[ECOS] HAL_DELAY_US()

["Øyvind Harboe" <oyvind dot harboe at zylin dot com>]

Going through my changes to eCos, I ran into HAL_DELAY_US().

Does this function belong in the application or eCos?

If it can be implemented in a correct manner that does not require any
configuration, then I believe it belongs in the HAL.

If it needs to be tuned to the hardware in question, then I believe it
belongs in the application space.

The implementation in EB40a is busted because it is not
multithreading/interrupt safe.


Ref. earlier discussions HAL_DELAY_US() should take *at least* that
many us, there is no upper limit or requirement on precision as such.

The implementation below could, with a CDL parameter that the user
must configure for his application, work for EB40a & similar HALs.


/* this fn takes at least 0.000001342194658704 seconds
* to execute one iteration.
*
* A number of factors affect this number:
*
* - Is the code in ram/rom, wait states
* - Iterrupts/task switches can slow it down
*/


static void busy(int j)
{
	__asm volatile (
	
	".L1: nop\n"
	"sub %0,%0,#1\n"
	"cmp %0,#0\n"
	"bne .L1\n"
	
	: "+r" (j));
}



/* Waits at least this many us.
*
* Does not require kernel to be running.
*/
externC void hal_delay_us(cyg_int32 us)
{
	float oneIteration=0.000001342194658704;
	int iterations;
	iterations=(int)(us*1000000*oneIteration);
	busy(iterations);
}




--
Øyvind Harboe
http://www.zylin.com

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