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+# freefit (also known as: FereFit, Lyne_Wearables, HomieFit, zwsvibe, WIRCASS)
+
+Reverse-engineering a sketchy Chinese watch app
+
+Documentation/protocols derived from decompiled FereFit Android app (jadx)  
+(com.czw.freefit, SHENZHEN ZHONGWEI INTELLIGENT TECHNOLOGY Co.,Ltd)
+
+## BLE characteristics
+
+### Main characteristics (presumably Nordic UART/Nordic semiconductor chips)
+- Service UUID: 6E40FC00-B5A3-F393-E0A9-E50E24DCCA9E
+- Write characteristic UUID: 6E40FC20-B5A3-F393-E0A9-E50E24DCCA9E
+- Notify characteristic UUID: 6E40FC21-B5A3-F393-E0A9-E50E24DCCA9E
+
+### Jieli/JL chip characteristics (untested)
+- Service UUID: 0000ae00-0000-1000-8000-00805f9b34fb
+- Write characteristic UUID: 0000ae01-0000-1000-8000-00805f9b34fb
+- Notify characteristic UUID: 0000ae02-0000-1000-8000-00805f9b34fb
+
+## Protocols/functions
+
+### Battery status/notification (read/6E40FC21)
+
+Sent by watch if command is invalid. TODO find proper trigger for this  
+byte[0] = 0x94 (BATTERY command/response)
+byte[1] = battery percentage (0-100)
+
+### syncTime
+
+Time sync packet structure (write/6E40FC20)
+byte[0]   = 0x01 (command)
+byte[1-4] = Unix timestamp (big endian, seconds)
+byte[5-8] = timezone offset (big endian, seconds)
+byte[9]   = i (unknown param, use 0x00)
+byte[10]  = language code
+byte[11]  = 0x01 if traditional Chinese, else 0x00
+
+Response: 0x81 0x00 (success, notify/6E40FC21)
+
+### enterMakeDial (watch face)
+
+Watch face header packet (write/6EFC20)
+byte[0]   = 0xE4 (ZK_DIAL command)
+byte[1]   = 0x51 (mode flag)
+byte[2]   = 0x01 (start)
+byte[3]   = 0x00
+byte[4-5] = total packet count (big endian)
+byte[6-9] = total image bytes (big endian)
+byte[10]  = 0x00
+byte[11-12] = MTU size (big endian)
+byte[13]  = rotation flag
+byte[14]  = 0x01
+byte[15]  = time text direction
+byte[16]  = 0x00
+byte[17-18] = transparent color (RGB565)
+byte[19-20] = checksum (sum of all image bytes, big endian)
+byte[21]  = show date (0x01 = yes, 0x00 = no)
+
+Header packet is followed by chunked RGB565 data, 1 chunk = MTU-14 bytes