diff --git a/applications/motor_controller/debug.conf b/applications/motor_controller/debug.conf
index a6bcef8..f1fa7e9 100644
--- a/applications/motor_controller/debug.conf
+++ b/applications/motor_controller/debug.conf
@@ -1,5 +1,19 @@
-# compiler
-CONFIG_DEBUG_OPTIMIZATIONS=y
+# No compiler section.
+#
+# This used to set CONFIG_DEBUG_OPTIMIZATIONS=y, which switches -Os to -Og and
+# adds ~56 KB to an image that already fills 99% of the SAMD21's 232 KB. It has
+# never linked on any revision of this app -- it only looked green because
+# twister aborted earlier, on the legacy USB stack's deprecation warnings.
+#
+# Nothing is needed to replace it: the normal build already compiles with
+# -gdwarf-4, so gdb has full symbols and line tables at -Os today. Stepping
+# through -Os is less pleasant than -Og, and on a part this full that is simply
+# the trade. If you need -Og, drop LVGL or the display from the build first.
+#
+# CONFIG_DEBUG_THREAD_INFO (RTOS-aware debugging) is deliberately not set here
+# either: it selects THREAD_MONITOR, which prj.conf turns off to make room for
+# the USB stack. Enable both together locally if you need thread awareness, and
+# expect to give up something else.
 
 # console
 CONFIG_CONSOLE=y
diff --git a/applications/motor_controller/prj.conf b/applications/motor_controller/prj.conf
index ce20d2c..9ab6304 100644
--- a/applications/motor_controller/prj.conf
+++ b/applications/motor_controller/prj.conf
@@ -8,14 +8,31 @@ CONFIG_LOG_MODE_DEFERRED=y
 CONFIG_LOG_PRINTK=y
 
 # Shell
+#
+# Trimmed to make room for the new USB device stack, which costs ~13 KB flash
+# and ~3.2 KB RAM more than the deprecated legacy one on a part that was
+# already at 98.9% flash / 99.5% RAM (160 bytes of RAM free). These were the
+# cheapest ~7.5 KB of flash available: the introspection shells and the thread
+# instrumentation they depend on. SHELL_HELP is pure help strings -- every
+# command still works, it just cannot describe itself.
+#
+# Restoring any of these means finding the flash elsewhere first. Do not
+# re-enable one without re-checking the size report.
+# NB: these need an explicit =n. KERNEL_SHELL, DEVICE_SHELL, INIT_STACKS,
+# THREAD_MONITOR and THREAD_STACK_INFO all default to y once SHELL is on, so
+# simply deleting the lines leaves them enabled and costs ~7.5 KB of flash.
 CONFIG_SHELL=y
-CONFIG_INIT_STACKS=y
-CONFIG_THREAD_STACK_INFO=y
-CONFIG_KERNEL_SHELL=y
-CONFIG_THREAD_MONITOR=y
+CONFIG_SHELL_HELP=n
+CONFIG_SHELL_HISTORY=n
+CONFIG_SHELL_TAB=n
+CONFIG_SHELL_TAB_AUTOCOMPLETION=n
+CONFIG_KERNEL_SHELL=n
+CONFIG_DEVICE_SHELL=n
+CONFIG_I2C_SHELL=n
+CONFIG_THREAD_MONITOR=n
+CONFIG_INIT_STACKS=n
+CONFIG_THREAD_STACK_INFO=n
 CONFIG_THREAD_NAME=y
-CONFIG_DEVICE_SHELL=y
-CONFIG_I2C_SHELL=y
 
 # Drivers
 CONFIG_I2C=y
@@ -26,7 +43,8 @@ CONFIG_MFD_ADAFRUIT_SEESAW=y
 CONFIG_ADC_ADAFRUIT_SEESAW=y
 CONFIG_LED_STRIP_ADAFRUIT_SEESAW=y
 CONFIG_ADAFRUIT_SEESAW_ENCODER=y
-CONFIG_DYNAMIXEL_LOG_LEVEL_DBG=y
+# DYNAMIXEL_LOG_LEVEL_DBG was on here; its debug strings are flash this board
+# no longer has. Re-enable it locally when debugging the bus, not in-tree.
 
