nRF52840 normally supplied via VDDH - can it be supplied just via VDD for SWD programming? (without connecting VDD and VDDH together)

[d.s.]

Hi all,

I am working on a project where the nRF52840 is used in USB powered mode. That means that it is normally supplied with 5V at the VDDH pin. In this case the nRF52840 generates internally its own VDD which is software adjustable and per default set to 1.8V according the datasheet.

But now the problem: the SWD programmer we want to use only supports 3.3V.

No matter which voltage I supply on the VDDH pin - before first programming the internal voltage regulator will generate 1.8V for VDD so that 3.3V at SWD pins would be much too high. I think the nRF52840 would get destroyed in this case.

So I got the idea to supply directly the VDD pins of the nRF52840 with an external 3.3V supply just for programming. But in normale voltage mode VDD and VDDH should be connected together according the datasheet - however they are not connected together in my design because in normal operation the nRF52840 will be supplied in high voltage mode with 5V at the VDDH pin.

Does anyone have an idea how to solve this problem?

What would happen if I just supply the VDD pins with 3.3V without connecting VDD and VDDH together?

Thank's a lot in advance for your help!

Cheers

[Simonr]

This is correct, but I don't see why you shouldn't be able to program it using a USB supply. Other ways to program the board would be to make a solder bridge between vdd/vddh to set it to normal voltage mode, to use the nRF52840 DK as a debugger, or (don't know if your debugger supports it) power the debugger externally.

Best regards,

Simon

[d.s.]

In high voltage mode the REG0 stage generates VDD. REG0 is set to 1.8V per default. Wouldn't we destroy the SWD pins of the nRF52840 if we drive them with 3.3V from the programmer while the nRF52840 is running at 1.8V?

 

The other way with the solder bridge is not possible I am afraid - we need to program thousands of them. We can't short the bridge, program and open the bridge again each time. But maybe I could connect VDD and VDDH together using a diode or a resistor that VDDH will be pulled to VDD if just VDD is supplied - would that work?

What's the exact reason, why VDDH needs to be connected to VDD for normal voltage mode operation? Is this just for detection that supply is provided at VDD and REG0 can be disabled or does the nRF52840 draw current from the VDDH pin even in normal voltage mode?