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mutthunaveen
Joined: 08 Apr 2009
Posts: 100
Location: Chennai, India
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| displaying float value |
 Posted: Wed Aug 12, 2009 8:26 am |
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Dear board, this is my code: When I run this I should get the value of 0.25 but when I compile this code in the PIC simulator IDE I get 0.00 in the USART screen.
My ccs compiler version is V4.023.
I tried like this " s=2.25" and printf. then it is displaying correctly as 2.25.
| Code: | #include <16F877A.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=14000000)
#include <math.h>
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)
void main(){
int i;
float s;
long mylong;
char mystring[4];
int line1[] = " ADC ";
int line2[] = "light intensity ";
while(1){
s=25/100;
printf("\n the float value is : %f", s);
}
} |
Did I make any mistake here ? Please help me actually I am working for PWM LED dimming.
I want to display % value in the LCD screen.
The formula for this is ((ADCdata/255)*100) gives 0-100% LED brightness.
How to program to work this MATH data? |
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Ttelmah
Guest
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| Posted: Wed Aug 12, 2009 9:01 am |
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The problem here, is numeric types.
'25' is an integer.
'100' is an integer.
25/100, using integer maths, gives '0'.
In C (not just CCS), the 'type' used for an operation, is the highest 'type' of the values involved. So, if you use:
25.0/100
25/100.0
or
25(float)/100
25/100(float)
you will get the right result. In each case, you are making one of the values into a 'float', so float arithmetic is used.
While this also applies in 'C' in general, CCS is 'stricter', than is normally the case. This is deliberate, to avoid wasted time (float arithmetic is much slower than integer), but on most modern compilers, there is implicit propogation of things like a constant evaluation as shown, to float by default...
Best Wishes |
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mutthunaveen
Joined: 08 Apr 2009
Posts: 100
Location: Chennai, India
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| thanks |
| Posted: Wed Aug 12, 2009 10:31 pm |
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thank u Ttelmah
u r right..... 
I defined all the variables to float then the float division is working.
| Code: |
float a,b=2.25,c=3.33;
a=b/c;
printf("\n the a = %f", a);
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This is working fine...... but the system working is slow.
Any idea to boost the time taken to calculate.
If you have time clarify this too..... what I have to do for the coding to get the float value like this (22.023621) after dot more than 2 values.  |
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SherpaDoug
Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA
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| Posted: Thu Aug 13, 2009 7:27 am |
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Look up the options for the printf() format string. That is where you determine how many digits to print.
Better yet, if you want speed, avoid floats entirely. If you need an answer to 1.234 digits then calculate integer 1/1000's ( i.e. 1234/1000) then use string functions to insert a decimal point where needed. A few years ago I did a system to measure ships. All the PIC math was done in 24 bit integer millimeters. At the very end I inserted a decimal to send lengths in meters to a PC.
_________________
The search for better is endless. Instead simply find very good and get the job done. |
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Ttelmah
Guest
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| Posted: Thu Aug 13, 2009 7:46 am |
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Or, assuming a reasonably recent compiler, look at %w in the printf.
So (for example):
| Code: |
#include <16F877A.h>
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use delay(clock=14000000)
#include <math.h>
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)
void main(){
int i;
int32 s;
while(1){
s=2500/100;
printf("\n the float value is : %5.2w", s);
}
}
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Float is slow.
However, it'll take about 30mSec, to print the output, and only about 5mSec to do the calculation involved.....
The limiting factor, in what is posted, will be the serial, not the calculation.
Best Wishes |
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