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[TUT] [C] Bit manipulation (AKA "Programming 101")
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BadGranola

Posted: Jul 04, 2010 - 12:40 AM

Joined: Sep 16, 2009
Posts: 39

Hi, I'm trying to build a 16-bit word made up from three integer variables; address, control (both in range 0x00-0x03), and data (0x00-0xFF).

The most significant four bits of the word are irrelevant, the next two hold the address, the next two the control information, and the final 8 hold the data.

Will something like this allow me to contruct the word from these variables?

Code:

word = (address<<10) | (control<<8) | data

JohanEkdahl

Posted: Jul 04, 2010 - 01:15 AM

Joined: Mar 27, 2002
Posts: 18599
Location: Lund, Sweden

Have you tried it out? (For this kind of stuff the simulator in AVR Studio is excellent.)

But yes, that looks reasonable.

Koshchi

Posted: Jul 04, 2010 - 04:57 AM

Joined: Nov 17, 2004
Posts: 13853
Location: Vancouver, BC

Or you could do this:

Code:

typefef struct

{

    unsigned int data :8;

    unsigned int control :2;

    unsigned int address :2;

} whateverYouWantToCallIt;

or:

Code:

typefef struct

{

    unsigned int :4; //4 bits of fiiler

    unsigned int control :2;

    unsigned int address :2;

    unsigned int data :8;

 } whateverYouWantToCallIt;

depending on endianness.

Code:

whateverYouWantToCallIt word;

...

word.address = address;

word.control = control;

word.data = data;

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Steve A.

The Board helps those that help themselves.

qli029

Posted: Jul 21, 2010 - 04:47 AM

Joined: Jun 30, 2010
Posts: 13

This one doesnot make sense to me,

To set bit 0 in foo and then store the result back into foo:
Code:

Code:

foo = foo | 0x01;

bit OR"|" with 0x01, we will set bit 1 in foo,,
why is it to set bit 0?
I am a newbie, forgive me if i am wrong. please explain it to me.

JohanEkdahl

Posted: Jul 21, 2010 - 09:16 AM

Joined: Mar 27, 2002
Posts: 18599
Location: Lund, Sweden

[Removed as I totally mis-understood the post above. Snigelen nailed the correct interpretation and answer below.]

Last edited by JohanEkdahl on Jul 21, 2010 - 10:08 AM; edited 1 time in total

snigelen

Posted: Jul 21, 2010 - 09:55 AM

Joined: Jan 08, 2009
Posts: 1160
Location: Lund, Sweden

qli029 wrote:

This one doesnot make sense to me,

To set bit 0 in foo and then store the result back into foo:
Code:

Code:

foo = foo | 0x01;

bit OR"|" with 0x01, we will set bit 1 in foo,,
why is it to set bit 0?

This is because bits in a byte are usually numbered from 0 to 7. So the first bit is number 0.

clawson

Posted: Jul 21, 2010 - 10:53 AM

Joined: Jul 18, 2005
Posts: 62371
Location: (using avr-gcc in) Finchingfield, Essex, England

Just to add a little more to that:

Code:

bit     shift     mask

0       (1<<0) =  0x01

1       (1<<1) =  0x02

2       (1<<2) =  0x04

3       (1<<3) =  0x08

4       (1<<4) =  0x10

5       (1<<5) =  0x20

6       (1<<6) =  0x40

7       (1<<7) =  0x80

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iA7med

Posted: Jul 24, 2010 - 10:22 AM

Joined: Feb 08, 2010
Posts: 12

That was useful , Thanks .

violin_rules

Posted: Jan 09, 2011 - 02:01 AM

Joined: Jan 02, 2011
Posts: 1

Thanks!, i was looking for this information.

yaswanth2008

Posted: Mar 02, 2011 - 01:27 PM

Joined: Jun 05, 2009
Posts: 67
Location: bangalore, India

how should i access the registers R0-R32? using c programming.
Just give me an example..

