Question

I am writing code for AVR ATmega32-A microcontroller. I am using switch case as shown below.

  unsigned char Command;
  unsigned int  Param;

  void runCom(void){

    switch(Command){

        case(NO_COM):
            Command = 0;
            break;

        case(INF):
            printf("\r\n\r\n");
            printf("university\r\n");
            printf("sweden\r\n");
            printf("Ver. 1.0A\r\n");

            Command = 0;
            break;

        case (DB):                        
            Command = 0;
            break;


        case(CLEARM):
            Command = 0;
            break;

        default:
            Command = 0;
            break;   
    }
}

the above code is working but now i want to add one more case in that switch like as shown below.

 unsigned char Command, Command1;
 unsigned int  Param;
 void runCom(void){

    switch(Command){

        case(NO_COM):
            Command = 0;
            break;

        case(INF):
            printf("\r\n\r\n");
            printf("university\r\n");
            printf("sweden\r\n");
            printf("Ver. 1.0A\r\n");

            Command = 0;
            break;

        case (DB):                        
            Command = 0;
            break;



         case(ADC):
             printf("ADC Value",ReadAd()); 

             printf("Enter Amplification stage");

                       switch(Command1){

                              case(stage1):
                                    PORTC=0x00;
                                    DDRC=0xC0; 
                                    printf("ADC Value",ReadAd());
                                    Command1 = 0;  
                                    break;

                                    case(stage2):
                                    PORTC=0x00;
                                    DDRC=0x03; 
                                    printf("ADC Value",ReadAd());
                                    Command1 = 0;  
                                    break;
                                   }    
            Command = 0;
            break; 

        case(MEM):
            Command = 0;
            break;

        case(CLEARM):
            Command = 0;
            break;

        default:
            Command = 0;
            break;   
    }
}

I am getting error like

   undefind symbol 'ADC'
   undefind symbol 'stage1'
   undefind symbol 'stage2'

then i have declared like this

unsigned char Command, ADC, Command1, stage1, stage2;
  unsigned int  Param;

now i am getting error like

constant integral expression required at line case(ADC)
constant integral expression required at line case(stage1)
constant integral expression required at line case(stage2)

so please can any one suggest me how to overcome this error. I haven’t declared WGP, WGF, INF any where but I am not getting “undefined symbol of WGF, WGP, INF. why this error occurring for ADC only. i have fallowed same switch case rules as i used before. any suggestions appreciated.

Thanks in advance.

Answer 1

The values you use in the cases of a switch statement must be compile-time constants. That is, they must have fixed values that are known to to the compiler at compile time.

There are four main ways to do this.

1. You can use a constant literal (a number).

   case 42:

Note that using unnamed values (“magic numbers”) isn’t recommended because it doesn’t document the code in any way, and makes the code fragile (what if you need to change the value to 43 in five different places and forget one?)

1. You can use a preprocessor constant.

   #define COMMAND_CODE 42
   ...

   case COMMAND_CODE:

That’s better because if you need to changet the command code value to 43 you only have to change the #define (which should only occur once, in a header file).

1. You can use an enum.

   typedef enum { COMMAND_ON=42, COMMAND_OFF=2, COMMAND RESET=77 } command_codes:
   ...

   case COMMAND_ON:

1. In C++, if your compiler is reasonably modern, you can use a constant:

   const int magic_number = 42;

   case magic_number:

(Note that the switch value shown in a case statement does NOT need to have parentheses around it.)

From the code you have shown, it seems that you have defined Command, ADC, etc., as variables (rather than as constants). In this case the compiler can’t know what value those variables will have at execution time when it compiles the code. The language specifies that case values must be known at compile time so that the compiler can generate fast code (faster then a whole series of if … else if … else if statements).