[Wiltshire] Random C coding question...

Tue Apr 22 18:13:55 BST 2008

Hi,

Thanks for the explaination and the links, ardunio looks pretty neat - 
I'll have to have a look into that - I like the idea of the bluetooth 
one...

I've actually been making an ethernet weather station [1] which I have 
working nicely but now I'm trying to extend it because I've bought a 
couple of his temp/humidity sensors [2] so I'm trying to get those to 
work on the original weather station hardware as well as the analog 
sensors it has.

I've think I've got the hardware sorted (apart from some soldering and 
tidying up - it's all on a bread-board at the minute).  So now I'm just 
updating the software to read both the sht11 sensors and the original 2x 
temp sensors and pressure on the board, plus drive the LCD and serve up 
a couple of web pages one for humans to read and one xml based one for a 
PC to grab and store.

Its been pretty interesting as I started knowing nothing about AVR / 
embedded stuff and now I feel I know a little!

Robert.

[1]http://tuxgraphics.org/electronics/200704/article07041.shtml
[2]http://tuxgraphics.org/electronics/200709/avr-webserver-sensirion-humidity-temperature.shtml

Lashley, Malcolm wrote:
> Hey Robert,
> 
> First - you might checkout www.arduino.cc if you're new to AVR coding -
> their open source hardware/software is a good learning ground for this
> stuff. (And hides some of the bare-metal concepts from the beginner,
> whilst allowing them full hardware access should they choose to.)
> 
> Secondly - you guessed right with the bitmask.
> 
> Lemme now breakdown the code bit-by-bit (pun intended) you posted for
> the benefit of anyone reading the archive.
> 
> #define LED1OFF PORTB|=(1<<PB1)
> 
> The right hand side of the above:  PORTB|=(1<<PB1) 
> Would expand to: PORTB = PORTB | (1<<PB1)
> | is bitwise OR, << is bitwise shift left.
> PORTB is an 8-bit register controlling 8 digital inputs/outputs
> (configurable)
> 
> Assume your LED is connected to an output pin corresponding to the least
> significant bit (LSB) of PORTB (thus PB1=0)
> 
> 1<<0  == 0000 0001b (b indicates binary representation)
> Similarly (if PB1 were 1 (which I guess it is in your case))
> 1<<1  == 0000 0010b
> 
> So - starting with all outputs off (I'm making the assumption they are
> all outputs)
> 
> PORTB= 0000 0000b
> 
> OR that value with 1<<1 
> 
> Gives
> 
> 0000 0010b
> 
> And your LED turns on. 
> 
> Repeat for PB5 and you would have
> 
> PORTB = 0010 0010b
> 
> Turning off - the reverse uses the same bitshift, a negation (~ invert
> all bits) and then logical and (&)
> 
> So starting with out PORTB as above.
> 
> 1<<PB1 == 1<<1 == 0000 0010b
> 
> ~ is negate
> 
> 1111 1101b
> 
> And then and that with our old PORTB
> 0010 0010b (old PORTB)
> 1111 1101b  ~(1<<1)
> ==========
> 0010 0000   Result - your led on PB1 is turned off, the led you might
> have had on PB5 retains it's previous state.
> 
> Other useful links:
> 
> http://www.arduino.cc/en/Reference/PortManipulation
> http://www.arduino.cc/en/Reference/HomePage
> 
> Cheers,
> Malc.
> 
> 
> 
> 
> -----Original Message-----
> From: wiltshire-bounces at mailman.lug.org.uk
> [mailto:wiltshire-bounces at mailman.lug.org.uk] On Behalf Of Robert
> Sent: 22 April 2008 13:51
> To: wiltshire at mailman.lug.org.uk
> Subject: [Wiltshire] Random C coding question...
> 
> 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
> extract?
> 
>      // 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
> the end result of the code is - the LED connected to pin 1 of port B on
> the atmega168 chip turns on or off.  And I can modify the code to turn
> stuff on and off on different ports by changing (eg) PB1 to PB2 and
> PORTB
> to PORTD, etc, etc and it works.  I just don't understand how it works!
> 
> My initial thoughts were some kind of bit mask, but the "1" just doesn't
> make sense to me.  I'm guessing that PORTB is an 8-bit int with each bit
> representing a pin, and that or-ing / and-ing (1<<PB1) somehow toggles
> the
> bit for pin1.  But I could be waaay off with that guess.
> 
> Anyway, I could go on all day about the things I don't know about C, so
> I'll leave it there..
> 
> Robert.
> 
> 
> 
> 
> 
> 
> _______________________________________________
> Wiltshire mailing list
> Wiltshire at mailman.lug.org.uk
> https://mailman.lug.org.uk/mailman/listinfo/wiltshire
> 
> _______________________________________________
> Wiltshire mailing list
> Wiltshire at mailman.lug.org.uk
> https://mailman.lug.org.uk/mailman/listinfo/wiltshire