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Guest Guest
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portb interrupt |
Posted: Sun Aug 28, 2005 7:51 pm |
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in the example EX_PBUTT.C, what is the purpose of the statement "last=current' in the detect_rb_change function? last is always set to 0 whenever an interrupt happens, is that right?
Code: | void detect_rb_change() {
int current;
static int last=0;
set_tris_b(0xF0);
current=input_b();
#if LOWTOHIGH
if ((!bit_test(last,4))&&(bit_test(current,4))) {dbutton4=1;}
if ((!bit_test(last,5))&&(bit_test(current,5))) {dbutton5=1;}
if ((!bit_test(last,6))&&(bit_test(current,6))) {dbutton6=1;}
if ((!bit_test(last,7))&&(bit_test(current,7))) {dbutton7=1;}
#elif HIGHTOLOW
if ((!bit_test(current,4))&&(bit_test(last,4))) {dbutton4=1;}
if ((!bit_test(current,5))&&(bit_test(last,5))) {dbutton5=1;}
if ((!bit_test(current,6))&&(bit_test(last,6))) {dbutton6=1;}
if ((!bit_test(current,7))&&(bit_test(last,7))) {dbutton7=1;}
#endif
last=current;
} |
regards |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Sun Aug 28, 2005 7:57 pm |
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Quote: | last is always set to 0 whenever an interrupt happens, is that right? |
It's only set to zero initially. |
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Guest Guest
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Posted: Sun Aug 28, 2005 8:18 pm |
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isn't this function executes at each interrupt so that "last" will always be 0 as it is initialized at each interrupt? |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Sun Aug 28, 2005 11:00 pm |
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It's not initialized at each interrupt. It's only initialized when the
program first starts running.
I'll show you how you can check this. First make a small test program:
Code: |
#include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 4000000)
#int_rb
void int_rb_isr(void)
{
int current;
static int last = 0;
last = current;
}
//=====================
void main()
{
while(1);
}
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Then compile it and look at the .LST file.
Here's the symbol table:
Code: |
int_rb_isr.last 00000028
int_rb_isr.current 00000029
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Below, notice that at address 0042, 'last' is cleared (set = 0).
But notice that address 0042 is part of the main() code.
If you follow the addresses in main(), they start at 0039
and then they go down to 0040, and there's a gap.
CCS puts the code for the isr inside the isr to make it
easy to read the .LST file, but the "last = 0" code isn't
executed inside the isr. It's just done once, in main(),
when the program starts.
Code: |
0000 ... #int_rb
0000 ... void int_rb_isr(void)
0000 ... {
0000 ... int current;
0000 ... static int last = 0;
0041 1283 BCF 03.5
0042 01A8 CLRF 28
0000 ...
0000 ...
0000 ... last = current;
0033 0829 MOVF 29,W
0034 00A8 MOVWF 28
0000 ... }
0000 ...
0000 ...//=========================
0035 100B BCF 0B.0
0036 118A BCF 0A.3
0037 120A BCF 0A.4
0038 281D GOTO 01D
0000 ... void main()
0000 ... {
0039 0184 CLRF 04
003A 301F MOVLW 1F
003B 0583 ANDWF 03,F
003C 1683 BSF 03.5
003D 141F BSF 1F.0
003E 149F BSF 1F.1
003F 151F BSF 1F.2
0040 119F BCF 1F.3
0000 ... while(1);
0043 2843 GOTO 043
0000 ... }
0044 0063 SLEEP |
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asmallri
Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia
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Posted: Mon Aug 29, 2005 3:34 am |
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Guest wrote: | isn't this function executes at each interrupt so that "last" will always be 0 as it is initialized at each interrupt? |
No. The variable is declared as static and therefore retains its value between interrupts. It is initially set to zero on program startup. _________________ Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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