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Symptom: stm32flash tool reported: handshaking... error unexpected response 0x68.
Analysis: The STM32 was not in bootloader mode (BOOT0 pin low). Instead, it was running user application code that was printing lowercase 'h' repeatedly in a loop.
Fix: Pulled BOOT0 high and reset the chip. The bootloader responded with 0x79 (ACK), and programming succeeded.
GSM/LTE modules require significant current bursts during network registration or transmission (often up to 2A for short bursts).
In many industrial and embedded protocols, 0x68 is a designated Start-of-Frame (SOF) or Sync Byte. handshaking... error unexpected response 0x68
| Check | Action | |-------|--------| | Baud rate | Ensure matching rates (e.g., 9600, 19200, 57600). Try auto-baud detection if supported. | | Data bits | Usually 8. | | Parity | Even, Odd, or None – must match device config. | | Stop bits | 1 or 2. | | Flow control | Disable hardware handshake (RTS/CTS) unless explicitly required. |
0x68 (hexadecimal) equals 104 in decimal, or the ASCII character 'h' (lowercase H). Symptom: stm32flash tool reported: handshaking
However—and this is crucial—0x68 is not a standard handshake character.
So if your device expected 0x06 (ACK – Acknowledge) or 0x15 (NAK – Negative Acknowledge) and instead received 0x68, the protocol is broken. Your device is basically screaming: "I asked if you were ready, and you replied with the letter 'h' – I have no idea what to do with that." In many industrial and embedded protocols, 0x68 is
Disconnect the peripheral. Connect a loopback plug (connect TX to RX on your PC’s serial port). Send a handshake command. Do you receive back exactly what you sent?
To isolate the problem, follow these steps: