Multiplication in CCS - v.4.093

[deperkin]

Hi all,

I was wondering what is the more accurate way to do a multiplication using CCS...

I am using a PIC18F4550.
I have a DC motor with a tachometer that gives me 3 logical ticks per revolution.

Thus, using an interrupt, i take a count every second, and want to display revolutions:

RPM = (count/3)*60.


(i am using an INT16 for RPM & count, so do i need to cast the other numbers into INT16's?)

Is using this equation adequate enough?

Thanks as always.

[collink]

[deperkin]

Thanks you,

that helps alot.

after my post I did change it to multiply by 20, but will try to shift instead, since I dont believe I am getting an accurate number displayed.

Thanks again.

[collink]

[deperkin]

I am forced to use an external interrupt because of the other pins being used.

I have:

INT_EXT2
void EXT2_isr(){
disable_interrupts(INT_EXT2); //disable interrupt
cnt++; //recieved input from tachometer
ds1307_get_time(hr,min,sec);
if (sec == (s + 1)){ //modify filter time every second
s = seconds;
enable_interrupts(INT_EXT2);
RPM =(cnt << 4)+(cnt << 2); //muliply by 20 to get RPM
cnt = 0;
}
}

in the above, the ds1307_get_time provides me with the hours, minutes, and seconds from an external RTC... i know that this part works, since I have been using it consistently in another interrupt. (in main I initially set s = seconds)
So I check on every interrupt if one second has gone by, and onces it has, I take my count and multiply by 20.

I should be getting about 2500 rpm, but only get 120 rpm.
I can see on an oscilloscope however that my tachometer is working properly... I must not be getting the interrupts correctly however.

Best regards,

[collink]

[Ttelmah]

Seriously, you have not got time to be reading the DS1307, inside the interrupt.
Forget this.
Use one of the PIC timers, to generate a 'tick'. Have this decrement a counter if the counter value is non zero (tick_counter)
Then set the value to the number of counts needed for one second (TICKS_PER_SECOND).
Your external interrupt code should be _short_ something like:

[deperkin]

Thank you!

This will be very helpful indeed.