I/O output - Electronic Circuits Projects Diagrams Free

Manie · 14th June 2006, 11:16 AM

How do I change a single bit value of a port? What is the syntax?

**Nigel Goodwin** · 14th June 2006, 11:36 AM

Quote:

Originally Posted by Manie

How do I change a single bit value of a port? What is the syntax?

Depends what processor you're talking about, and what laguage you're using, your question is far too vague to answer!.

Manie · 14th June 2006, 11:37 AM

I am using a PIC16f737 and programming in C

**Nigel Goodwin** · 14th June 2006, 11:39 AM

Quote:

Originally Posted by Manie

I am using a PIC16f737 and programming in C

Can't help you with C, but in assembler it's simply:

bsf Port, pin

to set a pin high

bcf Port, pin

to clear a pin.

As it's such a basic thing to do, you should have suitable examples with your C compiler?.

akg · 14th June 2006, 01:50 PM

Quote:

Originally Posted by Manie

I am using a PIC16f737 and programming in C

port_value = port_value | (1<< bitposition)

or

port_value |= (1<< bitposition)

GraveYard_Killer · 14th June 2006, 02:37 PM

example:

setting PORTB bit 0
PORTB|=(1<<0);

setting PORTB bit 2
PORTB|=(1<<2);

clearing PORTB bit 2
PORTB&=~(1<<2);

Papabravo · 14th June 2006, 02:49 PM

Check your C-compiler. Many of them have a declaration syntax for bits. If they do then the statement is just something like

Code:

bit  GIE  INTCON.7

GIE = 0 ; /* Disable Interrupts */
...
GIE = 1 ; /* Enable Interrupts */

The declaration syntax is not part of the C language standard, but several compilers for processors that support bit addressing have invented such extensions. You do have to look carefully in the compiler documentation though.

Look for #pragma keyword=extended or something similar to see if there is a switch that enables or disables the extended language features.

If you think you've found something then write a short test program and look at the compiler output to see if it matches both your understanding and Nigel's example.

On real ports with a mixture of inputs and outputs you need to be careful with bsf and bcf instructions because in hardware they read all 8 bits of a port, modify the single bit, and write back all 8 bits. This can be problematic and there are warnings in the PIC data sheets.

Let us know what you find.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

14th June 2006, 11:16 AM	#1
Manie	I/O output How do I change a single bit value of a port? What is the syntax?

14th June 2006, 11:37 AM	#3
Manie	I am using a PIC16f737 and programming in C

14th June 2006, 02:37 PM	#6
GraveYard_Killer	example: setting PORTB bit 0 PORTB\|=(1<<0); setting PORTB bit 2 PORTB\|=(1<<2); clearing PORTB bit 2 PORTB&=~(1<<2); __________________ Learning is a life-long process...

14th June 2006, 02:49 PM	#7
Papabravo	Check your C-compiler. Many of them have a declaration syntax for bits. If they do then the statement is just something like Code: bit GIE INTCON.7 GIE = 0 ; /* Disable Interrupts / ... GIE = 1 ; / Enable Interrupts / The declaration syntax is not part of the C language standard, but several compilers for processors that support bit addressing have invented such extensions. You do have to look carefully in the compiler documentation though. Look for #pragma keyword=extended or something similar to see if there is a switch that enables or disables the extended language features. If you think you've found something then write a short test program and look at the compiler output to see if it matches both your understanding and Nigel's example. On real ports with a mixture of inputs and outputs you need to be careful with bsf and bcf instructions because in hardware they read all 8 bits of a port, modify the single bit, and write back all 8 bits. This can be problematic and there are warnings in the PIC data sheets. Let us know what you find. Last edited by Papabravo; 14th June 2006 at 02:52 PM.*