nRF52840 VDDH and SWD

[Chris]

Dear All,

we use Reference configuration 4 and have connected the DK to our custom PCB. 3.3V of the DK go into VDDH and otherwise only GND and SWDIO/SWCLK are connected. We see 1.8V at VDD, but can't use SWD. In another thread I've read that the SWD needs voltage adjustment - so does it have to match VDD level? That would mean SWD requires 1.8V, right? 

Can we just use 

NRF_UICR->REGOUT0 = UICR_REGOUT0_VOUT_3V3;



to get 3.3V VDD level which then should in turn work with 3.3V logic at SWD?

[Chris]

well, sometimes I should just try. Was not sure how to do it because we're using Zephyr for this project, but simply adding a board.c (only important for those using Zephyr as well) with this code (copied from pca10059):

static int board_nrf52840_pca10056_init(struct device *dev)
{
	ARG_UNUSED(dev);

	/* if the nrf52840_pca10059 board is powered from USB
	 * (high voltage mode), GPIO output voltage is set to 1.8 volts by
	 * default and that is not enough to turn the green and blue LEDs on.
	 * Increase GPIO voltage to 3.0 volts.
	 */
	if ((nrf_power_mainregstatus_get() == NRF_POWER_MAINREGSTATUS_HIGH) &&
	    ((NRF_UICR->REGOUT0 & UICR_REGOUT0_VOUT_Msk) ==
	     (UICR_REGOUT0_VOUT_DEFAULT << UICR_REGOUT0_VOUT_Pos))) {

		NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen << NVMC_CONFIG_WEN_Pos;
		while (NRF_NVMC->READY == NVMC_READY_READY_Busy) {
			;
		}

		NRF_UICR->REGOUT0 =
		    (NRF_UICR->REGOUT0 & ~((uint32_t)UICR_REGOUT0_VOUT_Msk)) |
		    (UICR_REGOUT0_VOUT_3V3 << UICR_REGOUT0_VOUT_Pos);

		NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren << NVMC_CONFIG_WEN_Pos;
		while (NRF_NVMC->READY == NVMC_READY_READY_Busy) {
			;
		}

		/* a reset is required for changes to take effect */
		NVIC_SystemReset();
	}

	return 0;
}

SYS_INIT(board_nrf52840_pca10056_init, PRE_KERNEL_1,
	 CONFIG_KERNEL_INIT_PRIORITY_DEFAULT);

will give you 3.3V and make SWD attached to DK working nicely