extmod/modbluetooth: Allow config of scan interval/window.

This adds two additional optional kwargs to `gap_scan()`:
  - `interval_us`: How long between scans.
  - `window_us`: How long to scan for during a scan.

The default with NimBLE is a 11.25ms window with a 1.28s interval.

Changing these parameters is important for detecting low-frequency
advertisements (e.g. beacons).

Note: these params are in microseconds, not milliseconds in order
to allow the 625us granularity offered by the spec.

Author: jimmo

extmod/modbluetooth.c

@@ -501,23 +501,26 @@ STATIC mp_obj_t bluetooth_ble_gap_connect(size_t n_args, const mp_obj_t *args) {
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(bluetooth_ble_gap_connect_obj, 3, 4, bluetooth_ble_gap_connect);
 
 STATIC mp_obj_t bluetooth_ble_gap_scan(size_t n_args, const mp_obj_t *args) {
-    if (n_args == 2 && args[1] == mp_const_none) {
-        int err = mp_bluetooth_gap_scan_stop();
-        return bluetooth_handle_errno(err);
-    } else {
-        mp_int_t duration_ms = 0;
-        if (n_args == 2) {
-            if (!mp_obj_is_int(args[1])) {
-                mp_raise_ValueError("invalid duration");
+    // Default is indefinite scan, with the NimBLE "background scan" interval and window.
+    mp_int_t duration_ms = 0;
+    mp_int_t interval_us = 1280000;
+    mp_int_t window_us = 11250;
+    if (n_args > 1) {
+        if (args[1] == mp_const_none) {
+            // scan(None) --> stop scan.
+            return bluetooth_handle_errno(mp_bluetooth_gap_scan_stop());
+        }
+        duration_ms = mp_obj_get_int(args[1]);
+        if (n_args > 2) {
+            interval_us = mp_obj_get_int(args[2]);
+            if (n_args > 3) {
+                window_us = mp_obj_get_int(args[3]);
             }
-            duration_ms = mp_obj_get_int(args[1]);
         }
-
-        int err = mp_bluetooth_gap_scan_start(duration_ms);
-        return bluetooth_handle_errno(err);
     }
+    return bluetooth_handle_errno(mp_bluetooth_gap_scan_start(duration_ms, interval_us, window_us));
 }
-STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(bluetooth_ble_gap_scan_obj, 1, 2, bluetooth_ble_gap_scan);
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(bluetooth_ble_gap_scan_obj, 1, 4, bluetooth_ble_gap_scan);
 #endif // MICROPY_PY_BLUETOOTH_ENABLE_CENTRAL_MODE
 
 STATIC mp_obj_t bluetooth_ble_gap_disconnect(mp_obj_t self_in, mp_obj_t conn_handle_in) {

extmod/modbluetooth.h

@@ -186,7 +186,7 @@ int mp_bluetooth_gap_disconnect(uint16_t conn_handle);
 
 #if MICROPY_PY_BLUETOOTH_ENABLE_CENTRAL_MODE
 // Start a discovery (scan). Set duration to zero to run continuously.
-int mp_bluetooth_gap_scan_start(int32_t duration_ms);
+int mp_bluetooth_gap_scan_start(int32_t duration_ms, int32_t interval_us, int32_t window_us);
 
 // Stop discovery (if currently active).
 int mp_bluetooth_gap_scan_stop(void);

extmod/modbluetooth_nimble.c

@@ -614,16 +614,16 @@ STATIC int gap_scan_cb(struct ble_gap_event *event, void *arg) {
     return 0;
 }
 
-int mp_bluetooth_gap_scan_start(int32_t duration_ms) {
+int mp_bluetooth_gap_scan_start(int32_t duration_ms, int32_t interval_us, int32_t window_us) {
     if (duration_ms == 0) {
         duration_ms = BLE_HS_FOREVER;
     }
-    STATIC const struct ble_gap_disc_params discover_params = {
-        .itvl = BLE_GAP_SCAN_SLOW_INTERVAL1,
-        .window = BLE_GAP_SCAN_SLOW_WINDOW1,
+    struct ble_gap_disc_params discover_params = {
+        .itvl = MAX(BLE_HCI_SCAN_ITVL_MIN, MIN(BLE_HCI_SCAN_ITVL_MAX, interval_us / BLE_HCI_SCAN_ITVL)),
+        .window = MAX(BLE_HCI_SCAN_WINDOW_MIN, MIN(BLE_HCI_SCAN_WINDOW_MAX, window_us / BLE_HCI_SCAN_ITVL)),
         .filter_policy = BLE_HCI_CONN_FILT_NO_WL,
         .limited = 0,
-        .passive = 0,
+        .passive = 1,  // TODO: Handle BLE_HCI_ADV_RPT_EVTYPE_SCAN_RSP in gap_scan_cb above.
         .filter_duplicates = 0,
     };
     int err = ble_gap_disc(BLE_OWN_ADDR_PUBLIC, duration_ms, &discover_params, gap_scan_cb, NULL);