EE 308 Setting and Clearing Bits

EE 308

Setting and Clearing Bits
using Assembly and C

To put a specific number into a memory location or register (e.g., to put 0x55 into PORTA):
```
movb   #$55,PORTA             PORTA = 0x55;
```
To set a particular bit of a register (e.g., set Bit 3 of PORTA) while leaving the other bits unchanged do a bitwise OR of the register and a mask which has a 1 in the bit(s) you want to set, and a 0 in the other bits:
```
bset    PORTA,#$10           PORTA = PORTA | 0x10;
```
To clear a particular bit of a register (e.g., clear Bit 4 of PORTA) while leaving the other bits unchanged do a bitwise AND of the register and a mask which has a 0 in the bit(s) you want to clear, and a 1 in the other bits. You can construct this mask by complementing a mask which has a 1 in the bit(s) you want to set, and a 0 in the other bits:
```
bclr    PORTA,#$20           PORTA = PORTA & 0xDF;

                             PORTA = PORTA & ~0x20;
```
Write to all bits of a register when you know what all bits should be, such as when you initialize it. Set or clear bits when you want to change only one or a few bits and leave the others unchanged.
To clear a bit in a timer flag register (TFLG1 and TFLG2) write a 1 to the bit(s) you want to clear and a zero to all other bits. Do not do a bitwise OR of the register and a mask. To clear the Timer Overflow flag (Bit 7 of TFLG2):
```
movb    #$80,TFLG2           TFGL2 = 0x80;
```

Bill Rison
2000-02-14