The OKI M6260A (often marked as M6260A or 6260A) is a custom integrated circuit manufactured by OKI Semiconductor specifically for Honda Engine Control Units (ECUs). Officially designated as the Engine Control Counter IC, this chip was discontinued in the mid-1990s but remains a cornerstone of OBD0 and OBD1 ECU architectures.

Because official manufacturer documentation was never publicly released, the details here are the result of extensive hardware reverse-engineering by the Honda tuning community (notably J. Wong, Javier, Mark Lamond, and lestard).

Role and Core Functions#

The OKI M6260A acts as a multi-purpose I/O coprocessor and timing manager. It offloads real-time, high-frequency operations from the main MCU (such as the OKI 66207). Its primary duties include:

1. Engine Position & Speed Tracking: Monitoring raw distributor signals (TDC, CYP, CKP) to calculate RPM and trigger timing interrupts.
2. Vehicle Speed Processing: Reading speed sensor pulses (VSS) to calculate vehicle speed.
3. Injector Timing Output (Pre-drivers): Controlling the gate/base signals for the high-power fuel injector transistors (Q18–Q21).
4. Ignition Timing Output: Directing the igniter trigger pulse (ignition coil trigger) to control spark timing.
5. Auxiliary Output Driving: Driving auxiliary relays and solenoids (A/C clutch, main relay, EVAP purge, fast idle, Check Engine Light, etc.).
6. Hardware Feedback Loop Monitoring: Verifying output states by reading loop-back diagnostic inputs.

Software Interface & Memory Registers#

The main MCU communicates with the M6260A via its external data bus using memory-mapped I/O instructions (MOVX in 8051-style assembly). The M6260A occupies the address space from 0x2000h to 0x200Fh.

Write Operations to 0x2000h (Port Outputs)#

Writing a byte to address 0x2000h directly toggles digital output lines on the M6260A.

• Logic State: Active-Low control. If the high bit (Bit 7) is 0, the corresponding output is turned ON (enabled). If Bit 7 is 1, the output is turned OFF (disabled).
• Port Address: The lower 7 bits serve as an address or pin identifier.

Read Operations (Digital Inputs)#

Reading addresses 0x2001h and 0x2002h returns the live logic status of digital inputs wired to the M6260A.

Counter & Pulse Accumulator Registers#

Reading registers 0x2004 through 0x2007 retrieves hardware counters populated directly by the M6260A's distributor and VSS pulse-reading inputs.

Note

Writing to address 0x2000h with bits 4 and 5 set to 0 configures or resets these RPM/VSS counters and the internal Pulse Width Modulator (PWM) controllers. Address 0x200Fh behaves as a General Control Register for the M6260A's internal configuration.

M6260A Startup Initialization Sequence#

During ECU boot-up, the software runs a precise byte-exchange routine to initialize the M6260A chip:

WRITE 0x00 -> [0x2000h]   ; Clear/initialize outputs
WRITE 0x25 -> [0x2000h]   ; Initialize TDC sensor routing
WRITE 0x96 -> [0x2000h]   ; Command Error LED OFF (high bit active)
WRITE 0xA7 -> [0x2000h]   ; Turn off auxiliary port
WRITE 0xE9 -> [0x200Fh]   ; Configure general control register
READ          [0x2003h]   ; Read status register (expects 0x80)
WRITE 0xA0 -> [0x2000h]   ; Command Fast Idle Solenoid OFF
READ          [0x2003h]   ; Read status register (expects 0x80)
WRITE 0x00 -> [0x2001h]   ; Initialize Port 1
WRITE 0x01 -> [0x2003h]   ; Enable interrupts/write control
READ          [0x2003h]   ; Read status register (expects 0x81)
WRITE 0x01 -> [0x2002h]   ; Initialize Port 2
READ          [0x2003h]   ; Read status register (expects 0x80)

64-Pin Package Pinout Mapping#

The M6260A is housed in a 64-pin plastic QFP package. Below is the complete pinout configuration showing connections to the MCU and internal ECU components: