The tachometer (counting RPM) is a frequency counter. You can figure out the frequency from the number of cyclinders, and the fact that it is a 4 cycle engine. (Left as an exercise for the student). There are MANY frequency counter projects on the internet - Google "pic frequency counter" and then duck. Most of them use the 16F84. It all started with a Microchip app note on the subject, which is where to start. The basic idea is to count the input pulses for a fixed period of time (for example, 1 second) They all use tricks to be able to count to higher frequencies than the basic 8 bit timer of the 16F84 allows. My current project is a general purpose frequency counter using an 18F252 - the newer chips make life a lot easier. But the original F84 design is just fine for what you want, since you are only dealing with a few tens of Hertz. However, at low frequencies, such as what you are dealing with for a tach (e.g 3600 rpm = 60 revs/sec), you won't get very good resolution by measuring pulses for 1 second. 10 seconds would be better, but then the responsiveness to change is lousy. A better method is to measure the period of the signal. If you have a good clean signal, you can measure the time between two leading edges of the signal, then frequency = 1/period. To measure the period, you wait in a loop until the signal goes high. Start the timer running. Wait in a loop until the signal goes low. Wait in a loop until the signal goes back high again, then stop the timer. The timer value is now the period in "timer tics" which you have to convert into real wordl units (seconds, milliseconds, whatever) There are many ways to skin this particular cat. Have fun! Larry Original Message: > considering rpm ignition signal to be a square wave (in the case of a toyota ignition system at least), could someone please guide me as to how I could count the rpm when properly interfaced to a 16F84A. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist