PC Based EFI--RPM ?

[PHXSYS at aol.com]

Hello

I am trying to post this to the DIY list. I tried sending to Al lipper but
he's not accepting my mail. If this goes through could you respond to me
personally at phxsys at aol.com

And first, thankyou for making your project available to all of us interested
in EFI.

I have been studying the notes and am having a little difficulty understanding
the RPM input strategy. I am new to microcontroller and electronics.

In your explanation

>The output of the ignition pulse conditioner is fed into counter U2A
(74LS393) which >which divides the spark rate by four to reduce errors of
spark miscounting.  The >divided spark pulse (SP) goes to the latching
circuit.  

Is the SP rate divided by 4 or 2? Elsewhere in documentation it states 2 (SP2)

Is the rate divided i.e. if 4 SP counts are detected the output is = to one
SP. 
Does this division effect the pulsewidth or is it an average of the 4 pulse
widths (a+b+c+d)/4?

Your further explanation:

>The purpose of this circuit is to start the timer at the beginning of SP,
allow the >counter to count down while SP is HI and then stop the timer when
SP goes LO.  >Starting and stopping the timer is done by raising and lowering
gate (G0), which is >based on the output of U6A.  Furthermore (this is the
tricky part), the counter must >stay stopped until a software reset 

You stated above that RPM is measured by measuring the time between SP, but it
appears the timer starts when a low-hi pulse is detected and stops on a hi-
low. 

Is this SP your talking about actually the period of two pulses that has been
latched by a flip flop. i.e. the first low-hi SP triggers the flip flop
latching hi and it is the next SP that triggers the signal low hence a
measurement of the period of two SP? When we have the period we can then
determine rpm.

Thanks

Jon Petty
520-747-2020
fax 520-747-2424