What microcontroller/compiler do, when he see a "bit" which was already set before.

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Aleksey_ua
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Posted by Aleksey_ua: Sat. Jul 11, 2015 - 07:46 PM
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Hello programmers. Please help.

What microcontroller/compiler do, when he see a "bit" which was already set before. Is it bad programming style? Will a "bit" be overwritten again?

 

In the "else" we see as a " PB2 bit" could be rewritten to zero. Or maybe I should not think about it at all? The compiler does all the work for me. I guess I should learn assembler to solve similar questions by myself in the future, but may be somebody could explain. Thanks.

 

#include <avr/io.h>
#define BUTTON_PRESSED (PINB & (1<<PB1))
#define LED_ON PORTB |= (1<<PB2)
#define LED_OFF PORTB &= ~(1<<PB2)

void setup () {
	DDRB =  0b00011101;
	PORTB = 0b00100010;
}

int main(void)
{
	setup();
	
    while(1)
    {
        if (!BUTTON_PRESSED)
        {
			LED_ON;
        } 
        else
        {
			LED_OFF; // this place will executed again and again, while button not pressed. 
        }
    }
}

 

 

Tags:

AVR Microcontrollers, megaAVR and tinyAVR, compiler, c
Last Edited: Sat. Jul 11, 2015 - 07:52 PM
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alexru
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Posted by alexru: Sat. Jul 11, 2015 - 08:54 PM
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Compiler will generate the code to do what it was asked to do.

 

For simple variables it does not matter. But writing peripheral registers may have side effects. One good example is PINx registers on some AVRs. While normally the register reads the state of the inputs, if you write a one to any locations, corresponding pin will toggle. All those side effects are described in the documentation.

 

In this specific example you have noting to worry about.

 

NOTE: I no longer actively read this forum. Please ask your question on www.eevblog.com/forum if you want my answer.

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mikech
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Posted by mikech: Sat. Jul 11, 2015 - 09:11 PM
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Repeatedly setting a bit to 1 (or 0) does not harm anything and can simplify how you implement your logic.
In your example, you can ;
1) check if the button is pressed and then set the led on/off.
or
2) check if the button has changed state (from pressed to not-pressed, or from not-pressed to pressed) and then set the led off/on.
Either method will produce the same end result.


Having said that, for I/O or memory it does not matter if you repeatedly set/clear bits, but some internal modules *will* do something everytime you write to them (eg. the watchdog)

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Bronze Marktheusch
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Posted by theusch: Sat. Jul 11, 2015 - 09:11 PM
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// this place will executed again and again, while button not pressed.

So will this, whether the button is pressed or not:

if (!BUTTON_PRESSED)

A microcontroller program nearly always runs forever, so the "main loop" will constantly be executed unless the main processor clock is stopped (deep sleep).

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Aleksey_ua
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Posted by Aleksey_ua: Mon. Jul 13, 2015 - 10:45 AM
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Thanks to all!

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