feat(split): Increase split interval during idle

Slows down BLE connection interval between split halves when keyboard
goes idle, reducing central half battery consumption significantly
during idle periods.

When the keyboard becomes active again, connection parameters are
restored to their normal values. There may be brief latency (0.6-1s)
when waking from idle.

Based on: zmkfirmware#682

Co-authored-by: Nick Winans <nick@winans.codes>

Author: carrefinho

app/src/split/bluetooth/CMakeLists.txt

@@ -7,6 +7,7 @@ if (NOT CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
 endif()
 if (CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
   target_sources(app PRIVATE central.c)
+  target_sources_ifdef(CONFIG_ZMK_SPLIT_BLE_PREF_IDLE app PRIVATE central_idle.c)
 endif()
 
 if (CONFIG_ZMK_SPLIT_BLE_CENTRAL_BATTERY_LEVEL_PROXY)

app/src/split/bluetooth/Kconfig

@@ -74,6 +74,26 @@ config ZMK_SPLIT_BLE_PREF_TIMEOUT
     int "Supervision timeout to use for split central/peripheral connection"
     default 400
 
+config ZMK_SPLIT_BLE_PREF_IDLE
+    bool "Set slower split peripheral BLE params on idle to save power"
+    default y
+
+if ZMK_SPLIT_BLE_PREF_IDLE
+
+config ZMK_SPLIT_BLE_PREF_IDLE_INT
+    int "Idle connection interval to use for split central/peripheral connection"
+    default 18
+
+config ZMK_SPLIT_BLE_PREF_IDLE_LATENCY
+    int "Idle latency to use for split central/peripheral connection"
+    default 10
+
+config ZMK_SPLIT_BLE_PREF_IDLE_TIMEOUT
+    int "Idle supervision timeout to use for split central/peripheral connection"
+    default 400
+
+endif # ZMK_SPLIT_BLE_PREF_IDLE
+
 endif # ZMK_SPLIT_ROLE_CENTRAL
 
 if !ZMK_SPLIT_ROLE_CENTRAL

app/src/split/bluetooth/central_idle.c

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2025 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <zephyr/logging/log.h>
+LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
+
+#include <zephyr/bluetooth/bluetooth.h>
+#include <zephyr/bluetooth/conn.h>
+
+#include <zmk/event_manager.h>
+#include <zmk/events/activity_state_changed.h>
+
+static void set_sleep_params(struct bt_conn *conn, void *data) {
+    struct bt_conn_info info;
+
+    bt_conn_get_info(conn, &info);
+
+    if (info.role == BT_CONN_ROLE_CENTRAL && info.state == BT_CONN_STATE_CONNECTED) {
+        int err =
+            bt_conn_le_param_update(conn, BT_LE_CONN_PARAM(CONFIG_ZMK_SPLIT_BLE_PREF_IDLE_INT,
+                                                           CONFIG_ZMK_SPLIT_BLE_PREF_IDLE_INT,
+                                                           CONFIG_ZMK_SPLIT_BLE_PREF_IDLE_LATENCY,
+                                                           CONFIG_ZMK_SPLIT_BLE_PREF_IDLE_TIMEOUT));
+
+        if (err) {
+            LOG_DBG("Failed to set idle params on split connection: %d", err);
+        }
+    }
+}
+
+static void set_wake_params(struct bt_conn *conn, void *data) {
+    struct bt_conn_info info;
+
+    bt_conn_get_info(conn, &info);
+
+    if (info.role == BT_CONN_ROLE_CENTRAL && info.state == BT_CONN_STATE_CONNECTED) {
+        int err = bt_conn_le_param_update(
+            conn,
+            BT_LE_CONN_PARAM(CONFIG_ZMK_SPLIT_BLE_PREF_INT, CONFIG_ZMK_SPLIT_BLE_PREF_INT,
+                             CONFIG_ZMK_SPLIT_BLE_PREF_LATENCY, CONFIG_ZMK_SPLIT_BLE_PREF_TIMEOUT));
+
+        if (err) {
+            LOG_DBG("Failed to set active params on split connection: %d", err);
+        }
+    }
+}
+
+static void sleep_all(void) {
+    LOG_DBG("Setting idle connection parameters on peripherals");
+
+    bt_conn_foreach(BT_CONN_TYPE_LE, set_sleep_params, NULL);
+}
+
+static void wake_all(void) {
+    LOG_DBG("Waking up from idle connection parameters on peripherals");
+
+    bt_conn_foreach(BT_CONN_TYPE_LE, set_wake_params, NULL);
+}
+
+static int central_idle_listener(const zmk_event_t *eh) {
+    struct zmk_activity_state_changed *ev = as_zmk_activity_state_changed(eh);
+    if (ev == NULL) {
+        return -ENOTSUP;
+    }
+
+    switch (ev->state) {
+    case ZMK_ACTIVITY_ACTIVE:
+        wake_all();
+        break;
+    case ZMK_ACTIVITY_IDLE:
+        sleep_all();
+        break;
+    case ZMK_ACTIVITY_SLEEP:
+        break;
+    default:
+        LOG_WRN("Unhandled activity state: %d", ev->state);
+        return -EINVAL;
+    }
+    return 0;
+}
+
+ZMK_LISTENER(central_idle, central_idle_listener);
+ZMK_SUBSCRIPTION(central_idle, zmk_activity_state_changed);