these are notes by Thomas Watteyne about running the nRF52840-DK at 1.8V to be able to interface it directly to SCuM.

Thomas Watteyne I need Said Alvarado-Marin and Fil Maksimovic to proof-read this page

Target power supply configuration

We want the nRF52840 on the DK to operate at 1.8V. Per the nRF52840 Product Specification v1.5, we want to use this configuration:

This way, when providing 5V to “Supply” we end up with 1.8V at REGOUT0.

Quirks of the nRF52840-DK

Per the nRF52840-DK User Guide,:

That is, to set the VDD_nRF voltage by software:

Software configurations

With this in place, we want to set the following registers

Default configuration

When running project and observing the register settings on a fresh board, that is by default the configuration:

And I confirm that, when plugging in my micro-USB cable into the “nRF USB“ port and setting the “nRF Power Source“ switch to “USB“, I see the LEDs “turning“ (i.e. the program is running) but very dim

I only have a single USB cable, so I don’t have the main USB power plugged in; I’m pretty sure that wouldn’t change the power source of the nRF.

Notes about UICR

Of course, register REGOUT0 is non-volatile, i.e. it is part of the UICR registers which are mapped to a flash page, so its contents stays even after power cycling the board. If a different program was run which write a different value to REGOUT0 , the board would be running at a different voltage.

The recommended behavior of the scum-programmer is:

The user doesn’t have to worry about this, but should only plug in SCuM once she confirms the LEDs are dim

About Said’s technique

Said Alvarado-Marin 's technique is to provide add the following to flash_placement.xml

<MemorySegment name="UICR" start="$(UICR_START)" size="$(UICR_SIZE)">
    <ProgramSection alignment="4" keep="Yes" load="Yes" name=".uicr_regout0" start="0x10001304" size="4"/>

this “declares” the UICR memory segment so it can be referred to by a header file.

For this to work, the projet file (the main .emProj file) must declare the following variables

Then add a uicr_config.h file which contains teh following single line:

const uint32_t REGOUT0 __attribute__((section(".uicr_regout0"))) = 1; // 0 = 1.8v, 1 = 2.1v, 2 = 2.4v, 3 = 2.7v, 4 = 3.0v, 5 = 3.3v

In essence, this while technique just writes the REGOUT0 register. Using this super convoluted approach allows the program to change the voltage without touching the C code, which is useful if you have compile C code which isn’t yours and you want to change the voltage. But this technique is by no means AFAICT needed. I’d like to get input from Said Alvarado-Marin on this.

Also, I don’t understand why we use the setting to 1 (2.1V) and not 0 or 7 (1.8V). Again, would like input from Said Alvarado-Marin .