[Wiltshire] Random C coding question...

David Fletcher dave at thefletchers.net
Tue Apr 22 14:09:53 BST 2008

At 13:50 22/04/2008, you wrote:
>Hi all,
>Fairly random question, kind of related to linux, I'm hoping someone on
>here might be able to answer as it is C-related (and lots of linux things
>are C) and I can't think how to google for the answer or where else to
>ask. (putting "1<<" into google isn't helpful!)
>Can someone explain to me the significance of 1<<PB1 in the following
>      // gerneral purpose LED
>      /* set output to Vcc, red LED off */
>      #define LED1OFF PORTB|=(1<<PB1)
>      /* set output to GND, red LED on */
>      #define LED1ON PORTB&=~(1<<PB1)
>What is it doing?  Why "1" and what does << do / mean / what is going on.
>A bit of background may be required I guess, I'm not a C programmer, but I
>do recall from a C++ book I once read things like 'cout << "some text"',
>so I'm guessing it's some kind of redirect or pipe type thing.
>The code extract is from some AVR code I'm playing with and I know what

Thought it looked like embedded code.

See page 49 of "The C Programming Language" second edition by Brian W 
Kernighan and Dennis M Ritchie.

There you will see that << is the leftwards bit shifting operator, so 
if PB1 has value 0 the result is 1. If PB1 has the value 3 the result 
will be 8 i.e. bit 3 set. PORTB| performs a logical OR of the value 
of portB with the bit shift result to switch off the LED. As I 
recall, the AVR pins can sink current nicely but not source it, hence 
the LED will be connected via a resistor to Vcc, and setting the port 
high will therefore switch off the LED.

PORTB& performs a logical AND with the inverted bit shifted result. 
The ~ inverts all the bits of the shift operation. So the selected 
bit of the port is set low, switching on the LED.

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