C51: BIT ADDRESSABLE ARRAYS

Information in this article applies to:

• C51 Version 5.50

QUESTION

I have declared an array in bdata and I wish to use a loop to access each bit and assign it to a flag. I have written the following code but it doesn't work. What am I doing wrong?

unsigned char bdata barray[3] = {0xAA, 0x55, 0xA5};
bit flag;

void main(void)
{
  unsigned char i;

  for (i = 0; i < 8; i++)
  {
    flag = barray[0] ^ i;
  }

  while(1);
}

ANSWER

You cannot use the '^' character to access bits unless it is an sbit declaration. Outside of an sbit declaration, it is the Exclusive OR operator. Also you cannot use a variable after the '^' character when accessing bits, it must be a constant.

There are two solutions to this problem.

1. Declare an sbit with a unique name for each bit in the array. However, you cannot declare arrays of sbits which makes loops harder.
2. Use a mask. The following code demonstrates a simple and fast solution:

   unsigned char bdata barray[3] = {0xAA, 0x55, 0xA5};
   bit flag;
   
   void main(void)
   {
     unsigned char byte, mask;
   
     for (byte = 0; byte < 4; byte++)
     {
       for (mask = 0x01; mask != 0x00; mask <<= 1)
       {
         flag = barray[byte] & mask;
       }
     }
   
     while(1);
   }
   

MORE INFORMATION

• Refer to Bit-Addressable Objects in the Cx51 User's Guide.
• Refer to bdata in the Cx51 User's Guide.

Last Reviewed: Tuesday, March 27, 2007