How to prevent Arduino compiler from merging changing of port pin outputs into a single operation

Question

I want to set one pin's output value, and then set a different pin's output value so that the two pins change value one after the other. However, the compiler optimises the two calls to change both pins in one go.

PORTD |= _BV(0); //set pd0 to high PORTD |= _BV(1); //set pd1 to high PORTD &= ~(_BV(1)); //set pd1 to low

The two pins go high simultaneously. I tried inserting a NOP between these but it doesn't change behaviour.

(I originally intended to include an oscilloscope screengrab to show what was happening - that's how I knew (or got the wrong impression) that the two pins were changing at the same time. But the photo wasn't shifting from my Nexus 7 for me to access it from my laptop.)

I can't duplicate this behavior. Which version of GCC is in use? — Ignacio Vazquez-Abrams, Dec 11 '14 at 23:49
What application requires those pins to be turned on one after the other, but so quickly that you don't need any code to cause a delay between the two? — BrettFolkins, Dec 12 '14 at 00:38
Cool - there's an Arduino stackexchange! Thanks to the mods for migrating the question. — mackenir, Dec 12 '14 at 14:58
It sounds like I've just got myself confused here, since the behaviour I've seen isnt repro-able. — mackenir, Dec 12 '14 at 14:59
Granted I'm no expert on O-scopes, but given that the pin changes with separate calls will still happen in tens of nanoseconds, could a scope accurately detect the difference? — BrettFolkins, Dec 12 '14 at 20:28
Well, when testing this as its own sketch I can see everything working properly - the delays between signal changes are quite evident. — mackenir, Dec 12 '14 at 20:30

Majenko · Accepted Answer · 2014-12-12T00:45:49.837

7

I certainly can't duplicate what you are saying. The small code snippet:

void setup() {
    PORTD |= _BV(0);
    PORTD |= _BV(1);
    PORTD &= ~(_BV(1));
}

void loop() {
}

compiles into:

000000a6 <setup>:
  a6:   58 9a           sbi 0x0b, 0 ; 11
  a8:   59 9a           sbi 0x0b, 1 ; 11
  aa:   59 98           cbi 0x0b, 1 ; 11
  ac:   08 95           ret

000000ae <loop>:
  ae:   08 95           ret

That is clearly two separate bit sets followed by a bit clear.

I am using UECIDE that is set to use GCC 4.3.2

If I switch to GCC 4.8.1 instead, the code generated is identical.

Even turning on full -O3 optimization I still get the same results.

So how are you determining that it's happening at the same time?

edited Dec 12 '14 at 00:45

answered Dec 12 '14 at 00:37

Majenko

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Which makes sense since the memory location has been declared as volatile. – Ignacio Vazquez-Abrams Dec 12 '14 at 00:39
I think I'm using Arduino IDE 1.06 but I will need to check when I get home. – mackenir Dec 12 '14 at 15:09
I observed the two pins changing on an oscilloscope, but I will have to take a step back and do a test with the Arduino Micro removed from the project I'm working on. Sounds like I'm just mistaken. – mackenir Dec 12 '14 at 15:11
Same code generated here with avr-gcc 5.4.0. – Edgar Bonet Oct 25 '19 at 14:59

How to prevent Arduino compiler from merging changing of port pin outputs into a single operation

1 Answers1