zephyr: Switch nRF52840 DK to UART console, fix USB ordering.

The nRF52840 DK's USB CDC ACM console was unreliable — device enumeration
failures and stalls during raw-paste mode. Switch to UART via JLink OB
(uart0 with hw-flow-control) which is always available.

Move USB device stack init from mp_task (after console init) to
zephyr_start.c main() (before console init) so CDC ACM UART is ready
when the console subsystem opens the device. Add DTR wait for CDC ACM
console boards so output isn't lost before a host connects.

Reduce MICROPY_REPL_STDIN_BUFFER_MAX to 64 (raw-paste window=32 bytes)
to avoid overflowing USB-UART bridge buffers at 115200 baud.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

Author: pi-anl

ports/zephyr/boards/nrf52840dk_nrf52840.conf

@@ -1,9 +1,5 @@
 CONFIG_NETWORKING=n
 
-# USB CDC ACM console
-CONFIG_USB_DEVICE_STACK_NEXT=y
-CONFIG_USBD_CDC_ACM_CLASS=y
-
 CONFIG_BT=y
 CONFIG_BT_DEVICE_NAME_DYNAMIC=y
 CONFIG_BT_GATT_DYNAMIC_DB=y
@@ -43,7 +39,6 @@ CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=4096
 CONFIG_MICROPY_HEAP_SIZE=98304
 CONFIG_MAIN_STACK_SIZE=16384
 
-# CONFIG_DYNAMIC_THREAD=y
 CONFIG_THREAD_CUSTOM_DATA=y
 CONFIG_THREAD_MONITOR=y
 CONFIG_THREAD_STACK_INFO=y
@@ -52,7 +47,7 @@ CONFIG_THREAD_STACK_INFO=y
 # Full mbedTLS ECP causes DHKey check failure on nRF52840 native controller
 CONFIG_BT_LONG_WQ_STACK_SIZE=4096
 
-# SMP/pairing debug via RTT
+# RTT logging for debug (keeps log output off the UART console)
 CONFIG_LOG=y
 CONFIG_LOG_BACKEND_RTT=y
 CONFIG_LOG_BACKEND_UART=n

ports/zephyr/boards/nrf52840dk_nrf52840.overlay

@@ -0,0 +1,7 @@
+/* Enable UART hardware flow control (RTS/CTS) for reliable raw REPL.
+ * Pins already routed on nRF52840 DK: RTS=P0.5, CTS=P0.7.
+ * JLink OB UART bridge supports hardware flow control.
+ */
+&uart0 {
+	hw-flow-control;
+};

ports/zephyr/main.c

@@ -85,10 +85,6 @@ DT_FOREACH_STATUS_OKAY(micropython_heap, MICROPY_HEAP_DEFINE)
 
 static __noinit char heap[MICROPY_HEAP_SIZE];
 
-#if defined(CONFIG_USB_DEVICE_STACK_NEXT)
-extern int mp_usbd_init(void);
-#endif // defined(CONFIG_USB_DEVICE_STACK_NEXT)
-
 void init_zephyr(void) {
     // We now rely on CONFIG_NET_APP_SETTINGS to set up bootstrap
     // network addresses.
@@ -139,9 +135,8 @@ int real_main(void) {
     usb_enable(NULL);
     #endif
 
-    #ifdef CONFIG_USB_DEVICE_STACK_NEXT
-    mp_usbd_init();
-    #endif
+    // Note: CONFIG_USB_DEVICE_STACK_NEXT init is in zephyr_start.c main()
+    // so it runs before the console is initialised (required for CDC ACM).
 
     #if MICROPY_VFS && MICROPY_MODULE_FROZEN_MPY
     // Mount and/or create the filesystem

ports/zephyr/mpconfigport.h

@@ -59,6 +59,9 @@
 #define MICROPY_ENABLE_FINALISER    (MICROPY_VFS)
 #define MICROPY_HELPER_REPL         (1)
 #define MICROPY_REPL_AUTO_INDENT    (1)
+// Reduce raw-paste window to 32 bytes for UART-bridge connections (JLink OB).
+// Default 256 (window=128) overflows USB-UART bridge buffers at 115200 baud.
+#define MICROPY_REPL_STDIN_BUFFER_MAX (64)
 #define MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF  (1)
 #define MICROPY_KBD_EXCEPTION       (1)
 #define MICROPY_PY_BUILTINS_BYTES_HEX (1)

ports/zephyr/src/usbd.c

@@ -190,4 +190,4 @@ int mp_usbd_init(void) {
     return 0;
 }
 
-#endif // defined(CONFIG_USB_DEVICE_STACK_NEXT)
+#endif // defined(CONFIG_USB_DEVICE_STACK_NEXT) && !defined(CONFIG_CDC_ACM_SERIAL_INITIALIZE_AT_BOOT)

ports/zephyr/src/zephyr_start.c

@@ -24,17 +24,47 @@
  * THE SOFTWARE.
  */
 #include <zephyr/kernel.h>
+#include <zephyr/devicetree.h>
+#include <zephyr/drivers/uart.h>
 #include "zephyr_getchar.h"
 
 int real_main(void);
 int mp_console_init(void);
 
+#if defined(CONFIG_USB_DEVICE_STACK_NEXT)
+extern int mp_usbd_init(void);
+#endif
+
+// Check if the console device is a CDC ACM UART (USB serial).
+#define MP_CONSOLE_IS_CDC_ACM DT_NODE_HAS_COMPAT(DT_CHOSEN(zephyr_console), zephyr_cdc_acm_uart)
+
+#if MP_CONSOLE_IS_CDC_ACM && defined(CONFIG_UART_LINE_CTRL)
+static void mp_wait_for_usb_dtr(void) {
+    const struct device *dev = DEVICE_DT_GET(DT_CHOSEN(zephyr_console));
+    uint32_t dtr = 0;
+
+    while (!dtr) {
+        uart_line_ctrl_get(dev, UART_LINE_CTRL_DTR, &dtr);
+        k_msleep(100);
+    }
+}
+#endif
+
 int main(void) {
+    #if defined(CONFIG_USB_DEVICE_STACK_NEXT)
+    mp_usbd_init();
+    #endif
+
+    #if MP_CONSOLE_IS_CDC_ACM && defined(CONFIG_UART_LINE_CTRL)
+    mp_wait_for_usb_dtr();
+    #endif
+
     #ifdef CONFIG_CONSOLE_SUBSYS
     mp_console_init();
     #else
     zephyr_getchar_init();
     #endif
+
     real_main();
 
     return 0;