app: Enable UART application log by default

app/CMakeLists.txt

@@ -21,6 +21,7 @@ target_sources(app PRIVATE src/sm_at_icmp.c)
 target_sources(app PRIVATE src/sm_at_fota.c)
 target_sources(app PRIVATE src/sm_at_dfu.c)
 target_sources(app PRIVATE src/sm_uart_handler.c)
+target_sources(app PRIVATE src/sm_log.c)
 # NORDIC SDK APP END
 target_sources_ifdef(CONFIG_SM_SMS app PRIVATE src/sm_at_sms.c)
 target_sources_ifdef(CONFIG_SM_PPP app PRIVATE src/sm_ppp.c)

app/boards/nrf9151dk_nrf9151_ns.overlay

@@ -8,6 +8,7 @@
 	chosen {
 		ncs,sm-uart = &dtr_uart0;
 		nordic,pm-ext-flash = &gd25wb256;
+		zephyr,console = &uart1;
 	};
 };
 
@@ -69,3 +70,11 @@
 &i2c2 {
 	status = "disabled";
 };
+
+/* Enable UART1 as the shared backend for Zephyr logs and modem traces (VCOM1 on the DK).
+ * Modem traces require 1 000 000 baud. Hardware flow control is not used.
+ */
+&uart1 {
+	status = "okay";
+	current-speed = <1000000>;
+};

app/boards/thingy91x_nrf9151_ns.overlay

@@ -7,6 +7,7 @@
 / {
 	chosen {
 		ncs,sm-uart = &dtr_uart0;
+		zephyr,console = &uart1;
 	};
 };
 
@@ -27,3 +28,11 @@
 	/* Use PORT event for DTR (26) pin to save power */
 	sense-edge-mask = <BIT(26)>;
 };
+
+/* Enable UART1 as the shared backend for Zephyr logs and modem traces.
+ * Modem traces require 1 000 000 baud. Hardware flow control is not used.
+ */
+&uart1 {
+	status = "okay";
+	current-speed = <1000000>;
+};

app/overlay-trace-backend-uart.conf

@@ -4,14 +4,6 @@
 # SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
 #
 
-# Enable UART logging backend - shared with nRF modem traces.
-CONFIG_UART_CONSOLE=y
-CONFIG_RTT_CONSOLE=n
-CONFIG_LOG_BACKEND_RTT=n
-# Log backend starts disabled; the UART is off until activated via AT#XLOG=1.
-CONFIG_LOG_BACKEND_UART=y
-CONFIG_LOG_BACKEND_UART_AUTOSTART=n
-
 # Enable UART modem trace backend - shared with the UART log backend.
 CONFIG_SM_MODEM_TRACE_BACKEND_UART=y
 CONFIG_NRF_MODEM_LIB_TRACE=y

app/prj.conf

@@ -14,20 +14,19 @@ CONFIG_RING_BUFFER=y
 CONFIG_REBOOT=y
 CONFIG_EVENTFD=y
 
-# Segger RTT
-CONFIG_USE_SEGGER_RTT=y
-# Where console messages (printk) are output.
-# By itself, SM does not output any.
-CONFIG_RTT_CONSOLE=y
-CONFIG_UART_CONSOLE=n
-# Where logs are output.
-CONFIG_LOG_BACKEND_RTT=y
-CONFIG_LOG_BACKEND_UART=n
-# Increase the buffer so that all Serial Modem boot logs fit into the buffer.
-# Because J-Link RTT Viewer has to be connected after the bootup,
-# only logs that fit into the buffer are shown. This is especially useful in showing
-# errors if Serial Modem does not start properly.
-CONFIG_SEGGER_RTT_BUFFER_SIZE_UP=2048
+# Enable UART logging by default.
+CONFIG_UART_CONSOLE=y
+CONFIG_LOG_BACKEND_UART=y
+# Logging does not start until activated via AT#XLOG=1 and is deactivated with AT#XLOG=0.
+CONFIG_LOG_BACKEND_UART_AUTOSTART=n
+
+# Disable Segger RTT logging by default.
+CONFIG_USE_SEGGER_RTT=n
+CONFIG_RTT_CONSOLE=n
+CONFIG_LOG_BACKEND_RTT=n
+# Optional: when RTT logging is enabled, increase the RTT buffer so that
+# all Serial Modem boot logs fit into the buffer.
+#CONFIG_SEGGER_RTT_BUFFER_SIZE_UP=2048
 
