Reading Single Bit from a Char

Question

This is my first post here so please forgive anything I do wrong :) This is my situation. I am writing a simple data transfer operation for a PIC16F876 using MPLAB and the HI-Tech C compiler.

I have a char called data, I wanted to access the bits in data and send them to portB0, starting with the MSB.

In assembly I would simply do:

PORTB,0 = data,7 // to get the MSB and put it on port B0, I would then do this for all bits.

However in C this seems to be more complicated. I have done some research and have found a function which works:

getBit(char data, int bitNumber)
{
     return (data & (1 << bitNumber-1)) != 0;
}

Then I just use:

PORTBbits.RB0 = getBit(data,7);

This is OK, but messy and seems to take longer, I dont know why I need an extra function... So my question is: Is there not a simple method to access a bit in a register? like:

PORTBbits.RB0 = data,7

I cant understand why there would not be as the complier converts it in to assembly anyway??!!!!

Thanks in advance. Regards, Tim.

you can use bitfields in struct. but this will be probably worse. — Elazar, Apr 26 '13 at 15:19
possible duplicate of [Access bits in a char in C](http://stackoverflow.com/questions/8584577/access-bits-in-a-char-in-c) — Prof. Falken, Apr 26 '13 at 15:25
Hi there, thanks for your quick replies, Elazar, I did try that, but seeing as I want, in the end, to write a loop for the data send this doesn't work because I cant loop on a name. Amigable: I have looked at those (and other) topics, the ones I read seemed to be for the C language. Also, part of the question was why isn't there a simple solution, seeing as the language is based on ASM. — Tim M, Apr 26 '13 at 15:44
Most C compilers will let you use inline assembly. For your multiple bits operation, you probably want a repeated shift, and to take your output from the MSB or LSB (depending on the order you want). — Chris Stratton, Apr 26 '13 at 17:03

score 12 · Accepted Answer · answered Apr 26 '13 at 15:24

12

Another way to accomplish what you looking for is something like this:

PORTBbits.RB0 = (data >> 7) & 1;

Or more generically:

#define BIT(x,n) (((x) >> (n)) & 1)
PORTBbits.RB0 = BIT(data, 7);

answered Apr 26 '13 at 15:24

Remy Lebeau

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This looks good, I'll try it, thanks. Still don't know why there isn't a simple option to directly access an element in a register, I thought C for PIC was meant to simplify things.... – Tim M Apr 26 '13 at 15:45
Worked like a charm, and I suppose data,7 Vs. (data >> 7) & 1 is a fairly close match in typing terms. I still don't see why its needed though... Many thanks for your help, I wasn't allowed to vote up as I have no rep, but this was my chosen answer :). – Tim M Apr 26 '13 at 15:55

K Scott Piel · Answer 2 · 2013-04-26T15:27:13.570

3

Bear in mind that when you use a function, such as this, that means that you're going to push two values on the stack, perform a jump, execute operations, pop the stack and return a value.

You could get around it via a MACRO which would inline the code

#define GETBIT(x,n) ((x >> (n-1)) & 1)

Option #2 is the use of inline ASM if you're given to masochism and screaming to save cycles. ~smile~

edited Apr 26 '13 at 15:27

answered Apr 26 '13 at 15:21

K Scott Piel

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score 2 · Answer 3 · answered Apr 26 '13 at 15:23

If you just need the bit, you can get it by ( data & (1 << (bitNumber-1)) ) != 0. Or you could define a macro to make it more readable as

#define getBit(x,y) ((x) & (1 << ((y)-1))) != 0

and call it by getBit(data,bitNumber)

The macro will remove the function overhead. But there is no operator to give you the bit value the way you wanted.

score 0 · Answer 4 · answered May 02 '13 at 09:22

Some (most?) micros have several registers for the pins - if you know what you are going to do then (f'rexample) writing to the SET or CLEAR pin registers saves the whole read-modify-write/masking kerfuffle.

You probably know, but for the sake of clarence - SET/CLEAR registers work by only setting or clearing the pin if the bit written is 1, if 0 is written the pin state is not changed, which means the hardware is doing the work for you. To set pin 3 of a port you just write 0x04 to the port_SET register. To clear it, you write 0x04 to the port_CLEAR register.

It's not always possible to use these, but it shaves a lot of CPU cycles off if you can.

Reading Single Bit from a Char

4 Answers4