topic Re: How to manipulate one bit in a register? in 8-bit Microcontrollers

How to manipulate one bit in a register?

fredycolombia — Tue, 05 Feb 2008 21:41:23 GMT

Hi,
I have a little trouble.
I need to change one bit in one register on my program, if that bit is zero chage to one a vice-versa.

#include <hidef.h>
#include "derivative.h"
int dato;
DDRA=0x00;
PTA=dato;

how I can ask about seven bit y my variable "dato" and change only that bit?
I have tried to use this:

asm bclr 7, dato          //if I use this, I have a mistake when a compile   ( "invalid opcode or ":" expected)

also I try with this:

DDRA=0x00;          //first port as input
dato =PTA;
change();

void change (void){

DDRA=0xFF;      //then por as output
PTA=dato;

        if (PTA_PTA7==1){
        PTA_PTA7=0;
        dato=PTA;}
         else{
             if (PTA_PTA7==0){
             PTA_PTA7=1;
             dato=PTA;}
              else{}
             }
          dato=PTA;

}

When I simulate this program it works, but I think there isn't to useful and so complicated ¿I can manipule only one bit in the variable "dato"?

thanks for your answers.

Regars,
Fredy

Re: How to manipulate one bit in a register?

JimDon — Tue, 05 Feb 2008 22:09:47 GMT

PTA_PTA7 ^= 1; // XOR with a 1 toggles a bit

Re: How to manipulate one bit in a register?

Ake — Tue, 05 Feb 2008 22:33:43 GMT

Hi,

To set a bit use

PTA_PTA7 = 1;

To clear it, use

PTA_PTA7 = 0;

Regards,

Ake

Re: How to manipulate one bit in a register?

fredycolombia — Wed, 06 Feb 2008 01:25:06 GMT

hi,

that's ok, but can I manipulate bit by bit over my variable "dato" ?

I don´t need manipulate bits on porta, I need manipulate bits on "dato"

Is this posible?

Regards

Fredy

Re: How to manipulate one bit in a register?

JimDon — Wed, 06 Feb 2008 01:34:42 GMT

Yes,

XOR Function
| 0 | 1 |
------------
0 | 0 | 1 |
------------
1 | 1 | 0 |
------------

Where ever there is a 1 it will invert.
data = 0x5; // 0101
data ^= 0xf; // 1010 4 bits Inverted.

If you plan on being a coder, you should read and understand this:
Boolean Logic

Re: How to manipulate one bit in a register?

bigmac — Wed, 06 Feb 2008 01:49:44 GMT

Hello Fredy,

Your line of code -

asm bclr 7, dato

The error occurred because the BCLR instruction will only work for variables that reside in page 0. In all probability the variable dato will reside in DEFAULT_RAM, which is generally not page 0. The variable would need to be specifically assigned to MY_ZEROPAGE section.

For the variable to remain at the present location, it is possible to use a mask value instead of the bit number.-

To clear bit-7,

dato &= 0x7F;

To set bit-7,

dato |= 0x80;

To toggle bit-7,

dato ^= 0x80;

Alternatively, you could create a bit field for the variable, but this is more complex.

Regards,

Mac

Re: How to manipulate one bit in a register?

fredycolombia — Wed, 06 Feb 2008 04:32:48 GMT

hi again,

That's a great solution and very easy, about your alternative, I have tried that, but It really be complex.

Thanks for your help.

P.D. Can you help me about how I can use the correct location (zero_page), when I use instructions like "asm bclr 7,dato"

Re: How to manipulate one bit in a register?

bigmac — Wed, 06 Feb 2008 10:19:20 GMT

Hello Fredy,

The following assumes that MY_ZEROPAGE is appropriately defined within the Project.prm file -

#pragma DATA_SEG __SHORT_SEG MY_ZEROPAGE

byte dato;

#pragma DATA_SEG DEFAULT

Further to my previous post, you might consider use of the following macros that allow any bit to be manipulated. They should not be dependent on the variable location.

#define set_bit(bitnum,reg) ((reg) |= 1<<(bitnum))
#define clr_bit(bitnum,reg) ((reg) &= ~(1<<(bitnum)))
#define togl_bit(bitnum,reg) ((reg) ^= 1<<(bitnum))

To set bit-7 of dato, you would use the following line -

set_bit(7,dato);

Regards,
Mac

Message Edited by bigmac on 2008-02-06 01:26 PM

Re: How to manipulate one bit in a register?

fredycolombia — Thu, 07 Feb 2008 10:24:10 GMT

thanks bigmag,

Is interesting to know how I can do that things, your really help me a lot.
again... thanks.

regards,

Fredy