 # Network
 CONFIG_NETWORKING=y

app/sample.yaml

@@ -6,7 +6,7 @@ tests:
     build_only: true
     extra_args:
       - EXTRA_CONF_FILE="overlay-cmux.conf;overlay-ppp.conf;overlay-trace-backend-uart.conf;overlay-nrf-device-provisioning.conf"
-      - EXTRA_DTC_OVERLAY_FILE="overlay-nrf91m1.overlay;overlay-trace-backend-uart.overlay"
+      - EXTRA_DTC_OVERLAY_FILE="overlay-nrf91m1.overlay"
     extra_configs:
       - CONFIG_SM_LOG_LEVEL_DBG=y
     platform_allow:

app/src/sm_log.c

@@ -0,0 +1,146 @@
+/*
+ * Copyright (c) 2026 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+/*
+ * AT#XLOG command — enable/disable the Zephyr UART log backend at runtime.
+ *
+ * The UART (zephyr,console) is shared with the modem-trace backend
+ * (sm_trace_backend_uart.c).  AT#XLOG and AT#XTRACE are mutually exclusive:
+ * each refuses to activate while the other is in use.
+ *
+ */
+
+#include <zephyr/devicetree.h>
+
+/* Only compiled if zephyr,console is present and active in the devicetree. */
+#if DT_HAS_CHOSEN(zephyr_console) && DT_NODE_HAS_STATUS(DT_CHOSEN(zephyr_console), okay)
+
+#include <zephyr/drivers/uart.h>
+#include <zephyr/kernel.h>
+#include <zephyr/logging/log.h>
+#include <zephyr/logging/log_ctrl.h>
+#include <zephyr/pm/device.h>
+
+#include "sm_at_host.h"
+
+LOG_MODULE_REGISTER(sm_log, CONFIG_SM_LOG_LEVEL);
+
+/* Zephyr console UART is used both for application logs and modem traces. */
+#define UART_DEVICE_NODE DT_CHOSEN(zephyr_console)
+static const struct device *const uart_dev = DEVICE_DT_GET(UART_DEVICE_NODE);
+
+static bool log_active;
+
+static int uart_suspend(void)
+{
+	int ret = pm_device_action_run(uart_dev, PM_DEVICE_ACTION_SUSPEND);
+
+	if (ret && ret != -EALREADY) {
+		LOG_ERR("Failed to %s UART device: %d", "suspend", ret);
+		return ret;
+	}
+
+	return 0;
+}
+
+static int uart_resume(void)
+{
+	int ret = pm_device_action_run(uart_dev, PM_DEVICE_ACTION_RESUME);
+
+	if (ret && ret != -EALREADY) {
+		LOG_ERR("Failed to %s UART device: %d", "resume", ret);
+		return ret;
+	}
+	return 0;
+}
+
+static bool uart_is_active(void)
+{
+	enum pm_device_state state = PM_DEVICE_STATE_OFF;
+	int err = pm_device_state_get(uart_dev, &state);
+
+	if (err) {
+		LOG_ERR("Failed to get UART device state (%d).", err);
+		return false;
+	}
+	return state == PM_DEVICE_STATE_ACTIVE;
+}
+
+SM_AT_CMD_CUSTOM(xlog, "AT#XLOG", handle_at_log);
+STATIC int handle_at_log(enum at_parser_cmd_type cmd_type, struct at_parser *parser, uint32_t)
+{
+	const struct log_backend *log_be = log_backend_get_by_name("log_backend_uart");
+
+	if (!log_be) {
+		return -ENODEV;
+	}
+
+	if (cmd_type == AT_PARSER_CMD_TYPE_SET) {
+		int mode;
+		int ret = at_parser_num_get(parser, 1, &mode);
+
+		if (ret || (mode < 0) || (mode > 1)) {
+			return -EINVAL;
+		}
+
+		if (mode == (int)log_active) {
+			return 0;
+		}
+
+		if (mode == 1 && uart_is_active()) {
+			return -EBUSY;
+		}
+
+		if (mode == 1) {
+			ret = uart_resume();
+			if (ret) {
+				return ret;
+			}
+			if (!log_be->cb->initialized) {
+				log_backend_init(log_be);
+			}
+			log_backend_enable(log_be, log_be->cb->ctx, CONFIG_LOG_DEFAULT_LEVEL);
+			log_active = true;
+		} else {
+			log_backend_disable(log_be);
+			ret = uart_suspend();
+			if (ret) {
+				return ret;
+			}
+			log_active = false;
+		}
+		return 0;
+	} else if (cmd_type == AT_PARSER_CMD_TYPE_READ) {
+		rsp_send("\r\n#XLOG: %d\r\n", (int)log_active);
+		return 0;
+	} else if (cmd_type == AT_PARSER_CMD_TYPE_TEST) {
+		rsp_send("\r\n#XLOG: (0,1)\r\n");
+		return 0;
+	}
+
+	return -EINVAL;
+}
+
+static int sm_log_init(void)
+{
+	if (!IS_ENABLED(CONFIG_LOG_BACKEND_UART) ||
+	    !IS_ENABLED(CONFIG_LOG_BACKEND_UART_AUTOSTART)) {
+		/* Start with UART log backend disabled. */
+		if (uart_suspend()) {
+			LOG_ERR("Failed to suspend UART log backend");
+			sm_init_failed = true;
+			return -EFAULT;
+		}
+	} else {
+		log_active = true;
+	}
+
+	return 0;
+}
+
+SYS_INIT(sm_log_init, POST_KERNEL, CONFIG_APPLICATION_INIT_PRIORITY);
+
+#endif /* DT_HAS_CHOSEN(zephyr_console) */