Nissan Timing Wheels
Having looked at the Nissan Silvia and 200SX models (S12, S13 and S14) they all use a cam driven timing wheel with a speed sensor of 2degree resolution and portals of different sizes to denote the current cylinder.
Of all the options a trigger board is a better bet - getting the thin timing wheels will prove costly and may mean that the original ECU cannot be used should the user want to revert.
Trigger board code. This is just a rough idea of how we'd count interrupts on the outside degree wheel, and produce a cylinder trigger on port D0, and a cam sync home on port D1. This code won't work (but it compiles okay) as the interrupts are not setup correctly.\n
#include <inttypes.h>
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/signal.h>
#include <avr/wdt.h>
#define CYLINDER_WINDOW_OPEN 0x02
#define CYLINDER_TRIGGER 0x01
#define CAM_HOME_SYNC 0x02
#define SIGNAL_COUNT 0x05
int windowCount;
int camHomeSyncCount;
int cylinderTriggerCount;
void port_b0_ISR()
{
// check the cylinder window
if (PORTB & CYLINDER_WINDOW_OPEN)
{
// cylinder window is open
windowCount++;
}
else
{
// cylinder window is closed, check to see if it was open previously
if (windowCount != 0)
{
// which window is it? 16, 12, 8 or 4?
if (windowCount > 12)
{
// signal cam sync
PORTD |= CAM_HOME_SYNC;
camHomeSyncCount = SIGNAL_COUNT;
}
// signal cylinder
PORTD |= CYLINDER_TRIGGER;
cylinderTriggerCount = SIGNAL_COUNT;
// set window count to zero
windowCount = 0;
}
else
{
// we maintain the cam sync signal for its prescribed count
if (camHomeSyncCount > 0)
{
camHomeSyncCount--;
}
else
{
PORTD &= !CAM_HOME_SYNC;
}
// we maintain the cylinder trigger signal for its prescribed count
if (cylinderTriggerCount > 0)
{
cylinderTriggerCount--;
}
else
{
PORTD &= !CYLINDER_TRIGGER;
}
}
}
}
void system_init()
{
// setup globals
windowCount = 0x00;
camHomeSyncCount = 0x00;
cylinderTriggerCount = 0x00;
// setup ports
DDRB = 0xFC; // Pin 0 and pin 1 to input
PORTB = 0x00;
DDRD = 0xFF; // may as well make all of D output
PORTD = 0x00;
// enable interrupts
wdt_disable();
enable_external_int(0x01);
sei();
}
void main()
{
system_init();
for(;;); // Infinite loop
}