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hmmpic
Joined: 09 Mar 2010
Posts: 314
Location: Denmark
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| subtraction signed int8 |
 Posted: Tue Oct 21, 2014 1:14 pm |
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This is only a test program.
I have tested it in MPLAB V8.92. with the simulator.
It is compiled in CCS 5.027! and tested in real hardware (not the posted code)
Only the first OK1 is working all the other fail! But why?
I am Confused.
| Code: | #include <18F26k22.h>
#use delay(clock=8M,int)
#use RS232(Baud=57600,STOP=1,PARITY=N,BITS=8,UART1,ERRORS,stream=sim900)
/*Max dif: ad-adtmp=+-5 adtmp-ad=+-5*/
void GetTemp2(){
int8 ok=0;
int8 ad,adtmp;
int8 cnt;
ad=127;
adtmp=127;
for (cnt=0; cnt<10; cnt++){
ad++; /* Test with ad--, ad++, adtmp--, adtmp++ */
printf("AD:%u ADTmp:%u Cnt:%u\r\n",ad,adtmp,cnt);
ok=0;
if ( (((signed int8)ad-adtmp)<5) && (((signed int8)ad-adtmp)>-5) ) {ok=1;}
printf("OK1:%u\r\n",ok);
ok=0;
if ( ((-adtmp+ad)<5) && ((-adtmp+ad)>-5) ) {ok=1;}
printf("OK2:%u\r\n",ok);
ok=0;
if ( ((ad+(-adtmp))<5) && ((ad+(-adtmp))>-5) ) {ok=1;}
printf("OK3:%u\r\n",ok);
ok=0;
if ( ((ad+(-adtmp))<5) && ((adtmp+(-ad))<5) ) {ok=1;}
printf("OK4:%u\r\n",ok);
printf("******\r\n");
}
}
void main() {
GetTemp2();
} |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19552
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| Posted: Tue Oct 21, 2014 1:23 pm |
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A signed int8, can hold values from -128 to +127.
An unsigned int8, uses the same storage to hold 0 to 255.
You start at 127.
Then increment - now have 128. When this is converted to a signed int8, you get -1.
Then increment again - now have 129. When this is converted to a signed int8, you have -2.
etc. etc.
You are overflowing the signed int8 arithmetic, just about everywhere.... |
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hmmpic
Joined: 09 Mar 2010
Posts: 314
Location: Denmark
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| Posted: Tue Oct 21, 2014 2:06 pm |
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Now I ended up with this working on:
| Code: | | if (ad>adtmp) { ok=(ad-adtmp)<5; } else { ok=(adtmp-ad)<5; } |
Thanks for hints on the overflow:-) |
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