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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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 Posted: Mon Jun 24, 2013 4:20 pm |
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| temtronic wrote: | | You do the math of (288.15/65) and use the result, saving PIC doing it |
Actually the compiler does pre-calculate expressions like that at compile-time.
This means it's calculated on your Windows PC and not by the PIC micro.
How to prove this ? Make a test program. Using Windows calculator
(or your casio calculator) get the result of 288.15/65, which is 4.4330769.
Then download this zip and drag the floatconv.exe onto your Windows
desktop:
http://www.piclist.com/images/floatconv10.zip
Start up floatconv, select Microchip 32 bit and type in 4.4330769 in the top
box and press Convert to Byte. That gives: 81 0D DB C4
Then compile the program shown below (with vs. 4.141) and look through
the .LST file, looking for those bytes to be loaded as literals. Bingo:
| Code: |
00DB0: MOVFF 0C,FEA
00DB4: MOVFF 0B,FE9
00DB8: MOVLW C4 <===
00DBA: MOVWF 2B
00DBC: MOVLW DB <===
00DBE: MOVWF 2A
00DC0: MOVLW 0D <===
00DC2: MOVWF 29
00DC4: MOVLW 81 <===
00DC6: MOVWF 28
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The result of 4.4330769 is inserted into the ASM code. The PIC itself
does not calculate it. It's done by CCS at compile-time.
| Code: |
#include <18F4620.h>
#fuses INTRC_IO,NOWDT
#use delay(clock=4M)
#use rs232(baud=9600, UART1, ERRORS)
#include <math.h>
//======================================
void main(void)
{
const DOUBLE R = 287.052; // specific gas constant R*/M0
const DOUBLE g = 9.80665; // standard gravity
const DOUBLE t_grad = 0.0065; // gradient of temperature
const DOUBLE t0 = 273.15 + 15; // temperature at 0 altitude
const DOUBLE p0 = 1013.25; // pressure at 0 altitude
signed int16 P = 9200;
DOUBLE alti;
alti =(288.15/65)*(1 - exp(t_grad * (R / g)) * log(P / p0));
while(1); |
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RF_Developer
Joined: 07 Feb 2011
Posts: 839
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| Posted: Tue Jun 25, 2013 8:45 am |
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That behaviour - of the compiler pre-calculating results of expressions which only involve constants - into an equivalent constant is often called "constant collapse". Not all compilers do it, CCS C does, which is nice.
There are limits to what can be "collapsed" in to equivalent constants, for example math expressions that call C functions, e.g. log(10) is a constant, i.e. about 2.30258, but can't be collapsed by many compilers; CCS C certainly doesn't. That's a pity, because it would sometimes be useful, and such functions take a lot of PIC power. Its partly, at least, a result of such maths being implemented by (library) functions in C, rather than as operators built into the language. |
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