 # Actuator subsystem
 CONFIG_ACTUATOR=y
@@ -34,21 +52,21 @@ CONFIG_ACTUATOR_DXL=y
 CONFIG_PICOLIBC_IO_FLOAT=y
 
 # USB
-CONFIG_USB_DEVICE_STACK=y
-CONFIG_USB_DEVICE_VID=0x2F5D
-CONFIG_USB_DEVICE_PID=0x2202
-CONFIG_USB_DEVICE_MANUFACTURER="ROBOTIS"
-CONFIG_USB_DEVICE_PRODUCT="OpenRB-150"
-CONFIG_USB_MAX_POWER=250
-CONFIG_USB_DEVICE_INITIALIZE_AT_BOOT=y
-
-# USB Next
-# CONFIG_USB_DEVICE_STACK_NEXT=y
-# CONFIG_USBD_SHELL=y
-# CONFIG_USBD_CDC_ACM_CLASS=y
-# CONFIG_USBD_LOG_LEVEL_WRN=y
-# CONFIG_UDC_DRIVER_LOG_LEVEL_WRN=y
-# CONFIG_USBD_CDC_ACM_LOG_LEVEL_ERR=y
+#
+# The new device stack. The legacy CONFIG_USB_DEVICE_STACK selects DEPRECATED
+# and is built entirely from __deprecated APIs, so it cannot survive a -Werror
+# build (twister). VID/PID and the descriptor strings are no longer Kconfig;
+# they live in src/usb.c, which also reimplements the BOSSA 1200-baud reset
+# that soc/atmel/sam0/common/bossa.c only provides for the legacy stack.
+CONFIG_USB_DEVICE_STACK_NEXT=y
+CONFIG_USBD_CDC_ACM_CLASS=y
+CONFIG_USBD_LOG_LEVEL_WRN=y
+CONFIG_UDC_DRIVER_LOG_LEVEL_WRN=y
+# Must be OFF, not ERR: cdc_acm_uart0 is zephyr,shell-uart, so with
+# SHELL_LOG_BACKEND the CDC ACM driver logging about the transport it is
+# logging over recurses. usbd_cdc_acm.c makes this a hard #warning, which
+# -Werror turns into a build failure.
+CONFIG_USBD_CDC_ACM_LOG_LEVEL_OFF=y
 
 # CONFIG_SHELL_BACKEND_SERIAL_CHECK_DTR=y
 CONFIG_UART_LINE_CTRL=y
@@ -104,4 +122,11 @@ CONFIG_LV_Z_BITS_PER_PIXEL=16
 CONFIG_LV_COLOR_DEPTH_16=y
 CONFIG_LV_COLOR_16_SWAP=y
 
+# Rendering buffer, as a percentage of the 240x135x16bpp panel (64,800 B full).
+# The default 10% is ~6.5 KB and was the single largest RAM consumer; the new
+# USB stack needs ~2.7 KB of it. 5% is ~3.2 KB, still ~13 display lines per
+# flush (one line is 480 B), so this costs redraw cycles, not correctness or
+# any feature. It is the RAM knob to turn first if something else needs space.
+CONFIG_LV_Z_VDB_SIZE=5
+
 # CONFIG_LV_Z_MEM_POOL_SIZE=2048
diff --git a/applications/motor_controller/src/usb.c b/applications/motor_controller/src/usb.c
index b8bb97b..476de3e 100644
--- a/applications/motor_controller/src/usb.c
+++ b/applications/motor_controller/src/usb.c
@@ -31,43 +31,51 @@ static const struct usb_bos_capability_lpm bos_cap_lpm = {
 USBD_DESC_BOS_DEFINE(app_usbext, sizeof(bos_cap_lpm), &bos_cap_lpm);
 
 /*
+ * BOSSA 1200-baud touch reset.
+ *
+ * The legacy USB stack got this for free from soc/atmel/sam0/common/bossa.c,
+ * which hangs off CDC_ACM_DTE_RATE_CALLBACK_SUPPORT -- a legacy-stack-only
+ * symbol. Under the new stack BOOTLOADER_BOSSA_DEVICE_NAME is unsatisfiable,
+ * so bossa.c compiles to nothing and the behaviour has to live here instead.
+ *
+ * Without it `west flash` (bossac) cannot put the board into its bootloader,
+ * and every flash needs a manual double-tap of the reset button.
+ *
+ * The magic value and its SRAM location are a contract with the bootloader:
+ * keep them identical to bossa.c.
+ */
+#if defined(CONFIG_BOOTLOADER_BOSSA_ADAFRUIT_UF2)
+#define BOSSA_DOUBLE_TAP_MAGIC 0xf01669ef
+#elif defined(CONFIG_BOOTLOADER_BOSSA_ARDUINO)
+#define BOSSA_DOUBLE_TAP_MAGIC 0x07738135
+#endif
+
 static void usb_msg_cb(struct usbd_context *const ctx, const struct usbd_msg *msg)
 {
-	LOG_INF("USBD message: %s", usbd_msg_type_string(msg->type));
-
-	if (usbd_can_detect_vbus(ctx)) {
-		if (msg->type == USBD_MSG_VBUS_READY) {
-			if (usbd_enable(ctx)) {
-				LOG_ERR("Failed to enable device support");
-			}
-		}
-
-		if (msg->type == USBD_MSG_VBUS_REMOVED) {
-			if (usbd_disable(ctx)) {
-				LOG_ERR("Failed to disable device support");
-			}
-		}
+	if (msg->type != USBD_MSG_CDC_ACM_LINE_CODING) {
+		return;
 	}
 
