difference between "loop_until_bit_is_set" macro and while() expression

Question

this piece of code in running on ATmega2560, so what is the difference between these two:

while(UCSR0A & 0b00100000 == 0);  // check UDRE0 bit if it is 1

and:

loop_until_bit_is_set(UCSR0A, UDRE0);

as you can see this is for UART transmission, but if i use the first one I could't get desired output on terminal, for example if i do printf("Hello world\n"); the actual terminal display is: HeHeHe....... However the latter one works. You can find this piece of code in stdio.h.

Help stomp out pointless comments. `while(UCSR0A & _BV(UDRE0));` — Ignacio Vazquez-Abrams, May 16 '15 at 03:04
@IgnacioVazquez-Abrams shouldn't that be `while(!(UCSR0A & _BV(UDRE0)));` ? As far as I understand, he wanted to loop *until* the bit is set, not *while* it's set... — , May 24 '16 at 21:40
@vaxquis: That would be an answer, not a rewrite of the code in the question. — Ignacio Vazquez-Abrams, May 24 '16 at 21:41

score 3 · Answer 1 · edited May 24 '16 at 21:39

3

== has higher priority than &; also, no need to check for == 0 - you should try

while(!(UCSR0A & 0b00100000));

or rather, using bit macro

while(!(UCSR0A & _BV(UDRE0)));

instead.

edited May 24 '16 at 21:39

answered May 16 '15 at 12:26

Ruudje

31
1

oh yeah you are right, thanks!! I wish I could vote, but not enough reputation... – zrs5532 May 17 '15 at 20:51

difference between "loop_until_bit_is_set" macro and while() expression

1 Answers1