"Or"ing DDRB with itself

[cosmonavt]

According to a video tutorial I saw somewhere, here's the code for a blinking LED:

#include <avr/io.h>
#include <util/delay.h>

int main(void)
{
	DDRB = DDRB | 0b00000001;
	
    while(1)
    {
		PORTB = 0b00000000;
		_delay_ms(100);
		PORTB = 0b00000001;
		_delay_ms(100);
   }
}

Why do we have to say

DDRB = DDRB | 0b00000001;

in the beginning of the int function?

Does this mean that during the program, DDRB can accept values of either 0b00000000 or 0b00000001? Is DDRB set to 0b00000000 by default?

 

[alec_t]

I'm not familiar with AVR programming, so what follows may be rubbish:-
I assume the LED is attached to PORTB bit 0.
1) that line seems to be just ensuring that bit 0 of register DDRB (data direction?) is set to 1 (output?).
2) the while loop seems to keep PORTB bit 0 at 1 and all the other bits at 0. How's that supposed to blink a LED?

Is DDRB set to 0b00000000 by default?

The datasheet for the AVR should tell you.

 

[cosmonavt]

Oh sorry there was an error in code, corrected it. It should be as follows:

		PORTB = 0b00000000;
		_delay_ms(100);
		PORTB = 0b00000001;
		_delay_ms(100);

 

[cosmonavt]

Well??

Why do we Or DDRB by itself?

 

[Pommie]

You're ORing with 1, not with itself. ORing with 1 sets the bottom bit and makes it output.

Mike.

 

[cosmonavt]

Why OR it anyway? Why not simply make it DDRB = 0b00000001?

 

[3v0]

Why OR it anyway? Why not simply make it DDRB = 0b00000001?

You could if you knew that 0b00000001 was what you wanted. ORing preserves the direction of other bits. For example if DDRB was 0b00000100 ORing it with 0b00000001 gives 0b00000101.

 

[cosmonavt]

Thanks. It means that it will ease out the process of preserving other bits. Like if we want the pin 2 to change to output, we can simply type 0b00000100 instead of repeating the direction of pin 0 again and again, i.e. 0b00000101

Am I right?

 

[rushi53]

Thanks. It means that it will ease out the process of preserving other bits. Like if we want the pin 2 to change to output, we can simply type 0b00000100 instead of repeating the direction of pin 0 again and again, i.e. 0b00000101

Am I right?

thats right....

I use some other method... I use,
sbi(DDRB,2)
or
cbi(DDRA,0)

 

[misterT]

thats right....

I use some other method... I use,
sbi(DDRB,2)

That is the same method because:

#define sbi(port, bit) ((port) |= (1 << (bit)))

 

[Pommie]

thats right....

I use some other method... I use,
sbi(DDRB,2)
or
cbi(DDRA,0)

That demonstrates that you don't understand the way that code will be compiled.

Mike.