RPM - DIV32 Problem

[davewanna]

I am reading a tacho out line from an engine ECU with pulsin, scaling it, then displaying it on an LCD screen. I have had it working with the following code...

  PULSIN PORTD.6,1,rlow                             
  PULSIN PORTD.6,0,rhigh                            
  rpm = rhigh + rlow
  rpm = rpm / 50
  rpm = 60000 / rpm
  if rpm = 65535 then
    rpm = 0
  endif
...
  lcdout $fe,$ca,dec rpm,"rpm  "

in the higher RPM, the resolution drops, so I figured if I was to change my calculations to be ... rpm = 3000000 / rpm ... then the resolution would increase.

However when I make it ...

  PULSIN PORTD.6,1,rlow                             
  PULSIN PORTD.6,0,rhigh                            
  rpm = rhigh + rlow 
  tbase = 50 * 60000
  rpm = div32 rpm
  if rpm = 65535 then
    rpm = 0
  endif

The result is roughly 32370rpm
when the revs are increasing, the displayed rpm decreases, and visa-versa... but when the rpm is steady (at idle, or held steady at any RPM) the displayed rpm settles back to 32370

Is there something I am missing here? I am successfully using DIV32 many more times in my code for other functions.

Also I thought I might be overflowing the DIV32 integer (but according to my manual I am not) so I thought I would try....

  PULSIN PORTD.6,1,rlow                             
  PULSIN PORTD.6,0,rhigh                            
  rpm = (rhigh + rlow) / 10
  tbase = 500 * 600
  rpm = div32 rpm
  if rpm = 65535 then
    rpm = 0
  endif

This results in 0 being displayed all the time.

At idle, rlow, and rhigh (which are identical) read approximately 1400 each, which when I do my maths on paper, (either method I have shown) comes out to approximately 1071rpm which would be correct




Could somebody more intelligent or more observant than I please help me?


Also might add I'm using a 16F877A