DTM TX only active when RTT Viewer is connected (nRF52840, based on direct_test_mode)

Question

I am working with the DTM sample on nRF52840 using nRF Connect SDK (Zephyr) . 
 My goal is to run a continuous TX test automatically at boot (standalone) , without requiring UART or external commands 
 Setup 
 
 Board: nrf52840dk_nrf52840 (compatible based on direct_test_mode) 
 Hardware: Custom Board (based in Circuit configuration no. 2 for CKAA WLCSP) 
 SDK: nRF Connect SDK v2.6.1 
 Measurement: LiteVNA connected to ANT output 
 Programming: JTAG+SWD ARM Tool 
 
 What I observe 
 When the device boots without RTT connected: 
 
 Then TX stops completely

When I connect RTT Viewer: 
 
 Continuous TX starts immediately 
 RF signal is stable and visible on LiteVNA

What I am doing in code 
 I run DTM (based on direct_test_mode example) directly from main(): 
 dtm_tr_init();

dtm_setup_prepare();
dtm_setup_set_phy(DTM_PHY_2M);
dtm_setup_set_modulation(DTM_MODULATION_STANDARD);
dtm_setup_set_transmit_power(DTM_TX_POWER_REQUEST_MAX, 8, 0);

dtm_test_transmit(0, 37, DTM_PKT_PRBS9); // Select Channel 
 No external commands are used. 
 My prj.conf: 
#RTT console configuration
CONFIG_USE_SEGGER_RTT=y
CONFIG_RTT_CONSOLE=y
CONFIG_UART_CONSOLE=n
# CONFIG_PRINTK=n
CONFIG_LOG=y
# CONFIG_BT_DEBUG_MONITOR_RTT=y
# CONFIG_LOG_MODE_IMMEDIATE=y # Send logs immediately to backend (no internal buffering)
# CONFIG_LOG_MODE_DEFERRED=y # If Disabled: logs are not deferred/buffered
CONFIG_LOG_BUFFER_SIZE=8192
# CONFIG_BT_DEBUG_MONITOR_RTT_BUFFER_SIZE=4096
CONFIG_SEGGER_RTT_BUFFER_SIZE_UP=4096
CONFIG_SEGGER_RTT_MODE_BLOCK_IF_FIFO_FULL=y


# Configure assertions
CONFIG_ASSERT=n
CONFIG_ASSERT_NO_COND_INFO=y
CONFIG_ASSERT_NO_MSG_INFO=y

CONFIG_HW_STACK_PROTECTION=n

CONFIG_CONSOLE=y
CONFIG_UART_CONSOLE=n

CONFIG_LOG=y
CONFIG_LOG_PRINTK=y
CONFIG_USE_SEGGER_RTT=y
CONFIG_LOG_BACKEND_RTT=y
CONFIG_LOG_MODE_DEFERRED=y
CONFIG_LOG_BLOCK_IN_THREAD=n

# Use necessary peripherals
CONFIG_NRFX_TIMER0=y
CONFIG_NRFX_TIMER1=y
CONFIG_NRFX_TIMER2=y
CONFIG_NRFX_TIMER3=y

CONFIG_FEM_AL_LIB=y

# Power management configuration
CONFIG_PM_DEVICE=y
CONFIG_NRFX_POWER=y
CONFIG_POWEROFF=y

CONFIG_DTM_POWER_CONTROL_AUTOMATIC=n
CONFIG_DTM_RADIO_IRQ_PRIORITY=0
CONFIG_DTM_TIMER_IRQ_PRIORITY=0
CONFIG_DTM_FAST_RAMP_UP=n
# CONFIG_DTM_TRANSPORT_TWOWIRE=y 
 Questions 
 
 Is there any config in the DTM sample to keep TX running by default and stable? 
 Could the issue be related to:
 
 timers not running properly config? 
 missing clock configuration? 
 scheduler or idle behavior?

Marcos Carballo · Accepted Answer

Done, resolved — sorry for the late reply.

The issue also occurred when using the radio_test sample. After reviewing its implementation more closely, I found a section where the DC/DC configuration is handled.

In my current custom hardware design, the system operates in LDO/LDO mode, so I had to manually enable and configure the desired regulator mode. I executed this as a pre-kernel configuration.

Here’s the code in case it helps someone:

/*********************** EXTERNAL LIBRARIES **********************/
#include <hal/nrf_power.h>
#include <nrfx_power.h>
#include <hal/nrf_nvmc.h>
#include <nrfx.h>
#include <zephyr/logging/log.h>
#include <zephyr/init.h>

/**
 * @brief Sets the REGOUT0 voltage to 1.8V if not already set.
 *
 * This function checks the current voltage setting of the REGOUT0 pin and
 * updates it to 1.8V if it is not already set to that value. This setting is
 * required by the accelerometer and is not set by default in the nRF52840
 * development board. The function logs an info message if the setting needs
 * to be changed and performs a system reset after the change has been made.
 */
void config_regout0_to_1v8(){
  if ((NRF_UICR->REGOUT0 & UICR_REGOUT0_VOUT_Msk) != UICR_REGOUT0_VOUT_1V8) {
    LOG_INF("REGOUT0 not set to 1.8V. Updating...");

    // Enable write to UICR
    NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen;
    while (!NRF_NVMC->READY) {}

    // Set REGOUT0 to 1.8V
    NRF_UICR->REGOUT0 = (NRF_UICR->REGOUT0 & ~UICR_REGOUT0_VOUT_Msk)
                        | UICR_REGOUT0_VOUT_1V8;

    // Disable write to UICR
    while (!NRF_NVMC->READY) {}
    NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren;
    while (!NRF_NVMC->READY) {}

    LOG_INF("REGOUT0 updated. Triggering system reset.");
    NVIC_SystemReset();  // Required to apply change
  }
}

/**
 * @brief Initialize both REG0 and REG1 in LDO mode.
 *
 * This function sets REG0 and REG1 to LDO mode, providing low-noise startup.
 * Useful for sleep or initial startup to avoid high current peaks.
 */
void set_ldo_ldo(void) {
	NRF_POWER->DCDCEN0 = 0; // REG0 in LDO
	NRF_POWER->DCDCEN  = 0; // REG1 in LDO
	LOG_INF("REG0=LDO, REG1=LDO");
}

/**
 * @brief Pre-kernel initialization: configure REG0 output voltage and startup mode.
 *
 * This function sets REG0 to 1.8V and starts both regulators in LDO mode.
 */
int pre_kernel_power_config(void) {
	config_regout0_to_1v8();  // Configure REG0 to 1.8V

	// Uncomment the mode you want to test (Select one):
  set_ldo_ldo();
	// set_ldo_dcdc();
	// set_dcdc_ldo();
	// set_dcdc_dcdc();

	return 0;
}

/***************** INITIALIZATION PRE KERNEL ********************* */

SYS_INIT(pre_kernel_power_config, PRE_KERNEL_1, 1);

DTM TX only active when RTT Viewer is connected (nRF52840, based on direct_test_mode)

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