UART pins. Another workaround is to use other debugger that support wider voltage range, such as this or this

[Eliot Stock]

OK, thanks. I'll have to buy the J-Link and leave the whole device at 1.8V. Can you confirm though that it's not possible to run the UART pins at 1.8V while the nRF51822 itself is at 3.3V?

[Hung Bui]

It's correct, Eliot. You can find at section 8.23 in the nRF51822 Product Spec 2.0 that output high voltage for a GPIO pin is VDD-0.3V minimum.