Cars · Sensors
OBD1 8-bit High Cam RPM Conversion
Technical reference for converting 8-bit high cam table scale values to real-world RPM for OBD1 Honda ECUs.
This formula is used to convert 8-bit high cam table scale values to real-world RPM.
Conversion Formula
If x represents an 8-bit high-cam scale RPM value, the conversion to RPM is calculated as follows:
Hi Cam RPM(x) = (1,875,000 * x) / 53,248
Note
The divisor 53,248 is equivalent to D000h in hexadecimal.
Assembly Implementation
The following assembly logic demonstrates the fixed-point math used by the ECU to process these values:
| Instruction | Opcode / Notes |
|---|---|
MOV er2, 0ach |
0805 0 02 00 B5AC4A |
MOV er0, #0d000h |
0808 0 02 00 449800D0 |
CLR A |
080C 1 02 00 F9 |
DIV |
080D 1 02 00 9037 |
DIV Logic |
(er0, A) = (er0, A) / er2 |
LB A, r0 |
0817 0 02 00 78 |
JNE label_081f |
0818 0 02 00 CE05 |
label_081f: STB A, 0aah |
081F 0 02 00 D5AA |
Important
The register r0 contains the least significant byte of er0, which represents the most significant word of the quotient. Ensure overflow conditions are handled according to the specific ECU ROM structure when implementing this logic in custom code.