UART Frame Decoder with Resync
You are receiving a continuous byte stream from UART. Valid frames follow this format:
[0xAA][LEN][DATA...][CHK][0x55]
Where: - 0xAA → Start byte - LEN → Number of data bytes - DATA → Payload - CHK → Checksum = sum of DATA bytes & 0xFF - 0x55 → End byte
The stream may contain noise bytes before a valid frame.
Write a function that: 1. Scans the buffer for the first valid frame 2. Validates length, checksum, and footer 3. Returns the index of the frame start if valid 4. Returns -1 if no valid frame exists
This simulates real UART stream handling with resynchronization.
1int find_frame(unsigned char *buf, int n) {2 // Return index of valid frame start, or -13 return -1;4}