_delay_us(10) v. _delay_us(10.0)

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mapelec
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Posted by mapelec: Tue. May 21, 2013 - 09:25 PM
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Hi All,

I've inhereted some code with _delay_ms() and _delay_us() calls, however the author has chosen to use floating point constants as arguments rather than integer constants. I've read that this can bloat the code, but does it adversely affect the run time of these delays?

Usually I write _delay_us(10) for 10us delays and these work fine (verified with a countetr/timer), however what would you expect from _delay_us(10.0)? Does this also affect the _delay_ms() calls by the same degree?

Thanks,
Mark.

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AVR Microcontrollers, megaAVR and tinyAVR
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Koshchi
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Posted by Koshchi: Tue. May 21, 2013 - 09:46 PM
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Quote:
however the author has chosen to use floating point constants as arguments rather than integer constants
If you look at the definitions of _delay_ms() and _delay_us(), they do indeed take floating point value.
Quote:
I've read that this can bloat the code
You've read wrong. Those two functions use floating point regardless of the value or type of the parameter. What bloats the code is when the parameter is not a compile time constant. If it is a compile time constant, then all of the floating point math is done by the compiler, not the AVR. If it is not a compile time constant (or you for some reason choose to compile without optimization), then indeed floating point routines will be used at run time. That will bloat the code if those routines were not already included in the code. It will, however, make the routines inaccurate (particularly _delay_us()).

Regards,
Steve A.

The Board helps those that help themselves.

Last Edited: Tue. May 21, 2013 - 09:47 PM
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david.prentice
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Posted by david.prentice: Tue. May 21, 2013 - 09:46 PM
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_delay_us(10) should be the same as _delay_us(10.0)

The macros should evaluate the same number of _delay_loop_2()s. After all, 10.0 is the same value as 10. Both are compile-time constants

All the same, I see no advantage in using f-p constants. Do you ever want fractional us ?

However, it is not uncommon to want fractional _delay_ms() arguments. I would choose _delay_us(1234) in preference to _delay_ms(1.234). But whatever floats your boat !!

The important distinction is whether you have a runtime variable argument or a compile-time constant.

Untested. You can try the values for yourself.

David.

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Koshchi
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Posted by Koshchi: Tue. May 21, 2013 - 09:50 PM
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Quote:
All the same, I see no advantage in using f-p constants. Do you ever want fractional us ?
If you want a delay less than a microsecond, then yes.

Regards,
Steve A.

The Board helps those that help themselves.

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mapelec
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Posted by mapelec: Tue. May 21, 2013 - 11:25 PM
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Thanks for the replies, I understand now.

BTW is the reason for having to turn optimizations on because it that figures out if it is compile time constant parameter and optimizes to a constant? I've noticed the list file has calls to multiply routines with optimizations off, and they disappear when on.
Mark.

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Koshchi
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Posted by Koshchi: Wed. May 22, 2013 - 05:58 AM
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Quote:
BTW is the reason for having to turn optimizations on because it that figures out if it is compile time constant parameter and optimizes to a constant?
Yes.

Regards,
Steve A.

The Board helps those that help themselves.

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skeeve
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Posted by skeeve: Wed. May 22, 2013 - 06:10 AM
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david.prentice wrote:
All the same, I see no advantage in using f-p constants. Do you ever want fractional us ?
If the processor is running at 14.372 MHz, floating point arithmetic will be involved.
No reason not to allow floating point arguments.
It also prevents a deceptive asymmetry with _delay_ms.

Iluvatar is the better part of Valar.

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Koshchi
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Posted by Koshchi: Wed. May 22, 2013 - 06:36 AM
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Quote:
If the processor is running at 14.372 MHz, floating point arithmetic will be involved.
F_CPU is usually an integer, not a floating point number. And if you did make it a floating point number, it would be 14372000.0 since the value needs to be in Hertz.

Regards,
Steve A.

The Board helps those that help themselves.

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clawson
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Posted by clawson: Wed. May 22, 2013 - 09:33 AM
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Mark,

If you are ever in doubt about stuff like this just look at the .lss file. You should find that exactly the same code was generated whether you use (10) or (10.0).

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JohanEkdahl
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Posted by JohanEkdahl: Wed. May 22, 2013 - 10:30 AM
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Quote:

BTW is the reason for having to turn optimizations on because it that figures out if it is compile time constant parameter and optimizes to a constant?

Yes.

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