Thank you

abcminiuser

Posted: Mar 02, 2011 - 01:32 PM

Joined: Jan 23, 2004
Posts: 9832
Location: Trondheim, Norway

Quote:

how should i access the registers R0-R32? using c programming.
Just give me an example..

You don't - the point of C is that you don't need to directly access the CPU temporary registers. They are handled by the compiler in order to implement your code.

The I/O and peripheral registers (like the timer control registers) are manipulated by you, but not the CPU scratch registers.

- Dean Twisted Evil

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clawson

Posted: Mar 02, 2011 - 02:12 PM

Joined: Jul 18, 2005
Posts: 62371
Location: (using avr-gcc in) Finchingfield, Essex, England

To illustrate what Dean has said consider this program:

Code:

#include <avr/io.h>

#include <util/delay.h>

int main(void)

{

   uint8_t mask = 0x55;

   DDRB = 0xFF;

    while(1)

    {

         PORTB ^= mask;

       if (PIND & 1<<PD0) {

          mask ^= 0xFF;

       }

    }

}

when built by the compiler it generates:

Code:

00000080 <main>:

int main(void)

{

   uint8_t mask = 0x55;

   DDRB = 0xFF;

  80:   8f ef          ldi   r24, 0xFF   ; 255

  82:   84 b9          out   0x04, r24   ; 4

#include <avr/io.h>

#include <util/delay.h>

int main(void)

{

   uint8_t mask = 0x55;

  84:   85 e5          ldi   r24, 0x55   ; 85

   DDRB = 0xFF;

    while(1)

    {

         PORTB ^= mask;

  86:   95 b1          in   r25, 0x05   ; 5

  88:   98 27          eor   r25, r24

  8a:   95 b9          out   0x05, r25   ; 5

       if (PIND & 1<<PD0) {

  8c:   48 9b          sbis   0x09, 0   ; 9

  8e:   fb cf          rjmp   .-10        ; 0x86 <main+0x6>

          mask ^= 0xFF;

  90:   80 95          com   r24

  92:   f9 cf          rjmp   .-14        ; 0x86 <main+0x6>

In this code the compiler initially uses R24 to output 0xFF to DDRB. It then reuses R24 as 'mask' and initialises it to be 0x55. It reads PORTB into R25 then exclusive-or's this with 'mask' (R24). It then checks bit 0 of PIND and if that is set it complements the contents of 'mask' (so 0x55 switches to 0xAA).

You, the programmer, need never be aware which registers the compiler has chosen to put values or variables into.

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ghost800

Posted: Mar 30, 2011 - 06:51 PM

Joined: Mar 30, 2011
Posts: 1

What this program does not work help me

Code:

//***********************************************************

// Project:  Analog Comparator example   

// Author: winavr.scienceprog.com   

// Module description: Analog comparator example with positive comparator

// input connected to Vref 1.23V. When compared voltage exceed 1.23V LED lighst on.

// When voltage drops bellow - LED turns OFF. Comparator inputis connected to ADC3 pin.

// ***********************************************************

#include <avr\io.h>       

#include <avr\interrupt.h>  

#define AINpin PA3

#define LED PD0

void Init(){

   DDRA&=~(1<<AINpin);//as input

   PORTA&=~(1<<AINpin);//no Pull-up

   DDRD|=(1<<LED); //Led pin as output

   PORTD|=(1<<LED);//Initally LED is OFF

   SFIOR|=(1<<ACME);//enable multiplexer

   ADCSRA&=~(1<<ADEN);//make sure ADC is OFF

   ADMUX|=(0<<MUX2)|(1<<MUX1)|(1<<MUX0); //select ADC3 as negative AIN

   ACSR|=

   (0<<ACD)|   //Comparator ON

   (1<<ACBG)|   //Connect 1.23V reference to AIN0

   (1<<ACIE)|   //Comparator Interrupt enable

   (0<<ACIC)|   //input capture disabled

   (0<<ACIS1)| //set interrupt on output toggle

   (0<<ACIS0);

   sei();//enable global interrupts

}

// Interrupt handler for ANA_COMP_vect

//

ISR(ANA_COMP_vect) {

if bit_is_clear(ACSR, ACO)

