Add doc on enabling USB logging.

docs/docs/dev-guide-usb-logging.md

@@ -0,0 +1,51 @@
+---
+id: dev-guide-usb-logging
+title: USB Logging
+---
+
+## Overview
+
+If you are developing ZMK on a device that does not have a built in UART for debugging and log/console output,
+Zephyr can be configured to create a USB CDC ACM device and the direct all `printk`, console output, and log
+messages to that device instead.
+
+## Kconfig
+
+The following KConfig values need to be set, either by copy and paste into the `app/prj.conf` file, or by running
+`west build -t menuconfig` and manually enabling the various settings in that UI.
+
+```
+# Turn on logging, and set ZMK logging to debug output
+CONFIG_LOG=y
+CONFIG_ZMK_LOG_LEVEL_DBG=y
+
+# Turn on USB CDC ACM device
+CONFIG_USB_CDC_ACM=y
+CONFIG_USB_CDC_ACM_RINGBUF_SIZE=1024
+CONFIG_USB_CDC_ACM_DEVICE_NAME="CDC_ACM"
+CONFIG_USB_CDC_ACM_DEVICE_COUNT=1
+
+# Enable serial console
+CONFIG_SERIAL=y
+CONFIG_CONSOLE=y
+CONFIG_UART_INTERRUPT_DRIVEN=y
+CONFIG_UART_LINE_CTRL=y
+
+# Enable USB UART, and set the console device
+CONFIG_UART_CONSOLE=y
+CONFIG_USB_UART_CONSOLE=y
+CONFIG_UART_CONSOLE_ON_DEV_NAME="CDC_ACM_0"
+CONFIG_USB_UART_DTR_WAIT=n
+```
+
+## Viewing Logs
+
+After flashing the updated ZMK image, the board should expose a USB CDC ACM device, that you can connect to and view the logs.
+
+On Linux, this should be a device like `/dev/ttyACM0` and you can connect with `minicom` or `tio` as usual, e.g.:
+
+```
+sudo tio /dev/ttyACM0
+```
+
+From there, you should see the various log messages from ZMK and Zephyr, depending on which systems you have set to what log levels.

docs/sidebars.js

@@ -7,8 +7,6 @@ module.exports = {
       "dev-boards-shields-keymaps",
       "dev-posix-board",
     ],
-    "Dev Guides": [
-      "dev-guide-new-shield",
-    ],
+    "Dev Guides": ["dev-guide-new-shield", "dev-guide-usb-logging"],
   },
 };