feat(split): Add full-duplex wired split support

* Depends on full-duplex hardware UART for communication.
* Supports all existing central commands/peripheral events, including
  sensors/inputs from peripherals.
* Only one wired split peripheral supported (for now)
* Relies on chosen `zmk,split-uart` referencing the UART device.

app/boards/shields/zmk_uno/zmk_uno_split.dtsi

@@ -11,8 +11,13 @@ left_encoder: &encoder {
     status = "disabled";
 };
 
+&arduino_serial {
+    status = "okay";
+};
+
 / {
     chosen {
+        zmk,split-uart = &arduino_serial;
         zmk,physical-layout = &split_matrix_physical_layout;
     };
 

app/dts/bindings/zmk,wired-split.yaml

@@ -0,0 +1,20 @@
+# Copyright (c) 2024 The ZMK Contributors
+# SPDX-License-Identifier: MIT
+
+description: |
+  Complete specification of wired split connetion
+
+compatible: "zmk,wired-split"
+
+properties:
+  device:
+    type: phandle
+    required: true
+    description: The UART device for wired split communication
+
+  half-duplex:
+    type: boolean
+    description: Enable half-duplex protocol mode.
+
+  dir-gpios:
+    type: phandle-array

app/src/split/CMakeLists.txt

@@ -5,6 +5,10 @@ if (CONFIG_ZMK_SPLIT_BLE)
     add_subdirectory(bluetooth)
 endif()
 
+if (CONFIG_ZMK_SPLIT_WIRED)
+    add_subdirectory(wired)
+endif()
+
 if (CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
     target_sources(app PRIVATE central.c)
     zephyr_linker_sources(SECTIONS ../../include/linker/zmk-split-transport-central.ld)

app/src/split/Kconfig

@@ -9,16 +9,21 @@ if ZMK_SPLIT
 config ZMK_SPLIT_ROLE_CENTRAL
     bool "Split central device"
 
-choice ZMK_SPLIT_TRANSPORT
-    prompt "Split transport"
-
 config ZMK_SPLIT_BLE
-    bool "BLE"
+    bool "BLE Split"
+    default y
     depends on ZMK_BLE
     select BT_USER_PHY_UPDATE
     select BT_AUTO_PHY_UPDATE
 
-endchoice
+DT_CHOSEN_ZMK_SPLIT_UART := zmk,split-uart
+
+config ZMK_SPLIT_WIRED
+    bool "Wired Split"
+    default $(dt_chosen_enabled,$(DT_CHOSEN_ZMK_SPLIT_UART)) if !ZMK_SPLIT_BLE
+    select SERIAL
+    select RING_BUFFER
+    select CRC
 
 config ZMK_SPLIT_PERIPHERAL_HID_INDICATORS
     bool "Peripheral HID Indicators"
@@ -29,3 +34,4 @@ config ZMK_SPLIT_PERIPHERAL_HID_INDICATORS
 endif # ZMK_SPLIT
 
 rsource "bluetooth/Kconfig"
+rsource "wired/Kconfig"

app/src/split/Kconfig.defaults

@@ -2,3 +2,5 @@
 # SPDX-License-Identifier: MIT
 
 rsource "bluetooth/Kconfig.defaults"
+rsource "wired/Kconfig.defaults"
+

app/src/split/wired/CMakeLists.txt

@@ -0,0 +1,3 @@
+target_sources(app PRIVATE wired.c)
+target_sources_ifdef(CONFIG_ZMK_SPLIT_ROLE_CENTRAL app PRIVATE central.c)
+target_sources_ifndef(CONFIG_ZMK_SPLIT_ROLE_CENTRAL app PRIVATE peripheral.c)

app/src/split/wired/Kconfig

@@ -0,0 +1,44 @@
+if ZMK_SPLIT_WIRED
+
+choice ZMK_SPLIT_WIRED_UART_MODE_DEFAULT
+    prompt "Default UART Mode"
+
+config ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_ASYNC
+    bool "Async (DMA) Mode"
+    depends on SERIAL_SUPPORT_ASYNC
+    select UART_ASYNC_API
+
+config ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_INTERRUPT
+    bool "Interrupt Mode"
+    depends on SERIAL_SUPPORT_INTERRUPT
+    select UART_INTERRUPT_DRIVEN
+
+config ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_POLLING
+    bool "Polling Mode"
+
+endchoice
+
+if ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_POLLING
+
+config ZMK_SPLIT_WIRED_POLLING_RX_PERIOD
+    int "Ticks between RX polls"
+
+endif
+
+if ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_ASYNC
+
+config ZMK_SPLIT_WIRED_ASYNC_RX_TIMEOUT
+    int "RX Timeout (in microseconds) before reporting received data"
+
+endif
+
+config ZMK_SPLIT_WIRED_CMD_BUFFER_ITEMS
+    int "Number of central commands to buffer for TX/RX"
+
+config ZMK_SPLIT_WIRED_EVENT_BUFFER_ITEMS
+    int "Number of peripheral events to buffer for TX/RX"
+
+config ZMK_SPLIT_WIRED_HALF_DUPLEX_RX_TIMEOUT
+    int "RX timeout (in ticks) when polling peripheral(s)"
+
+endif

app/src/split/wired/Kconfig.defaults

@@ -0,0 +1,28 @@
+if ZMK_SPLIT_WIRED
+
+config ZMK_SPLIT_WIRED_CMD_BUFFER_ITEMS
+    default 4
+
+config ZMK_SPLIT_WIRED_EVENT_BUFFER_ITEMS
+    default 16
+
+
+if ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_POLLING
+
+config ZMK_SPLIT_WIRED_POLLING_RX_PERIOD
+    default 10
+
+endif
+
+
+if ZMK_SPLIT_WIRED_UART_MODE_DEFAULT_ASYNC
+
+config ZMK_SPLIT_WIRED_ASYNC_RX_TIMEOUT
+    default 100
+
+endif
+
+config ZMK_SPLIT_WIRED_HALF_DUPLEX_RX_TIMEOUT
+    default 50
+
+endif