Read multiple RPM fans through shift-register

Using Arduino Programming Questions
1

I am reading a 12V PC fan's RPM, the signal is connected to an arduino pin and to +5v via a 10K pullup resistor. this is the code i am using:

const int rpmPin = 2;

void setup() {
    Serial.begin(9600);
    pinMode(rpmPin, INPUT);
}

void loop() {
    unsigned long interval = pulseIn(rpmPin, HIGH) + pulseIn(rpmPin, LOW);
    int RPS = 1000000UL / interval; // Pulses per second
    int RPM = RPS*60;
    Serial.println(RPM);
}

ok this is working ok, but now i need to read the rpm from 4 different fans, through a parallel in-serial out shift-register CD4021B and i have no idea where to start... any help will be appreciated.

2

I do not think a shift register is appropriate. Read the general use of such logic here:

You may find that an interrupt driven implementation would be best, you only have 4 inputs.

Ray

3

Are you really that desperate for input pins? The implementation would be far simpler without the shift register and cheaper too.

4

mrburnette:
I do not think a shift register is appropriate. Read the general use of such logic here:
http://www.arduino.cc/en/Tutorial/ShiftIn

You may find that an interrupt driven implementation would be best, you only have 4 inputs.

Ray

well, in fact i will be connecting some other things aside from the fans... i need to control also 4 water pumps, read 4 flow sensors (they work the same as the fans, so i need to read the pulses too), and some other things. (some buttons, sensors etc).

i have tried reading RPM of just one fan using interrupts, but the problem with this is (as far as i know) i have to use delay() or millis() and this blocks program execution... and also, i have no idea if i can use interrupts through a shift register...

KenTF:
Are you really that desperate for input pins? The implementation would be far simpler without the shift register and cheaper too.

yes I am... my project involves many other devices too... i was hoping i would be able to connect everything with daisy-chained shift registers (one chain for inputs with CD4021B registers, one chain for outputs with 74HC595 registers)