Shifting r1 by 9 has the right effect. Do you think there is a way to
incorporate HZ into this udelay routine? I got the impression that you
don't want to add any more instructions to this routine due to timing
constraints.
-Jaspreet
Russell King - ARM Linux wrote:
>
> On Tue, Apr 24, 2001 at 02:01:59PM -0400, Jaspreet Singh wrote:
> > There is a comment in that file which states:
> >
> > loops = (r0 * 0x10c6 * 100 * loops_per_jiffie) / 2^32.
> >
> > Now I couldn't dissemble the above expression from the code.
>
> Unfortunately, its overly complicated to keep the arithmetic within
> acceptable precision. The code is:
>
> mov r2, #0x6800
> orr r2, r2, #0x00db
> mul r1, r0, r2 r1 = r0 * 0x68db
> ldr r2, LC0
> ldr r2, [r2]
> mov r1, r1, lsr #11 r1 = r1 >> 11
> mov r2, r2, lsr #11 r2 = loops_per_jiffy >> 11
> mul r0, r1, r2 r0 = r1 * r2
> movs r0, r0, lsr #6 r0 = r0 >> 6
>
> Plugging those all into each other:
>
> r0 = (((r0 * 0x68db) >> 11) * (loops_per_jiffy >> 11)) >> 6
>
> Now, simplify:
>
> r0 = ((r0 * 0x68db * loops_per_jiffy) >> 11) >> 6
>
> r0 = (r0 * 0x68db * loops_per_jiffy) >> 28
>
> r0 = (r0 * loops_per_jiffy) * (26843 / (2 ^ 28))
>
> r0 = (r0 * loops_per_jiffy) * (26843 / 268435456)
>
> r0 = (r0 * loops_per_jiffy) / 10000.2
>
> Now, taking the original expression:
>
> loops = (r0 * 0x10c6 * 100 * loops_per_jiffy) / 2^32.
>
> loops = r0 * 429400 * loops_per_jiffy / 2^32
>
> loops = r0 * loops_per_jiffy / 10002.3
>
> The code we now have is more accurate than the comment (which actually
> reflects the old code).
>
> The constant you will need to adjust is the 0x6800 and 0x00db to
> something around 0x1a36c. Unfortunately you need an extra instruction
> to load it, you might as well just shift right the r1 value by two.
>
> mov r2, #0x6800
> orr r2, r2, #0x00db
> mul r1, r0, r2
> ldr r2, LC0
> ldr r2, [r2]
> mov r1, r1, lsr #13 11 for 100Hz, 13 for 400Hz
> mov r2, r2, lsr #11
> mul r0, r1, r2
> movs r0, r0, lsr #6
> _____
> |_____| ------------------------------------------------- ---+---+-
> | | Russell King [EMAIL PROTECTED] --- ---
> | | | | http://www.arm.linux.org.uk/ / / |
> | +-+-+ --- -+-
> / | THE developer of ARM Linux |+| /|\
> / | | | --- |
> +-+-+ ------------------------------------------------- /\\\ |
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