-	if (msg->type == USBD_MSG_CDC_ACM_CONTROL_LINE_STATE) {
-		uint32_t dtr = 0U;
+#if defined(BOSSA_DOUBLE_TAP_MAGIC)
+	uint32_t baudrate = 0U;
 
-		uart_line_ctrl_get(msg->dev, UART_LINE_CTRL_DTR, &dtr);
-		if (dtr) {
-			LOG_INF("Got DTR from USB");
-		}
+	if (uart_line_ctrl_get(msg->dev, UART_LINE_CTRL_BAUD_RATE, &baudrate)) {
+		return;
 	}
 
-	if (msg->type == USBD_MSG_CDC_ACM_LINE_CODING) {
-		uint32_t baudrate = 0U;
+	if (baudrate == 1200) {
+		uint32_t *top = (uint32_t *)(DT_REG_ADDR(DT_NODELABEL(sram0)) +
+					     DT_REG_SIZE(DT_NODELABEL(sram0)));
 
-		uart_line_ctrl_get(msg->dev, UART_LINE_CTRL_BAUD_RATE, &baudrate);
-		if (baudrate) {
-			LOG_INF("Baudrate %u", baudrate);
-		}
+		/* Detach before resetting, as bossa.c did, so the host sees the
+		 * device go away rather than stop responding mid-enumeration. */
+		(void)usbd_disable(ctx);
+		top[-1] = BOSSA_DOUBLE_TAP_MAGIC;
+		NVIC_SystemReset();
 	}
+#endif
 }
-*/
+
 static int app_usb_init(void)
 {
 	int err;
@@ -87,7 +95,7 @@ static int app_usb_init(void)
 		return err;
 	}
 
-	err = usbd_register_all_classes(&app_usbd, USBD_SPEED_FS, 1);
+	err = usbd_register_all_classes(&app_usbd, USBD_SPEED_FS, 1, NULL);
 	if (err) {
 		LOG_ERR("Failed to add register classes");
 		return err;
@@ -98,13 +106,13 @@ static int app_usb_init(void)
 	 * available, use an appropriate triple to indicate it.
 	 */
 	usbd_device_set_code_triple(&app_usbd, USBD_SPEED_FS, USB_BCC_MISCELLANEOUS, 0x02, 0x01);
-	/*
-		err = usbd_msg_register_cb(&app_usbd, &usb_msg_cb);
-		if (err) {
-			LOG_ERR("Failed to register message callback");
-			return err;
-		}
-	*/
+
+	err = usbd_msg_register_cb(&app_usbd, &usb_msg_cb);
+	if (err) {
+		LOG_ERR("Failed to register message callback");
+		return err;
+	}
+
 	(void)usbd_device_set_bcd_usb(&app_usbd, USBD_SPEED_FS, 0x0201);
 
 	err = usbd_add_descriptor(&app_usbd, &app_usbext);
diff --git a/applications/motor_controller/tests.yaml b/applications/motor_controller/tests.yaml
index 083d390..6110f66 100644
--- a/applications/motor_controller/tests.yaml
+++ b/applications/motor_controller/tests.yaml
@@ -6,6 +6,13 @@ sample:
   name: motor-controller-application
 common:
   build_only: true
+  # The app is board-specific: it needs the motor0/seesaw/display nodes from
+  # boards/robotis_openrb_150.overlay, and mise's `app` task allows no other
+  # board. Without this, twister fans out over ~60 unrelated platforms and
+  # reports failures ("'__device_dts_ord_..._motor0_ORD' undeclared") that only
+  # mean "this app was never meant to build there".
+  platform_allow:
+    - robotis_openrb_150
   integration_platforms:
     - robotis_openrb_150
 tests: