Set a one bit in an 8-bit PIN without changing the other bits

Question

Using winAVR for the following code.

I got an 8-bit PIN here that I need to OR it with 00000010 or 0x02 to set the 7th bit.

Now what I have to do is the following :

• Set Port B bit 7
• Wait for 1ms
• Clear port B bit 7
• wait 19ms

BUT, I shouldn't change the other bits in these steps.

So I have to :

• Read port B
• Set bit needed
• write the modified value back to the port
• Clear bits
• Write back to Port B

So my test code is :

B=PINB|0x02
Loop delay for 1ms
BP=PINB&0x00
Loop for 19ms

But I think that the other bits are going to be altered in this process, my question is, HOW am I supposed to manipulate one bit of an 8 bit port without changing the other bits ?

Thanks alot !!

Answer 1

You need BP=PINB & ~0x02 The ~ operator is logical NOT. The and operator keeps only the bits other than 2.

Answer 2

You use the bitwise negation of the setting mask, and AND that:

B = PINB & ~0x02

For the selected bit, the bitwise negation sets that bit to zero; all the others are one. The ones do not change the value in PINB when ANDed.

Answer 3

This page has a good summary of several tricks with bitwise operators. http://www.catonmat.net/blog/low-level-bit-hacks-you-absolutely-must-know