For the Nissan CAS trigger (360 pulses) 3 possible solutions (one working, one should be easy and one hard)
Current solution is a 24+1 slot trigger wheel as described here: VehicleFitment/Nissan
We have a number of cars running on these trigger disks.
With leaving the 360 impulse disk, a hardware divby8 divider can help. We have divby8 implemented with HCF4024 circuit (originally for high toothcount ABS wheels). Since this circuit has (active-high) reset input, this can be useful to make a precise 45 pulse toothtrain from the 360 tooth.
- TODO: find out if the length of the sync pulse accompanying the 360 teeth, and if it is mostly low (good) or mostly high state
- possibly another counter/divider chip is needed with edge-activated reset input ?
- is the sync pulse falling edge not racing with (the rising or falling edge of) one of the 360 pulses ? (I assume not, because that would be very stupid. This would make a nice counter-type implementation impossible, but a helper "assistant CPU" could still help)
- TODO: actually try the resettable divby8 counter (on bench, and on engine)
- divby4 (360 => 90 pulses) is trivial to set up. Firmware could (should) be configured to divby3 or even divby15 (via divby-ANY setting)
- divby8 (360 => 45 pulses) for 4 cyl engines requires a small trick of 4 crankdegrees ignition trimming (because delaying by 23 teeth is not exactly the same as the required 22.5 teeth)
Alternative solution would be software divide by the main processor. The processor is definitely capable. But unfortunately we have some tasks in interrupt that would need to be rewritten. (not the trigger input capture interrupt, but other interrupts that could interfere). This has been evaluated but found to be too much work that would delay other developments unacceptably.
See also