document write characteristic MTU size limit and implement frame spli…

…tting on write

ikawa_protocol.md

@@ -23,17 +23,23 @@ Communication between the app and the roaster is facilitated over Bluetooth Low
 1. Write characteristic `851A4582-19C1-4E6C-AB37-E7A03766BA16` for sending commands to the roaster.
 2. Notify characteristic `948C5059-7F00-46D9-AC55-BF090AE066E3` for receiving responses to commands sent over the write characteristic.
 
+To complete a request-response interaction, a request must be written to the write characteristic, and a listener on the notify characteristic must await the result. Each request and response is tagged with a sequence number to ensure they are correctly matched up.
+
 > [!IMPORTANT]
-> To complete a request-response interaction, a request must be written to the write characteristic, and a listener on the notify characteristic must await the result. Each request and response is tagged with a sequence number to ensure they are correctly matched up.
+> The write characteristic only accepts 20 bytes[^1] at a time, so longer data frames need to be split into multiple writes.
+> Similarly the notify characteristic provides at most 20 bytes at a time, so a notify may not contain a complete frame and the frame may need to be assembled from multiple notify events.
+
+[^1]: MTU size of 23 - 3 bytes for header
 
 ## Frames
 
 Data sent and received are encapsulated in frames, structured as follows:
 
-- Start and end with FRAME_BYTE: `0x7E`
-- Encapsulated payload:
+- Start FRAME_BYTE: `0x7E`
+- Escaped payload:
     - Message data
     - Custom CRC16 checksum
+- End FRAME_BYTE: `0x7E`
 
 Payload escaping is performed by replacing `0x7E` with `0x7D 0x5E` and `0x7D` with `0x7D 0x5D`. This applies to both the message data and the CRC checksum.
 

libikawa.py

@@ -15,11 +15,12 @@ class Ikawa:
 	ESCAPE_MAPPING = {0x7D: 0x5D, 0x7E: 0x5E}
 	UNESCAPE_MAPPING = {0x5D: 0x7D, 0x5E: 0x7E}
 
-	def __init__(self, reconnect=True, retry_timeout=10):
+	def __init__(self, reconnect=True, reconnect_timeout=60, retry_timeout=10):
 		self.seq = 1 # start with 1 because seq=0 makes Cmd(cmd_type=BOOTLOADER_GET_VERSION) an empty message which the firmware does not seem to handle
 		self.resp_queue = asyncio.Queue()
 		self.recv_buf = bytearray()
 		self.reconnect = True
+		self.reconnect_timeout = reconnect_timeout
 		self.retry_timeout = retry_timeout
 
 	async def __aenter__(self):
@@ -45,7 +46,7 @@ def filter_device(device, advertisement_data):
 	async def connect(self):
 		self.client = BleakClient(self.target_device, disconnected_callback=self.on_disconnect)
 		t = time.time()
-		while time.time() - t < self.retry_timeout:
+		while time.time() - t < self.reconnect_timeout:
 			print("Trying to connect")
 			try:
 				await self.client.connect()
@@ -86,7 +87,7 @@ def on_disconnect(self, client):
 			asyncio.get_running_loop().create_task(self.connect())
 
 	async def on_notify(self, sender, data):
-		# print(f"notify recieved: {data}")
+		# print(f"notify recieved: (len={len(data)}) {data}")
 		self.recv_buf += data
 		while True:
 			try:
@@ -119,7 +120,10 @@ async def send_cmd(self, cmd):
 		data = cmd.SerializeToString()
 		cmd.seq = 0
 		frame = self.encode_frame(data)
-		await self.send_frame(frame)
+		for i in range(0, len(frame), 20):
+			frame_part = frame[i:i+20]
+			# print(f"sending (len={len(frame_part)}) {frame_part}")
+			await self.send_frame(frame_part)
 		# print("waiting for response")
 		resp = await asyncio.wait_for(self.resp_queue.get(), self.retry_timeout)
 		return resp