ecg: Add 16 bit sample counter to BLE stream

Documentation/bluetooth/ecg.rst

@@ -25,3 +25,5 @@ ECG samples characteristic
 
 List of 16 bit samples (big endian). Enable notifications to
 receive a stream of samples while the ECG app is open.
+
+The first 16 bit are a sample counter (big endian).

preload/apps/ecg/__init__.py

@@ -85,6 +85,7 @@ class ECG:
                         self.disp.backlight(20)
                 else:
                     self.ble_streaming = True
+                    self.ble_sample_count = 0
                     if not config.get_option("BLE Disp"):
                         self.disp.backlight(0)
 
@@ -173,10 +174,12 @@ class ECG:
 
     def callback_ecg(self, datasets):
         if self.ble_streaming:
+            self.ble_sample_count += len(datasets)
             self.b.gatts_notify(
                 1,
                 self.ecg_data_handle,
-                struct.pack(">" + ("h" * len(datasets)), *datasets),
+                struct.pack(">h", self.ble_sample_count & 0xFFFF)
+                + struct.pack(">" + ("h" * len(datasets)), *datasets),
             )
 
             # Don't update the screen if it should be off during a connection