   PORTD&=~(1<<LED);//LED is ON

   else    PORTD|=(1<<LED);//LED is OFF

}

// ***********************************************************

// Main program

//

int main(void) {

     Init();

    while(1) {     // Infinite loop; interrupts do the rest

   }

}

clawson

Posted: Mar 30, 2011 - 07:54 PM

Joined: Jul 18, 2005
Posts: 62371
Location: (using avr-gcc in) Finchingfield, Essex, England

ghost800,

How does your post extend the discussion of the article in the first post of this thread?

Moderator.

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JohanEkdahl

Posted: Mar 30, 2011 - 09:10 PM

Joined: Mar 27, 2002
Posts: 18599
Location: Lund, Sweden

Cliff!

When those post totally unrelated to the thread occur I think we should tossin an explanation of the difference between the "new topic" and the "new reply" buttons. I have a feeling that the distinction is not clear for many newcomers, or thet they even do not see the buttons and think that the only way to post on the forum is the "Quick Reply" area.

ghost800!

If you want to start a discussion on a new topic, use the "new topic" button at the top or bottom thread pages and forum listings here at AVRfreaks. Each discussion should commence in a separate "thread" (topic) or things become very confused and out of order. When creating a new thread please take the time to think out a really smart title (subject) so that it reflects the content of your post (extremely bad subjects are things like "Help!", "Why does my code not work" etc).

Hope this helps!

reaper420

Posted: Apr 01, 2011 - 07:27 AM

Joined: Jul 21, 2010
Posts: 5

can the macros be used with ports?
for eg

Code:

int totalcomplete()

{

int retval;

if ((bit_get(PORTC, BIT(1))) && (bit_get(PORTC, BIT(0)))==0)

{

retval = 1;

}

else retval = 0;

return retval;

}

clawson

Posted: Apr 01, 2011 - 10:33 AM

Joined: Jul 18, 2005
Posts: 62371
Location: (using avr-gcc in) Finchingfield, Essex, England

I'm curious. What would make you want to type:

Code:

if ((bit_get(PORTC, BIT(1))) && (bit_get(PORTC, BIT(0)))==0)

{

when you could have typed:

Code:

if ((PORTC & 3) == 0)

{

Do the tortuous macros really help?

(BTW do you mean to read PORTC or PINC?)

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reaper420

Posted: Apr 01, 2011 - 08:15 PM

Joined: Jul 21, 2010
Posts: 5

thanks! ya my mistake it should have been PINC .sorry but am new to programming an ya thanks i guess i could use

Code:

if ((PORTC & 3) == 0)

{

but what i meant was does this macro

Code:

#define bit_get(p,m) ((p) & (m))

work when a pin is specified in place of variable p. Or do we have to use macro such as

Code:

#define READ(port,pin) ((PIN ## port & (1<<pin))

clawson

Posted: Apr 01, 2011 - 08:44 PM

Joined: Jul 18, 2005
Posts: 62371
Location: (using avr-gcc in) Finchingfield, Essex, England

Yes and my question is why obfuscate code with layers of macros when the plain C to achieve the same would actually be simpler. There seems to be a recent trend to try and wrap code in N levels of macro at the drop of a hat. As someone who has to read/maintain other programmers code I find that it very rarely aids the mainatainability/readability of the code but instead just makes it more difficult to follow. Perhaps you can explain what how your bit_get() macro makes the code easier to follow. ALL programmers know what an AND operation is!

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reaper420

Posted: Apr 01, 2011 - 09:02 PM

Joined: Jul 21, 2010
Posts: 5

thanks, and well as u said it will be simpler to use plain c but the reason is that, i am working on a project and the panel where i have to present it to has a few members who have well little or no knowledge of programming so i am hoping the macros will help in making them understand more easily.

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