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volatile modifier for bit?

I have a flag bit variable that is set by an interrupt routine. Then in my main loop the flag is checked like:

 adc0_flag = 0;
 while(1){
    if(adc0_flag == 1){
       adc0_flag = 0;
       process_data()
    }
 ...

 }

I tried declaring adc0_flag as:

volatile bit adc0_flag;

But this did not fix the problem that call to process_data() was optimized out [using no optimization--ie opt(0)]

Changing it to:
volatile unsigned char adc0_flag;

fixes the problem--but wastes memory. I have several such bit flags being modified by interrupts. Is there an alternative?

Is it true that volatile does nothing for bit variables or am I doing something wrong?

Parents

0 Jon Ward over 20 years ago

I tried a simplified example that would compile and got the following:

line level    source

   1          bit my_bit;
   2
   3
   4          void main (void)
   5          {
   6   1      my_bit = 0;
   7   1
   8   1      while(1)
   9   1        {
  10   2        if (my_bit == 1)
  11   2          {
  12   3          my_bit = 0;
  13   3          }
  14   2        }
  15   1      }
  16

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
0000 C200        R     CLR     my_bit
0002         ?C0001:
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
                                           ; SOURCE LINE # 10
0002 3000FD      R     JNB     my_bit,?C0001
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 12
0005 C200        R     CLR     my_bit
                                           ; SOURCE LINE # 13
0007         ?C0003:
                                           ; SOURCE LINE # 14
0007 80F9              SJMP    ?C0001
0009         ?C0002:
                                           ; SOURCE LINE # 15
0009         ?C0004:
0009 22                RET
             ; FUNCTION main (END)

This look AOK to me. Can you perhaps show us the exact C source code and the generated assembly code so we can see what you mean?

Jon

Reply

0 Jon Ward over 20 years ago

I tried a simplified example that would compile and got the following:

line level    source

   1          bit my_bit;
   2
   3
   4          void main (void)
   5          {
   6   1      my_bit = 0;
   7   1
   8   1      while(1)
   9   1        {
  10   2        if (my_bit == 1)
  11   2          {
  12   3          my_bit = 0;
  13   3          }
  14   2        }
  15   1      }
  16

ASSEMBLY LISTING OF GENERATED OBJECT CODE


             ; FUNCTION main (BEGIN)
                                           ; SOURCE LINE # 4
                                           ; SOURCE LINE # 5
                                           ; SOURCE LINE # 6
0000 C200        R     CLR     my_bit
0002         ?C0001:
                                           ; SOURCE LINE # 8
                                           ; SOURCE LINE # 9
                                           ; SOURCE LINE # 10
0002 3000FD      R     JNB     my_bit,?C0001
                                           ; SOURCE LINE # 11
                                           ; SOURCE LINE # 12
0005 C200        R     CLR     my_bit
                                           ; SOURCE LINE # 13
0007         ?C0003:
                                           ; SOURCE LINE # 14
0007 80F9              SJMP    ?C0001
0009         ?C0002:
                                           ; SOURCE LINE # 15
0009         ?C0004:
0009 22                RET
             ; FUNCTION main (END)

This look AOK to me. Can you perhaps show us the exact C source code and the generated assembly code so we can see what you mean?

Jon

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