This guide details the procedure for dumping the internal program memory of the Oki 83C154 microcontroller, commonly found in OBD0 Honda ECUs.

Hardware Requirements#

• External Connection: The target MCU must be connected to an external EPROM.
• Serial Interface: A MAX233 (or compatible) serial level shifter is required to facilitate communication between the ECU and a PC.
• Hardware Modification:
  1. Open the _EA (External Access) jumper on the ECU board.
  2. Connect the _EA line of the MCU to pin P1.7 of the MCU.

ROM Dumping Procedure#

1. Preparation: Configure your serial terminal software to 4800 baud, No parity, 8 data bits, 1 stop bit (4800, N, 8, 1).
2. Code Execution: Run the specialized dumping code which performs the following:
   • Jumps to memory address 4100h.
   • Flips the state of the _EA pin (using P1.7 to mask/unmask the internal ROM).
   • Enters a delay loop to allow the internal ROM to be mapped into the memory space at 0000h–4000h.
3. Data Extraction: Copy the contents of the memory range via the serial port.
4. Verification: Analyze the downloaded dump to verify the integrity of the data, specifically looking for fuel and ignition tables.

Note: This process relies on hardware design vulnerabilities to bypass internal mask-ROM read protection. Ensure your wiring is correct before applying power to avoid damaging the MCU.