[ECOS] HAL_DELAY_US() default implementation

[Øyvind Harboe]

Getting rid of calibration is not easy.

Take the AT91 HAL variants specifically. There are 5'ish different
sub-hals * 3 types of memory(flash + internal/external RAM). Thats
15'ish combinations.

How about this approach?

- No automatic calibration
- Add a machine code busy loop/CPU type. Machine code is
  used to make the busy loop independent of compiler optimisations
- Some(many?) HAL's have a HAL_DELAY_US() that works out
  of the box according to the newest HAL_DELAY_US() specification and 
  hence won't need any calibration.
- If the user provides a CDL parameter which calibrates the busy loop
  he gets HAL_DELAY_US(), otherwise he doesn't. It is better with a
  compile time error than a broken HAL_DELAY_US().
- If a HAL knows that it can accuratly estimate this parameter in all
  cases, it can provide a default value and HAL_DELAY_US() works out of
  the box.


-- 
Øyvind Harboe
http://www.zylin.com


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