ARM split - detect USB to select master/slave (#6424)

* Initial split refactor to allow usb master detection

* Add split USB detect docs

* Add SPLIT_USB_DETECT demo mode limitation

* fix rebase issues

* clang-format

2 files changed, 20 insertions, 0 deletions

diff --git a/docs/config_options.md b/docs/config_options.md
index 9e43f47c2..ec3d1a1c8 100644
--- a/docs/config_options.md
+++ b/docs/config_options.md
@@ -267,6 +267,14 @@ There are a few different ways to set handedness for split keyboards (listed in
     * 4: about 26kbps
     * 5: about 20kbps
 
+* `#define SPLIT_USB_DETECT`
+  * Detect (with timeout) USB connection when delegating master/slave
+  * Default behavior for ARM
+  * Required for AVR Teensy
+
+* `#define SPLIT_USB_TIMEOUT 2500`
+  * Maximum timeout when detecting master/slave when using `SPLIT_USB_DETECT`
+
 # The `rules.mk` File
 
 This is a [make](https://www.gnu.org/software/make/manual/make.html) file that is included by the top-level `Makefile`. It is used to set some information about the MCU that we will be compiling for as well as enabling and disabling certain features.
diff --git a/docs/feature_split_keyboard.md b/docs/feature_split_keyboard.md
index 42dd838d0..efc65a4c7 100644
--- a/docs/feature_split_keyboard.md
+++ b/docs/feature_split_keyboard.md
@@ -190,6 +190,18 @@ This sets how many LEDs are directly connected to each controller.  The first nu
 ?> This setting implies that `RGBLIGHT_SPLIT` is enabled, and will forcibly enable it, if it's not.
 
 
+```c
+#define SPLIT_USB_DETECT
+```
+This option changes the startup behavior to detect an active USB connection when delegating master/slave. If this operation times out, then the half is assume to be a slave. This is the default behavior for ARM, and required for AVR Teensy boards (due to hardware limitations).
+
+?> This setting will stop the ability to demo using battery packs.
+
+```c
+#define SPLIT_USB_TIMEOUT 2500
+```
+This sets the maximum timeout when detecting master/slave when using `SPLIT_USB_DETECT`.
+
 ## Additional Resources
 
 Nicinabox has a [very nice and detailed guide](https://github.com/nicinabox/lets-split-guide) for the Let's Split keyboard, that covers most everything you need to know, including troubleshooting information.