PWM from PIC

[Guest analouge]

Hey,

I'm trying to design a PWM cuircut from the PIC18F452, and got some questions I hope you guys could be kind and help me with.

I got a fan from my car heater, and I've designed a MOSFET motor driver with a EL7212CN with others to drive the MOSFET. Got my tips from picotech.com. excelent tips in there!

But I ran into some stuff I couldn't resolve on my own. I know I gotta drive the motor at high frequencies so that it "thinks" it runs on a almost clean DC power supply to make it work optimal.... At the picotech.com site the tutorials said that the frequency should be about 5 times the RPSecond of the motor (6000rpm = 100rps). And here is my problem,

I have NO idea of what rpm the motor is running. And I know very little about motors generally. I know how to find out the current it draws, and how to find the resistance of the motor - I think... It could be done with a simple multimeter.... right? So when I measure the motor all I gotta do is to find a MOSFET that works and adjust the motor powersupply thereafter...

But is there a way of calculating the rpm of the motor?

I guess I could rig the fan with some kind of optical tachometer cuircut and read the rpm of a display or osciloscope or whatever, but I haven't got a osciloscope, and I haven't got the knowledge (YET) how to build a tachometer cuircut, so if there is a way of calculating the rpm without rigging a tachometer onto the motor I would appreciate if someone that knows how could tell me :)

Also, I wondered what the limitations of the PIC chip are. Especially how many motors I can run from a PIC. I tried to find the datasheet for the 18F452, but instead I ended up with the 18F4520 datasheet, and I didn't understand much of it, I read it fast though, so maybe I should get the right datasheet and read it properly :P. But if someone knows I would appreciate if you just told me ;). Let's say my motor is "PWM channel 1". - is there a "PWM channel 2" etc. so that I can run more motors at different pulse width at the same time from the PIC? I need it to run some servos too, so it really makes no sense to build a lot of pwm cuircuts if I could just run them all from the pic, only with each own mosfet and motor driver cuircuts - know what I mean?

I really appreciate any answers - someone in here told me to look into PWM for my projects, and thanks to him/her I'm totally hooked on digital electronics :) It's GOOD! :D see you guys!

THANKS!

Ingebret