Clearing and Setting Bits ATMega16

Setting and clearing a single bit, without changing any other bits, is a common task in AVR microcontroller programming. You will use these techniques over and over again.
When manipulating a single bit, it is often necessary to have a byte value in which only the bit of interest is set. This byte can then be used with bitwise operations to manipulate that one bit. Let's call this a bit value mask. For example, the bit value mask for bit 2 would be 00000100 and the bit value mask for bit 6 would be 01000000.
Since the number 1 is represented in binary with only bit 0 set, you can get the bit value mask for a given bit by left shifting 1 by the bit number of interest. For example, to get the bit value mask for bit 2, left shift 1 by 2.

To set a bit in C, OR the value with the bit value mask.

uint8_t a = 0x08; /* 00001000 */
                  /* set bit 2 */
a |= (1<<2);      /* 00001100 */

Use multiple OR operators to set multiple bits.

uint8_t a = 0x08;   /* 00001000 */
                    /* set bits 1 and 2 */
a |= (1<<2)|(1<<1); /* 00001110 */

To clear a bit in C, NOT the bit value mask so that the bit of interest is the only bit cleared, and then AND that with the value.

uint8_t a = 0x0F; /* 00001111 */
                  /* clear bit 2 */
a &= ~(1<<2);     /* 00001011 */

Use multiple OR operators to clear multiple bits.

uint8_t a = 0x0F;      /* 00001111 */
                       /* clear bit 1 and 2 */
a &= ~((1<<2)|(1<<1)); /* 00001001 */

To toggle a bit in C, XOR the value with the bit value mask.

uint8_t a = 0x0F; /* 00001111 */
                  /* toggle bit 2 */
a ^= (1<<2);      /* 00001011 */
a ^= (1<<2);      /* 00001111 */

Source http://www.micahcarrick.com/tutorials/avr-microcontroller-tutorial/avr-c-programming.html