PBPro Maths Help

[retepsnikrep]

Thanks for that Charles i probably should have added i am using a 16F88 pic running at 8mhz, TMR1 (16bit) is already in use for another interrupt driven routine. Darrell's SSPWM routine.

[Charles Linquis]

It would still work with that part (I think). I normally run this on a TMR0 interrupt (in an 18F part). You could make the speed interrupt a low priority interrupt, and let the SSPWM be high priority. That way, the speed interrupt wouldn't screw up your PWM.

You could get by with 8 bit counters (ex fan1counter in my example) in your application. I used a 16 bit counter because I often need to measure the speed of
15,000 RPM fans.

[retepsnikrep]

I will try that later Charles.
I was hoping for some help using DIV32 and that sort of thing to reduce errors in my 16 bit maths

[retepsnikrep]

Still hoping for some guidance on the div32 type maths to improve the accuracy of my current 16 bit maths

[Acetronics2]

Hi, Retep ...

first you have to declare a dummy var ... and a Div flag

then ...

 
 
IF Local > 32767 then    ' see PBP manual ... for the Why ???
   Local = Local/2
   Div = 1
Else
   Div = 0
ENDIF
 
Dummy = 11
Dummy = Dummy*7908 ; operation needed by PBP ...
ActualSpeed = DIV32  Local
 
IF Div THEN ActualSpeed = Actualspeed / 2    ' Ooops ! not *2 ...
 
ActualSpeed = ActualSpeed + 1

[retepsnikrep]

Thanks for the help so far, here is a different formula though.
How to convert that to use max resolution and div 32 command etc.

3600sph / (count * 0.000005 * 4550)

Count will be a maximum of 10,000

[retepsnikrep]

Does this code look correct to implement the formula?

'Formula Speed = 3600 / (Local * 0.000005 * 4550)

Dummy = Local * 5
Local = 10000
Dummy = Dummy * 4550
Dummy = DIV32 Local
Count1 = 36000
Local = Count1 * 10000
Speed = div32 Dummy
Speed = Speed / 1000