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 divby4 divider can help. We have divby4 / 8 implemented with HCF4024 circuit (under PCB manufacturing, originally for high toothcount ABS wheels). Since this circuit has (active-high) reset input, this can be useful to make a precise 180, 90 or 45 pulsetrain from the 360 tooth.
- TODO: find out if the exact pattern of the 4 sync pulses accompanying the 360 teeth (from a reliable source or measurement).
- because the sync pulse is mostly 0 (and high for only a short time), the active-high reset input of the HCF4024 seems suitable
- 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 counter-type implementation impossible, but a helper "assistant CPU" could still help)
- configuring sectrig input=both edges and firmware modification is necessary with the 4-window sync pulsetrain
- by blocking 3 of the 4 windows, (and using a HW divider) firmware mod would not be needed (good for experiments, but this should not be needed finally)
- TODO: actually try the resettable divby4 counter (on bench, and on engine)
- with divby4 (360 => 90 pulses).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)
- according to MembersPage/JannoKusman/NissanTrigger both sectrig edges must be sensed because of the 4 slot syncpulse
Alternative solution would be (without HW divider) a software divide by the main processor. The processor is definitely capable (up to at least 17000 RPM). 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).
From auditrigger bench experiments that worked at 11000 RPM (pulse=2.66 crankdeg), we know that 8300 RPM (pulse=2 crankdeg) is possible without major rewrite. Going above the "all pulse always sensed" threshold could delay ignition by 1 tooth (2 crankdeg) or more (not advance that could blow engine). We speculated that without HW divider going above 9300 RPM could be too much work that would delay other developments unacceptably. So we will only implement fw mod that works well with HW divider.
- we think >9300 RPM is minimum reasonable goal to say that some trigger is supported (even though much lower than the 11k RPM of 135 tooth auditrigger or 25k RPM of lower toothcount wheels)
See also